Matthew J Hodgson
B.PHE. Physical Education Queen's University 2001 A thesis Submitted in Partial Fulfillment of the
Requirements of the Degree of MASTER OF SCIENCE In the School of Physical Education
O Matthew J Hodgson, 2005 University of Victoria
All rights reserved. This thesis may not be reproduced in whole or in part, by photocopy or other means, without permission of the author.
Co-Supervisors: Dr David Docherty & Dr. Paul. Zehr
ABSTRACT
The purpose of this study was to understand how the previous contractile history of muscle influences volitional force production. An additional objective was to quantify the effect of contractile history on specific indices of neuromuscular output, in order to examine potential underlying mechanisms which may contribute to alterations in force production following muscle contractions. Plantar flexor twitch torque (P3, soleus (SOL) H-reflexes and the RFD of explosive plantarflexions (exPFs) were obtained from 13 trained, university aged men before and after 4 separate trials consisting each of a conditioning series of 3, 5-second maximal voluntary isometric contractions (MVIC) of the plantar flexors. P, significantly increased @<0.05) relative to control values, while H- reflexes and the RFD of exPFs demonstrated no significant differences from controls. There was also no significant correlation between the temporal profiles P, and exPF RFD or the temporal profiles of H-reflex amplitude and exPF RFD. The results suggest that although P, was significantly higher post-conditioning it had no associated effect on volitional force production. It is also suggested that any post-activation potentiation (PAP) effect is isolated to electrically-evoked conditions.
Table of Contents Abstract Table of Contents List of Tables List of Figures Acknowledgements Introduction
Statement of the problem and purpose Research questions Hypotheses Operational definitions Limitations Delimitations Assumptions Methodology Participants Experimental design Familiarization sessions Protocol
Electromyography and H-reflex measurements Muscle twitch measurements
Recordings of isometric force Statistics
Results
Twitch contractile properties
11 ... 111
v
vi vii 2 5 6 7 8 9 10 10 11 11 11 14 14H-reflex measurements Contractile RFD Discussion
Change in muscle twitch force H-reflex measurements Contractile RFD
Implications for motor performance Conclusions
References
Appendix A: Tables
Appendix B: Informed Consent
Appendix C: Literature Review: Effect of Contractile History on Neuromuscular output
Vita
List of Tables Table 1 Group mean P, and M,, values for ISOt Table 2 Group mean P, and M,, values for COMt
Table 3 Group mean M wave and H-reflex values for ISOh and COMh Table 4 Group mean Tpeak and RFD,,, values for COMt and COMh Table 5 Group mean RFD absolute values for COMt
List of Figures Figure 1 Experimental timeline
Figure 2 Experimental set up for isometric plantar flexion Figure 3 Temporal profile of P, group data for ISOt and COMt Figure 4 Percentage change in P, for ISOt versus COMt
Figure 5 Temporal profile of SOL H-reflex group mean data for ISOh and COMh
Figure 6 Temporal profile of SOL M wave group mean data for ISOh and COMh
Figure 7 Temporal profile of Tpeak group mean data for COMt and COMh
Figure 8 Temporal profile of RFD,, group mean data for COMt and COMh
Figure 9 Temporal profile of absolute RFD group mean data for COMt and COMh
Acknowledgments
I would like to thank my graduate supervisors, Dr. David Docherty and Dr. Paul Zehr for their guidance and insight. I also owe tremendous gratitude to all the members of the Rehabilitation Neuroscience Laboratory: Sandra, Pam, Holly, Olle, Jen, Kristy, Erin and Jackie.
Effects of Contractile History on Neuromuscular Output Matthew J. Hodgson
Introduction
The response of skeletal muscle to volitional command or electrically induced stimuli is affected by its contractile history. Neuromuscular fatigue, which can briefly be defined as the decrease in force observed after a period of repeated muscle activation (Rassier & Macintosh, 2000), is the most obvious effect of contractile history. In contrast to fatigue which serves to impair force production, there is evidence that the contractile history of skeletal muscle may facilitate the volitional production of force - this
phenomenon is known as post-activation potentiation (PAP) (Sale, 2002). Although fatigue and potentiation have opposing effects on force production in skeletal muscle, these two mechanisms can coexist (Rassier & Macintosh, 2000). Force output following contractile activity reflects the net balance between processes that enhance force
development and those that diminish it (Vandenboom & Grange, 1993). Identification of the possible physiological mechanisms mediating alterations in force production will promote the development of strategies which are effective in optimizing such production.
The two most prevalent measures of neuromuscular output used to quantify the effect of previous activation history on subsequent force production have been: a) muscle twitch force (for a review see Sale, 2002) and b) H reflex amplitude (Gollhofer, Schopp, Rapp & Stroinik, 1998, Gullich & Schmidtbleicher, 1996; Trimble & Harp, 1998). In addition to examining the mechanisms that may account for PAP, there have been applied movement studies that have investigated the effect of contractile history (induced by means of a maximal or near-maximal preloading exercise) on subsequent dependent measures of mechanical power performance, such as, vertical jump height and rate of force development (RFD) in an explosive movement (Duthie, Young & Aitken, 2002; Gilbert, Lees & Graham-Smith, 2001; Gossen & Sale, 2000; Gullich &
A twitch is a brief muscle contraction in response to a single presynaptic action potential or a single, synchronized volley of action potentials (Latash, 1998). The force of a twitch contraction is increased following: 1) a sustained maximal voluntary contraction (MVC) ) (Gossen & Sale, 2000; Hamada, Sale, MacDougall & Tarnopolsky, 2000; Vandervoort, Quinlan, & McComas, 1983); 2) an evoked tetanic contraction (O'Leary, Hope, & Sale, 1997); or 3) repeated sub-fusion stimuli (Macintosh & Willis, 2000). In addition to enhancing peak twitch force, preceding forms of contractile conditioning (listed above) have also been shown to increase the rate of force development (RFD) in a twitch response and decrease its time to peak force (Grange, Vandenboom & Houston,
1993; Sale, 2002). This effect, known commonly as twitch potentiation (TP), is a well established and reproducible phenomenon, although its functional relevance to human motor performance is less clear. One proposed mechanism of TP is the phosphorylation of myosin regulatory light chains via myosin light chain kinase, which theoretically renders actin-myosin interaction more sensitive to ca2+released from the sarcoplasmic reticulum (Sweeny, Bowman & Stull, 1993).
Although the application of TP and its mechanism(s) to human performance has yielded indeterminate results, there is evidence to suggest that strength and speed performance can theoretically be affected by TP (Sale, 2002). As previously noted, TP has been shown to increase the RFD of isometric force even at high stimulation frequencies greater than 100 Hz (Abbate, Sargeant, Verdijk, & de Haan, 2000;
Vandenboom et al., 1993), analogous to those observed during voluntary ballistic muscle actions (Van Cutsem, Duchateau & Hainaut, 1998). It is theoretically possible, that TP, through subsequent increased RFD and acceleration, would increase peak velocity and power attained during the performance of dynamic muscle contractions (Gossen & Sale 2000). This hypothesis has yet to be confirmed, however, and further research attempts using strategies to produce TP and its subsequent effects are required.
The H reflex is another measurement tool utilized by researchers to study the effects of contractile history on neuromuscular response. The H reflex is traditionally defined as a monosynaptic reflex (MSR) induced by an electrical stimulation of Group Ia afferents of the muscle nerve (Latash, 1998). Initially viewed as a purely MSR,
modification of H reflex amplitude via oligosynaptic pathways (e.g., Ib inhibitory affects from Golgi tendon organs and large cutaneous afferents) (Burke, Gandevia & Mckeon,
1984) and through mechanisms that influence levels of presynaptic inhibition (PSI) has disconfirmed its use as a direct measure of alpha-motoneuron excitability (Zehr, 2002). It is also important to note that even though it is frequently employed as an estimate of spinal reflex processing, a number of methodological issues exist which can influence interpretation of the H reflex and, in some instances, undermine its validity as a measurement. Thus, rigorous experimental controls must be implemented to safeguard against a multitude of confounding factors which can affect the accurate interpretation of H reflex measurements (for a detailed list see Zehr, 2002).
In regard to H reflex modulation following volitional activation, two main effects have been found a) post-activation depression (PAD); and b) post-activation potentiation or reflex potentiation (RP). PAD develops immediately upon muscle relaxation and, depending on the nature of the preceding contractile activity, its time course can be relatively brief, lasting 10 to 60 s (Crone & Neilsen, 1989; Enoka et al., 1980, Gollhofer et al., 1998), or can persist for several minutes (Gullich & Schmidtbleicher, 1996; Trimble & Harp, 1998).
H reflex potentiation post-volitional activation has received considerably less attention than either PAD or potentiation induced via high-frequency electrical stimulation of the la afferents of the homonymous muscle, known more commonly as post-tetanic potentiation (PTP) (Corrie & Hardin, 1964; Van Boxtel, 1986). It is theorized that if some attributes of tetanic electrical stimulation, such as, the sustained recruitment
of high threshold motor units, are reflected in the volitional conditioning stimulus, the potentiation of the reflex could occur in response to previous voluntary activation (Trimble & Harp, 1998). Studies that have demonstrated this effect are limited and subject to methodological issues (Gullich & Schmidtbleicher, 1996; Trimble & Harp,
1998).
With reference to PAP, the existing literature has classically ascribed this phenomenon to physiological events localized within the muscle, such as the
phosphorylation of myosin light chains (Sweeny, Bowman & Stull, 1993). However, in lieu of findings regarding H reflex potentiation following contractile activity, PAP also appears to occur at the spinal level, through an increased synaptic efficacy between Ia afferent terminals and alpha-motoneurons of the homonymous muscle (Corrie & Hardin, 1964; Gullich & Schmidtbleicher, 1996; Trimble & Harp, 1998; Van Boxtel, 1986). It is also possible that PAP is the result of both myogenic and neurogenic mechanisms (Tubman, Macintosh & Maki, 1996). Further, although theoretically possible (Sale, 2002) there is no evidence to date that supports a direct functional benefit of twitch potentiation. It would, therefore, appear instructive to establish experimental protocols that provide a valid index of force production with other concurrent measures of
neuromuscular output (i.e., twitch force and H reflex) which may reveal the locus or loci of mechanisms mediating potential alterations in volitional force production post- contractile conditioning.
Statement of the Problem
Research investigating the effects of contractile history, induced via volitional activation, on various measures of neuromuscular output, have documented equivocal results regarding a functional role in human performance for the PAP phenomenon.
In
light of the limited and contradictory findings regarding H reflex potentiation following amaximal voluntary conditioning stimulus, further clarification of H reflex modulation post-volitional activation is required. In addition, there is no evidence to date which supports a direct functional role of twitch potentiation. Finally, only two studies (Gossen & Sale, 2000; Gullich & Scmidtbleicher, 1996) have included concurrent physiological measures of neuromuscular output when examining the effect of contractile history on human performance.
Statement of the Purpose
The purpose of this study was to investigate the acute effect of contractile history on the temporal profiles of:
i) plantar flexor twitch torque
ii) H reflex amplitude of the LG and SOL muscles
iii) Explosive isometric plantar flexion force development
An additional objective of this study was to examine the correlation between the temporal profiles of twitch force and explosive isometric force development; and, secondly, the correlation between the temporal profiles of H reflex amplitude and explosive isometric force development.
Research Questions
1. Are there differences in the mean plantar flexor twitch torque response post- conditioning compared with control values?
2. Are there differences in the mean LG and SOL H reflex amplitudes post- conditioning compared with control values?
3. Are there differences in the mean rate of explosive isometric plantar flexion force development post-conditioning compared with control values?
demonstrating H reflex potentiation following a series of sustained maximal voluntary isometric contractions incorporating more rigorous internal validity?
5. Is there a significant correlation between the temporal profiles of plantar flexor twitch torque and the rate of explosive isometric plantar flexion force development post- conditioning?
6. Is there a significant correlation between the temporal profiles of LG or SOL H reflex amplitude and the rate of explosive isometric plantar flexion force development post- conditioning?
Hypotheses
1. A significant potentiation of plantarflexor (PF) twitch force is expected post- conditioning. The peak value of PF twitch force will be evident immediately (5 s) following conditioning and will diminish gradually with time.
2. A significant depression of H reflex amplitude immediately post-conditioning followed by a return to baseline values and subsequent significant potentiation occurring approximately 2-4 min post-conditioning.
3. A significant positive correlation between the temporal profiles of H reflex amplitude and explosive isometric plantar flexion force development (expPF).
4. A non-significant correlation between the temporal profiles of PF twitch force amplitude and expPF.
Operational definitions
Maximum Voluntary Isometric Contraction (MVIC): a sustained maximal effort voluntary contraction performed at a fixed muscle length. The MVIC consists of a maximal isometric plantarflexion contraction held for a 10 s duration.
Muscle Twitch: a brief muscle contraction in response to a single presynaptic action potential (AP) induced via electrical stimulation of the muscle nerve. Twitch torque recorded is that produced by the triceps
surae muscle group in response to electrical stimulation of the tibial nerve.
Hoffmann (H) Reflex: reflex induced by electrical stimulation of the muscle nerve. It is an estimate of spinal reflex processing involving the synaptic interaction of Ia afferent terminals and alpha- motoneurons of the homonymous muscle. The H reflex is elicited in the triceps surae muscle via electrical
stimulation of the tibial nerve.
Rate of Explosive Force Development (expPF): the peak rate of isometric plantar- flexion force development (+dFldt,,) (mN/s), determined by differentiating the force profile.
Triceps Surae: muscle group of the posterior lower leg involved in plantarflexion. This consists of the medial (MG) and lateral gastronemius (LG) muscles and the soleus (SOL) muscle.
Post-Activation Twitch Potentiation (PATP): an increase in the force of a twitch
contraction relative to a constant stimulation intensity following previous contractile activity (series of MVICs).
Post-Activation Reflex Potentiation (PAW): an increase in the peak to peak amplitude of the H reflex relative to a constant stimulation intensity following previous contractile activity (series of MVICs).
Post-Activation Depression (PAD): a decrease in the peak to peak amplitude of the H reflex relative to a constant stimulation intensity following previous contractile activity (series of MVICs).
Limitations
1. The results will describe the effect of contractile history (series of MVICs) on measures of neuromuscular output (twitch force, H reflex amplitude and expFD) obtained from the triceps surae muscles of trained males between the ages of 19-25. 2. If alterations in force production are found post-conditioning, results are relevant only
for isometric contractions.
3. Validity and reliability of H reflex measurements (see Zehr, 2002).
4. Mobilization of the PAP mechanism(s) with performance of a series of explosive isometric plantar flexions (Sale, 2002), such that, the temporal profiles of twitch force and H reflex amplitude are modified relative to the performance of successive
contractions.
5. M wave potentiation post-conditioning; this translates to an unstable stimulation condition which in turn reduces the accuracy of interpretation of H reflex modulation following contractile activity.
6. Large inter-individual variability with respect to dependant measures
7. Muscle fiber characteristics (i.e., % of type I1 fibers) of the triceps surae muscles of subjects.
Delimitations
1. Participants are trained males aged 19-27.
2. A series of MVICs of 5 s duration with a 50 s rest interval represents the conditioning stimulus.
3. The rate of explosive isometric PF force development (expPF) used to represent volitional force production.
4. Muscle twitch force and H reflex amplitude and expPF are indices of neuromuscular output.
Assumptions
1. Muscle twitch force, H reflex amplitude and expPF are valid measures of neuromuscular output.
2. The performance of a series of 3 MVICs of 5 s duration involve the sustained recruitment of high-threshold MUs, thus reflecting an attribute of electrically induced tetanic stimulation and the subsequent effects of PTP.
3. Any changes in the dependent measures represent an effect of prior contractile conditioning rather than an error in measurement or a change in protocol. 4. The periodic performance of explosive isometric PFs will not significantly alter
Methodology Participants
Thirteen trained university-aged male subjects volunteered to participate in the study (body mass 86.1 k 9.6 kg, height 179 k 5 cm, age 23.5 k 2.4 yr, means k SD). A
trained participant was defined as having at least 2 years experience with heavy lower body resistance training andlor playing a sport at the inter-university level or above that requires the repeated explosive action of plantar flexion. Participants had no known neurological or orthopaedic pathologies and were free from lower leg injury at the time of data collection. Dominant lower extremity was determined by asking participants which leg they preferred to use when kicking a ball. Participants were instructed to refrain from caffeine consumption or intense lower body exercise 24 hours prior to data collection. Written consent explaining the purpose and possible risks associated with the study was obtained from all participants prior to participation, following approval from the University of Victoria Human Ethics Committee.
Experimental Design
The temporal profile of PAP and its effect on the rate of volitional force development in the human tricep surae musculature were studied. A quasi-experimental design was used. This consisted of a repeated measures within-subjects design @re-test, post-test) in which subjects served as their own controls. The experimental protocol is illustrated in Figure 1. This protocol was devised so that the temporal profile and extent of PAP could be quantified and recorded via evoked isometric twitches and H-reflexes. The effect of PAP on the rate of volitional force development could then be assessed by the performance of explosive isometric plantar flexions (exPFs). An experimental session was approximately 2.5 hours in duration and consisted of the following: a standard warm up (5 minute stationary bike) followed by the collection of an M-H recruitment curve
which was recorded approximately 2-5 minutes post-warm up (following electrode placement and apparatus adjustment), then 4 test trials each approximately 15 minutes in duration. These were performed in random order and each separated by a 15 minute rest period. Each trial required participants to complete a series of 3 maximal voluntary isometric contractions (MVICs) of 5 second duration, with a 55 second recovery interval between successive contractions. This was followed by an 11 minute post-conditioning recording period in which subsequent indices of neuromuscular output (twitches, H reflex and exPFs) were obtained for evaluation. Twitch and H-reflex protocols were performed in isolation (IS0 trials) or in combination with exPFs (COM trials). The COM trials were identical to the I S 0 trials except that exPFs were performed at certain times when twitches or H-reflexes would have occurred in the IS0 trials. This was done in order to provide an estimate of the twitch or reflex response in the absence of successive exPFs. Participants were also required to participate in one familiarization session in order to acquaint themselves with the experimental procedures and performance measures (MVICs and exPFs).
Protocol ISOt Muscle max twitch characteristics (post conditlonlng recording tlrne
-
10 min)3x5s MVCS (I mln rest
blw reps)
P T O ~ O C O ~ C O M t Muscle max twltch characteristics + balllstic Isometric plantar flexions (BCs)
3x5s MVCS (1 mln rest btw reps)
-
Protocol ISOh; H reflex d a t aPre Conditlonlng 29 1 Om
I I
I
I
I 1 r - - - - - - ICon H reflex - 1 0
tnals
H reflex acqvlsltlon (rand- stlm aag 6lmln) ror 10 rnln post-condltlonlng
Protocol COMh: H rerlex + Ballistic Contractions (BCS) - p e r f o r m e d at 1 5 s p o s t - c o n d l t l o n ~ n g a t 1
rnln I n t e r v a l s u p to 10 rnln w c BC BE B C BC B C B C ec B C wc BC Pre Condltlonlng 12s 68s 128s 180s ~ b k 380s 368s 428s 4sbs 520s 680s-
8
=
1 c o n H r.4.-10 t r l d r c Con BlPFs F3x5e MVCs (, rest H reflex acqulsltlon (randomstlm avg 61mln) for 1 Omln post-condltlonlng
btw reps)
Figure IB. Experimental timeline for H-reflex trials (ISOh and COMh)
Apparatus
Testing sessions required participants to be seated in a chair with their backs supported. Hip, knee, and ankle angles were set at approximately 90•‹, 150 O, and 90•‹, respectively (see figure 2). Restraints were placed around the foot to minimize
movement. Subject posture was maintained throughout recording periods in order to control for task dependency of reflex modulation. The temperature, noise, and lighting were held as constant as possible between sessions.
Familiarization session
The primary purpose of this session was to familiarize the subjects with the experimental apparatus, the electrical stimulation procedures and performance of the MVICs and exPFs. Subjects were required to perform three 5 second MVICs and as many exPFs as required so that rate of force production became easily reproducible.
Protocol
Electromyography and H-reflex measurements- M-H recruitment curves, ISOh & COMh protocols
Electromyography (EMG) was recorded with bipolar surface recording electrodes. EMG signals were pre-amplified and band pass filtered at 30-300 Hz. Surface recording electrodes were placed over the lateral gastrocnemius (LG) and soleus muscle (SOL) of the test leg. Additional electrodes were placed over the tibialis anterior (TA) of the test leg to monitor antagonist muscle activity. M-H recruitment curve data were obtained for each subject immediately following the warm up. To evoke the SOL and LG H-reflex, the tibia1 nerve was stimulated by a cathode placed in the popliteal fossa and an anode positioned just inferior to the patella of the test leg. Before testing, the skin was lightly abraded and prepped with alcohol to ensure a stable skin-electrode interface. The nerve was localized by the electrode placement which produced a muscle twitch response with the lowest amplitude current pulse. Once localized, an H-reflex and M-response recruitment curve were obtained by progressively increasing the amplitude of the current pulses until a maximum H-wave and then a maximum M-wave were obtained. Subjects were instructed to maintain a constant level of background EMG (approx 5% of max) throughout all H-reflex data acquisition. EMG levels were visually provided on an oscilloscope. The test reflex intensity to be used in ISOh and COMh trials was selected to produce a small M-wave and an H-wave on the rising phase of the H-reflex recruitment curve. Before performing the conditioning MVICs in both the ISOh and COMh trials
subjects received 10 stimuli at the select intensity (approximately 10-15% of M,,) elicited pseudo randomly with an interval of 2-3 seconds. Subjects then performed the conditioning exercise. The H-reflex was elicited in the same manner as before conditioning and recorded in the inter-conditioning intervals (ICl & IC2) and for 11 minutes post-conditioning. The subjects remained in the seated position with hip, knee and ankle angles kept constant (see above) throughout the duration of the pre- and post- conditioning periods. In the COMh trial, subjects also performed a brief series of 3 exPFs, each separated by a 2 second interval. These force recordings were obtained as a control (pre) before the conditioning exercise and control H-reflex recordings and again at 15 seconds, 1 minute and at 1 minute intervals for the duration of the post-conditioning period. The H-reflex and evoked M-response for each stimulation were analyzed for peak-to-peak amplitude.
All H-reflex data were analyzed off-line using a custom LABVIEW data acquisition program. Data were sampled at 2000 Hz with a 12 bit A/D converter controlled by the LABVIEW program. All M-H recruitment curves used 50 sweeps of data collection with a 20 ms pre stimulus window. To compare the pre- and post- conditioning H-reflex amplitude, data were blocked into pre-conditioning control trials (10 sweeps lasting approx. 1-2 minutes) and in 1 minute blocks of trials (8-9 sweeps) during the inter- and post-conditioning period. Not all blocks contained the same number of sweeps. If a sweep occurred when background EMG levels deviated from control levels by
*
5%, this sweep was deleted from further analysis. To analyze data across subjects, data were normalized by dividing the H-wave amplitude by the corresponding- in-time maximum M-wave (M,,) amplitude obtained during the isometric twitch trials.Muscle twitch measurements (ISOt and COMt protocols)
Electrode configuration and tibia1 nerve stimulation used to evoke supramaximal twitch responses in the plantar flexor muscles are the same as those described in the H- reflex measurement section (see above). A control maximal twitch response was evoked in subjects by delivering a series of single stimuli of increasing intensity until a plateau of twitch torque and M-wave amplitude was observed. This intensity was recorded and used for subsequent pre- and post-conditioning twitch responses evoked during ISOt and COMt trials. The ISOt trial consisted of one control maximal twitch stimulation immediately before the conditioning MVICs. Max twitches were evoked again immediately following each MVIC, and at 15, and 30 seconds and at 30 second intervals for the remainder of the 11 minute recording period. The COMt trial required subjects to perform a series of 3 exPFs trial at exactly the same times as the COMh trial, thus at 15s and at 1 minute intervals post-conditioning subjects performed a series of 3 exPFs instead of receiving maximal twitch stimulation.
Recordings of isometric force
Torque values were established via strain gauge (Omegadyne Ltd. Model 101- 500, range 0-226.7 kg) and amplified by a custom made high gain amplifier system. The force was displayed using custom built continuous acquisition software utilizing LABVIEW. Torque was calculated after MVIC, max twitch or exPF by converting voltage output into kg (1.00V = 45.3 kg). Plantar flexion force was consistently applied with a moment arm length of 0.15m (measured from the adjustable heel block to the center of the strain gauge). In addition to peak torque (P,), isometric twitches were also analyzed for time to peak torque (TPT) and half relaxation time (HRT). The RFD of the explosive plantar flexions was evaluated in 3 ways: time to peak force (Tpeak); the average RFD (RFD,,,), derived as the average slope of the moment-time curve over 5
equal time intervals differentiated relative to the onset of force and Tpeak; and the RFD in discrete time intervals of 0-30, 0-60, 0-90, 0-120, 0-150 ms relative to the onset of contraction.
Statistics
A one-way ANOVA with repeated measures over time was employed to analyze pre- and post-conditioning values for P, H-reflex amplitude, and the RFD of exPF. In the presence of significant F values a Tukey's post hoc comparison was used to compare each post-conditioning value to pre-conditioning values. A pearson correlation was also performed to determine whether the predictor variables (P, and H-reflex amplitude) were significant predictors of the criterion variable (RFD of exPF). The alpha level was set at pC0.05 for significance.
Results Twitch contractile properties
Mean control (pre) P, values ranged from 32.3 N-m (ISOt) to 34.1 N-m (COMt), with no significant differences between mean values. The lack of a significant difference between twitch trials indicates that the 15 minute rest interval between trials was
sufficient for a return to resting twitch conditions, such that any TP induced during previous trials had subsided before the beginning of subsequent trials. Correspondingly, there were also no significant differences in M,, amplitude between twitch trials. Values ranged from 5524.7 mV (ISOt) to 5183.1 mV (COMt). There were significant (p< 0.05) differences between pre and post P, values for both the ISOt and COMt. A Tukey's post hoc comparison revealed significant differences between pre and ICI
,
IC2,2 s, 15 s for ISOt (p< 0.05) and between pre and IC2,2 s, 30 s and 90 s for the COMt trials (pC0.05). Figure 4 shows the twitch responses (normalized to control values) throughout the course of the recording period. P, values were highest immediately following the conditioning activity in ISOt (20.7% increase at 2 s post-conditioning) and at 30 s (1 8.8% increase) in COMt. A significant (p< 0.01) difference was also found between post-conditioning P, values in ISOt and COMt. This suggests that the performance of voluntary exPFs interspersed with twitch stimulations (COMt trials) caused an increase in the amount of TP relative to the same stimulations in-time during ISOt. There was no significant difference between pre and post normalized M,, values for ISOt, however there was a significant difference (p< 0.05) in pre and post M,, values obtained during COMt (see Table 2).H reflex measurements
H-reflexes were normalized to corresponding-in-time M,, values obtained during ISOt and COMt trials and expressed as a percentage of M,,. SOL H-reflex
amplitude demonstrated a trend of initial H-reflex depression post-conditioning (see figure 5), although this effect failed to reach statistical significance. There was also a trend toward an initial M wave depression post-conditioning (see figure 6), although it also failed to reach statistical significance. There were no significant differences in SOL pre- and post-conditioning H-reflex amplitude in either ISOh or COMh trials (see figure 5). There were also no significant differences in absolute or normalized M wave
amplitude (direct motor response accompanying H-reflex). Reliable reflex responses in SOL were obtained in all subjects. A reliable LG reflex response was found in only 5 of 12 subjects and results were similar to that of SOL such that there were no significant differences between pre- and post-conditioning values. Further, there were no significant differences in either M wave or H reflex values between ISOh and COMh trials. An ANOVA revealed no significant difference in TA activation within subjects across all H- reflex trials; therefore antagonist muscle activity did not affect H-reflex recordings.
Contractile RFD
There were no significant differences in mean exPF RFD,,, for either COMt or COMh (see table 4). There was a significant difference (p< 0.05) in mean Tpeak in COMt at 420 s post-conditioning, however this actually revealed a significant decrease in contractile performance relative to control values and was accompanied by variable responses in Tpeak throughout the post-conditioning recording period (see figure 7A), therefore it is unlikely that this effect was due to the previous activation of the muscle. Moreover, there were also no significant differences in Tpeak in COMh (see table 4). There were significant differences (p< 0.05) in mean exPF RFD when normalized into discrete time intervals (see table 5). These differences however, revealed no systematic temporal pattern (see figure 9) and were unlikely to be the caused by the conditioning MVICs.
A. ISOt Time (s) 42 - 40 - 38 -
3 3 6 -
Z
&- 34 - 32 - 3 0 - 28 - 26 B. COMt 44 - 42 - 40 -3
3 8 - z V 3 6 - 34 - 32 - * - * * - I I I I I I I I , I I I 30 I I I 8 I Ipre IC1 IC2 2 30 90 15 210 270 330 390 450 510 570 630 pre ICl IC2 2 30 90 15 210 270 330 390 450 510 570 630
Time (s)
Figure 3. A: Temporal profile of P, data (means
*
SE) for ISOt (N=13). B: similar P, data (means SE) for COMt (N=12). Asterisks (*) indicates significant difference from Pre value (p < 0.05).-0- ISOt
...O... COMt
n
Time (s)Figure 4. Percentage increase in P, relative to Pre values for ISOt (N=13) and COMt (N=12). Asterisks (*) indicates significant difference between ISOt and COMt value recorded at same time interval (p < 0.05).
A. ISOh 6 5
1
3 0 I I I u I I I I I I Pre I C 1 I C 2 M I M 2 M 3 M 4 M 5 M 6 M 7 M 8 M 9 M I 0 M I 1 T i m e ( m i n ) B. COMh 7 0 -8
6 5 - s V Q -3 .-
2 6 0 - .-
-
E
5 5 - 82
5 0 -3
9. 4 5 -3
a 4 0 I I 1 I I I I I I 1 I , P r e I C l I C 2 M I M 2 M 3 M 4 M 5 M 6 M 7 M 8 M 9 M 1 0 M I 1 T i m e ( m i n )Figure 5.
A:
Temporal profile of H-reflex SOL data (means*
SE) for ISOh (N=12). B: Temporal profile of H-reflex SOL data (means + SE) for COMh (N=l 1). H-reflex values are expressed as a % of M,
,
obtained during corresponding twitch trials (ISOt and COMt).A. ISOh 2 6 1 6 1 I I 1 I I I I I I I I I 8 P r e I C I I C 2 M I M 2 M 3 M 4 M 5 M 6 M 7 M 8 M 9 M I 0 M I 1 T i m e ( m i n ) B. COMh 2 2
-
p
2 0 - 2s.
V 4 1 8 - 3 u .-
-
1 6 -<
0 1 4 - 2 A 8 12 -3
u % l o -a"
8 I I I I I 1 I I I I I 1 I Pre I C 1 I C 2 M I M 2 M 3 M 4 M 5 M 6 M 7 M 8 M 9 M 1 0 M 1 1 T i m e ( m i n )Figure
6.
A:
Temporal profile ofM
wave SOL data (means t SE) for ISOh (N=12). B: Temporal profile of M wave SOL data (means+
SE) for COMh (N=l 1). M wave values are expressed as a % of M,, obtained during corresponding twitch trials (ISOt and COMt).A. COMt 2 2 0
-
2 1 0 - 2 0 0 - 1 9 0 - V 1 8 0 - 0c-"
1 7 0 - 1 6 0 - 1 5 0 - 1 4 0 I I 1 I I Pre 15 6 0 1 2 0 1 8 0 2 4 0 3 0 0 3 6 0 4 2 0 4 8 0 5 4 0 6 0 0 T i m e (s)B. COMh
Pre 15 6 0 120 180 2 4 0 3 0 0 3 6 0 4 2 0 4 8 0 5 4 0 6 0 0 T i m e (s)Figure
7.
A:
Temporal profile of Tpeak data (means k SE) for COMt (N=l2).B: Temporal profile of Tpeak data (means
*
SE) for COMh (N=9). Asterisks (*) indicates significant difference from Pre value (p < 0.05).A. COMt T i m e ( s ) B. COMh 2.8 1 , P r e 1 5 6 0 1 2 0 1 8 0 2 4 0 3 0 0 3 6 0 4 2 0 4 8 0 5 4 0 6 0 0 T i m e ( s )
Figure
8.
A:
Temporal profile of RFD,, data (means*
SE) for COMt (N=12). B: Temporal profile of RFD,, data (means -t SE) for COMh (N=9). ). Asterisks (*) indicates significant difference from R e value (p < 0.05).A . COMt
" 1 I , I I
Pre 15 60 120 180 240 300 360 420 480 540 600
Time (s)
1.
Figure 9A: Temporal profile of absolute RFD data for COMt (N=12). RFD was calculated in time intervals of 0-30, 60, 90, 120 and 150 ms from onset of contraction. Asterisks (*) indicates significant difference from Pre value (p < 0.05).
B. COMh
0 I I 1 1 I I I I I I I I 1
Pre 15 60 120 180 240 300 360 420 480 540 600
Time (s)
1
Figure 9B: Temporal profile of absolute RFD data for COMh (N=9). Asterisks (*) indicates significant difference from Pre value (p < 0.05).
Discussion
Although the present study demonstrated a significant increase in plantarflexion twitch force, there was no significant difference in the soleus and lateral gastrocnemius H-reflex amplitude following a series of 3 MVICs of 5 second duration. These measures of neuromuscular output, which were used to quantify and localize the effect of previous activation history on subsequent volitional force production, suggest that PAP appears to reside physiologically in the muscle. While an increase in twitch force was present post- activation, it does not appear to influence the RFD of voluntary explosive isometric contractions or to be affected by enhanced spinal excitability.
Changes in muscle twitch force (TF). TF increased significantly in both ISOt and COMt trials following performance of the conditioning activity. Consistent with the literature, TF values were highest immediately following the MVICs, remained significantly elevated up to approximately 1 min post-conditioning, and then quickly returned to control levels (Hamada, Sale, MacDougall & Tarnopolsky, 2000; Sale, 2002; Vandervoort et al., 1983). This increase, commonly referred to as twitch potentiation (TP), has been classically ascribed to events localized within the muscle, specifically, the phosphorylation of myosin regulatory light chains, which renders the actin-myosin interaction more sensitive to ca2+released from the sarcoplasmic reticulum (Sale 2002; Sweeny et al., 1990).
In addition to the significant increase in TF post-conditioning, the current study demonstrated a significant increase in TF in COMt trials when compared to the same in- time TF values obtained during the ISOt trials. The present findings show that during the COMt trials, when subjects performed a series of three, brief explosive isometric
plantarflexions interspersed between twitch stimulations, these contractions were
conditioning recording period (see fig 4). This event, which has previously been termed the mobilization of the PAP mechanism (Sale, 2002), was also demonstrated when the effects of a 10 second maximal voluntary leg extension on dynamic kicks (used to assess alterations in movement velocity) were additive, such that the TP assessed at 30 s and 50 s post-MVC was greater with, than without (twitch control trial) the added kicks performed in between twitch stimulations (Gossen & Sale, 2000). It remains to be determined whether this effect can be exploited in order to influence functional motor performance.
H-reflex measurements. The results of the present study did not reveal a
significant difference in SOL or LG H-reflex amplitude post-activation relative to control values. These findings are in contrast to those previously presented which found a significant depression of H-reflex amplitude immediately following the conditioning activity (Enoka et al., 1980; Gollohofer, 1998; Gullich & Schrnidtbleicher, 1996; Trimble & Harp, 1998) and a subsequent potentiated H-reflex response evident between 4-1 1 minutes post-exercise (Gullich & Schmidtbleicher, 1996; Trimble & Harp, 1998). The possible reasons for this discrepancy could be related to: 1, the conditioning activity employed; and 2, H reflex stimulation protocol. The conditioning activity used in the present study was selected on the assumption that if some attributes of tetanic stimulation- specifically, the sustained recruitment of high-threshold motor units at stimulation frequencies greater than 100 Hz- were reflected in the conditioning activity, then post-activation potentiation of the H-reflex may occur, as is observed following tetanic electrical stimulation which produces post-tetanic potentiation or PTP in human subjects (Corrie 1973; Van Boxtel, 1986). It has also been suggested that repetitive bouts of a maximal effort exercise may produce continuous descending inhibitory input onto the interneurons controlling PSI of the Ia afferent fibers (Trimble & Harp, 1998). Excluding the Trimble and Harp (1998) study, which employed a cyclic, plantarflexion-
to-dorsiflexion movement, both the Gullich and Schrnidtbliecher (1996) and the Enoka et al. (1980) studies used a similar 5 second MVIC as the conditioning stimulus, the only difference being the number of repetitions performed (5 and 1 respectively). Therefore, it appears unlikely that the conditioning activity would account for the variation in results between these and the present study.
It is believed that methodological differences in H-reflex stimulation parameters are most likely to account for the variation in findings of the present study and the three studies highlighted above (Enoka et al., 1980; Gullich & Schmidtbleicher, 1996; Trimble & Harp, 1998). H-reflexes in the present study were evoked upon a background level of muscle activation- approximately 5 % of MVIC EMG levels. In contrast, the three previous studies evoked H-reflexes while the target muscle was electromygraphically silent. It is recommended that the H-reflex is evoked upon a tonic level of background muscle activity whenever possible (Zehr, 2002). The reason for this is that at rest the relative state of depolarization of the motoneuron pool is unknown. Therefore, in order to ensure a similar level of motoneuron excitability across conditions (pre-post), a constant level of background muscle activation must be maintained. This type of low level contraction has additionally been shown to reduce the variability in both the latency and amplitude of the H reflex (Burke et al., 1989; Funase & Miles, 1999), thus increasing the validity of its use as a measurement. Moreover, it is also important to note that from a functional perspective motor tasks are not performed in quiescent muscles; consequently, H reflexes evoked upon a background level of tonic muscle activity represent a more functionally-relevant measurement.
It is also suggested that when comparing H reflexes between subjects and conditions that reflex amplitude be normalized to M,, in order to account for time- dependant or movement dependant changes (Misiaszek, 2003; Zehr, 2002). H-reflex data in the present study were normalized by dividing the H-wave amplitude by the
corresponding-in-time M,, amplitude recorded during ISOt and COMt. Gullich and Schmidtbliecher (1996) did not include normalized H-reflex data, which, may in turn partially account for the difference in results of the current study. Moreover, due to this reason, their inter-subject comparisons must be interpreted with caution.
Other methodological differences concerning H-reflex stimulation parameters between this and previous experiments include: duration of the post conditioning
recording period (Enoka et al., 1980) in which the post-conditioning recording period was limited to 50 s; and the predominant muscle investigated (Trimble & Harp, 1998; Gullich & Schmidtbleicher, 1996). A significant potentiated reflex response was only observed in the LG muscle in both the Trimble and Harp (1998) and Gullich and Schmidtbliecher (1 996) studies. SOL H-reflexes, although recorded, did not reach statistical significance. H reflexes in the present study were elicited primarily in the SOL muscle, due to ease and reliability of the reflex response. Attempts were made to evoke concurrent reflexes in the LG of all subjects, although only 5 of 12 subjects provided valid H-reflex recordings from the LG. The data was similar to that of the SOL, such that, unlike findings from previous studies (Trimble & Harp, 1998; Gullich & Scmidtbleicher, 1996), H-reflexes post-conditioning did not show any significant deviations from control values.
It is also possible that subject positioning may account for variation in H-reflex amplitude reported across studies (Gullich & Schmidtbleicher, 1996; Trimble & Harp,
1998). Alterations in H-reflex amplitude associated with variable postures and movement are well documented in the literature, and considerable soleus H-reflex modulation occurs when changing posture from lying, to sitting, to standing (see Schieppati, 1987; Zehr, 2002). This variability is considered to be a result of changes in presynaptic inhibition (PSI) which steadily increases as subjects change from lying down, to standing, to walking, in turn decreasing H-reflex amplitude relative to a constant stimulation intensity (Zehr, 2002). The above studies employed different subject positioning compared with
the current study, including lying prone (Trimble & Harp, 1998), lying supine (Gullich & Schmidtbleicher, 1996) and seated (Enoka et al., 1980). As the functional implications of PAP was a basic question under investigation, a seated posture was selected such that it allowed for rigorous controls to be applied to ensure a valid reflex response, while also maintaining a sufficient level of functionality. Although a potentiated reflex response was found in both studies which employed lying postures, the functional relevance of these findings must be questioned, as few performance-related motor tasks are executed while in these positions.
Contractile
RFD.
In addition to increasing peak twitch force, post-activation TP has been shown to increase the isometric rate of force development of an electrically evoked twitch (Vandenboom et al., 1993). This raises the possibility that these findings may be replicated under voluntary conditions. The present study, however found no significant difference in Tpeak or FWDavg in all 4 experimental trials following the conditioning activity. A significant effect was observed when the force profile of the exPFs was analyzed in discrete units of time (30,60,90, 120, 150 msec); however unlike the findings reported by Gullich and Schmidtbliecher (1996) this effect did not reveal any systematic temporal pattern (see fig 9), and was therefore deemed unlikely to be related directly to performance of the conditioning activity.Implications for motorperformance. A number of applied studies have
investigated the effect of a maximal or near-maximal series of conditioning contractions on subsequent indices of motor performance or mechanical power output, and have yielded equivocal results (Duthie et al., 2002; French, Kraemer & Cooke, 2003; Gilbert et al., 2001; Hrysomallis & Kidgell, 2001; Young et al., 1998). However, there are only two other studies (Gullich & Scmidtbleicher, 1996; Gossen & Sale, 2000) which have
provided concurrent physiological measures of PAP in addition to the performance of a motor task, in order to determine the possible mechanisms underlying alterations in
volitional force production following pre-activation of muscle. Gossen and Sale (2000) found that when 10 subjects performed dynamic leg extensions after TP had been induced with a 10 s MVC, the TP failed to increase the attained peak velocity with any load. In fact, results suggested that fatigue produced by the 10s MVC suppressed any benefit that could be derived from the induced TP. In contrast, Gullich and
Schrnidtbleicher (1996) reported a significant pearson correlation (r = .89) between the times of the highest expression of H-reflex amplitude and the rate of explosive force of voluntary plantar flexions following a series of 5 MVICs of 5 second duration. These findings present a strong contrast to the results of the current study which demonstrated no significant effect of conditioning on H-reflex amplitude and no correlation between the temporal profiles of H-reflex amplitude and the RFD of exPFs, or the temporal profiles of TF and the RFD of exPFs. The results of the present study tend support the findings presented by others (Gossen & Sale, 2000), such that any post-activation potentiation effect is isolated to electrically-evoked conditions. The results suggest that there is no observable transfer of PAP to volitional force production.
Conclusions. The present study demonstrated significant increases in muscle twitch force following a conditioning series of maximal voluntary contractions. There was no significant modulation of H-reflex amplitude following the same conditioning protocol, and results failed to reveal a systematic effect of conditioning on the RFD of explosive isometric plantarflexions. These findings suggest that the post-activation phenomenon resides peripherally at the level of the muscle, and is probably due to physiological events localized within the muscle such as the phosphorylation of myosin regulatory light chains (Sale 2002, Sweeny et al. 1990). Moreover, under the conditions of the present experiment, there was no observable correlation between twitch
potentiation and voluntary motor performance. Additional research is required in order to further elucidate a functional role in human performance for the PAP phenomenon.
References
Abbate, F.A., Sargeant, A.J., Verdijk, P.W., & de Haan, A. (2000). "Effects of high-f requency initial pulses and posttetanic potentiation on power output of skeletal muscle." Journal of Applied Physiology, 88,3540.
Burke, D., Gandevia, S.C., & McKeon, B. (1984). "Monosynaptic and oligosynaptic contributions to the human ankle jerk and H reflex". Journal of Neurophysiology, 42,435-447.
Corrie, W.S., & Hardin, W.B. (1964). "Post-tetanic potentiation of H reflex in normal man." Archives of Neurology, 1 1,3 17-323.
Crone, C., & Neilsen, J. (1989). "Methodological implications of the postactivation depression of the soleus H reflex in man." Experimental Brain Research, 78, 28-32.
Duthie, G.M., Young, W.B., & Aitken, D.A. (2002). "The acute effects of heavy loads on jump squat performance: An evaluation of the complex and contrast methods of power development." Journal of Strength and Conditioning Research, 16(4), 530-538.
Enoka, R.M., Hutton, R.S., & Eldred, E. (1980). "Changes in excitability of tendon tap and Hoffmann reflexes following voluntary contractions."
Electrocephalogrphy and Clinical Neurophysiology, 48,664-672.
French, D.N., Kraemer, W.J., & Cooke, C.B. (2003). "Changes in dynamic exercise performance following a sequence of preconditioning isometric muscle actions." Journal of Strength and Conditioning Research, 17(4), 678-685.
Funase, K., & Miles, T.S. (1999). "Observations on the variability of the H reflex in human soleus." Muscle Nerve, 22(3), 34 1-6.
Gilbert, G., Lees, A., & Graham-Smith, P. (2001). "Temporal profile of post-tetanic potentiation of muscle force characteristics after repeated maximal
exercise." Journal of Sport Science, 19,6.
Gollhofer, A., Schopp, A., Rapp, W., & Stoinik, V. (1998). "Changes in reflex excitability following isometric contraction in humans." European Journal of Applied Physiology and Occupational Physiology, 77, 89-97.
Gossen, E.R., & Sale, D.G. (2000). "Effect of postactivation potentiation on dynamic knee extension performance." European Journal of Applied Physiology, 83,524-530.
Grange, R.W., Vandenboom, R., & Houston, M.E. (1993). "Physiological
significance of myosin phosphorylation in skeletal muscle." Canadian Journal of AppliedPhysiology, 1 8,229-242.
Gregory, J.E., Wise, A.K., Wood, S.A., Prochazka, A., & Proske, U. (1998). "Muscle history, fusimotor activity and the human stretch reflex." Journal of Physiology,l3(3), 927-934.
Gullich, A., & Schmidtbleicher, D. (1996). "MVC-induced short-term potentiation of explosive force." New Studies in Athletics, 11(4), 67-81.
Hamada, T., Sale, D.G., MacDougall, J.D., & Tarnopolsky, M.A. (2000)."Postactivation potentiation, fiber type, and twitch contraction time in the human knee extensor muscles." Journal of Applied Physiology, 88,2 13 1-2 137.
Hultborn, H., Illert, M., Neilsen, J., Paul, A., Ballegaard, M., & Wiese, H. (1996). "On the mechanism of the post-activation depression of the H reflex in human subjects." Experimental Brain Research, 108,450-462.
Hyrsomallis, C., & Kidgell, D. (2001). "Effect of heavy dynamic resistive exercise on acute upper-body power." Journal of Strength and Conditioning Research, 15(4), 426-430.
Latash, M.L. (1 998). Neurophysiological Basis of Movement. Champaign
IL:
Human Kinetics.MacIntosh, B.R., & Willis, J.C. (2000). "Force-frequency relationship and potentiation in mammalian skeletal muscle." Journal of Applied Physiology, 88, 2088-2096. Misiaszek, J.E. (2003). "The H-reflex as a tool in neurophysiology: its limitations and
uses in understanding nervous system function." Muscle & Nerve, 28, 144- 160.
O'Leary, D.D., Hope, K., & Sale, D.G. (1997). "Posttetanic potentiation of human dorsiflexors." Journal of Applied Physiology, 83,213 1-2138.
Rassier, D.E., & MacIntosh, B.R. (2000). "Coexistence of potentiation and fatigue in skeletal muscle." Brazilian Journal of Medicine and Biology Research, 33,499-' 508.
Sale, D.G. (2002). "Postactivation potentiation: role in human performance." Exercise and Sport Sciences Reviews, 30(3), 138- 143.
Schieppati, M. (1987). "The Hoffinann reflex: a means of assessing spinal reflex excitability and its descending control in man." Progress in Neurobiology, 28(4),345-76.
Sweeny, H.L., Bowman, B.F., & Stull, J.T. (1993). "Myosin light chain posphorylation in veterbrate striated muscle: regulation and function." American Journal of
Physiology, 264, 1085-1095.
Trimble, M.H., & Harp, S.S. (1998). "Postexercise potentiation of the H-reflex in humans." Medicine and Science in Sports and Exercise, 30(6), 933-941.
Tubman, L.A., Macintosh, B.R., & Maki, W.A. (1996). "Myosin light chain phosphorylation and posttenic potentiation in fatigued skeletal muscle." European Journal of Applied Physiology, 43 1, 882-887.
Van Boxtel, A. (1 986). "Differential effects of low-frequency depression, vibration- induced inhibition, and posttetanic potentiation on H-reflexes and tendon jerks in the human soleus muscle." Journal of Neurophysiology, 55(3), 55 1-568.
Van Cutsem, M., Duchateau, J., & Hainaut, K. (1998) "Changes in single motor unit behavior contribute to the increase in contraction speed after dynamic training in humans." Journal of Physiology, 513,295-305.
Vandenboom, R., Grange, R.W., & Houston, M.E. (1993). "Threshold for force potentiation associated with skeletal myosin phosphorylation." American Journal of Physiology, 265(6 Pt I), 1456-1462.
Vandervoort, A.A., & McComas, A.J. (1983). "A comparison of the contractile
properties of the human gastronemius and soleus muscles." European Journal of Applied Physiology, 5 1,435-440.
Vandervoort, A.A., Quinlan, J., & McComas, A.J. (1983). "Twitch potentiation after voluntary contraction." Experimental Neurology, 8 1, 141 -1 52.
Young, W.B., Jenner, A., & Griffiths, K. (1998). "Acute enhancement of power performance from heavy load squats." Journal of Strength and Conditioning Research, 12(2), 82-84.
Zehr, E.P. (2002). "Considerations for use of the Hofhann reflex in exercise studies."European Journal of Applied Physiology, 86,455-468.
Appendix
A
Table I. Mean (*SE) twitch torque (PJ and Mm, values for ISOt (N=13). *Indicates significant difference fiom pre value (p < .05).
Stimulation P, (Nm) SE M,, SE
Table 2. Mean (iSE) twitch torque (PJ and M,, values for COMt (N= 12). *Indicates significant difference from pre value, tindicates significant difference from ISOt value (p < .05).
Stimulation P, (Nm) SE M,, SE Pre 34.1 2.0 5183.1 550.6 IC 1 37.5 2.3 5636.0* 599.6 IC2 39.3* 1.9 5731.2* 612.4 2s 40.5* 2.1 5814.2* 646.8 30s 40.5*~ 2.0 5469.6* 577.9 90s 38.8*t 2.0 5340.6 563.9 150s 37.6 2.0 5248.2 558.1 210s 37.8t 2.2 5256.8 566.3 270s 36.6t 1.8 5191.2 561.1 330s 35.8 2.1 5197.3 574.2 390s 36.3t 1.7 5157.1 564.3 450s 35.0 2.1 5199.8 559.8 510s 34.6t 1.9 5122.9 552.9 570s 34.2t 2.0 5151.8 558.4 630s 33.6t 1.9 5195.3 557.1
Table 3. Mean (*SE) M wave and H-reflex values for ISOh (N=12) and COMh (N=l 1). All M wave and H-reflex values expressed as a % of M,, obtained during corresponding twitch trials (ISOt and COMt).
Time
1
M wave SE H-reflex SEI
M wave SE H-reflex SEPre 11.8 1.6 49.8 6.9 16.7 2.3 54.2 5.2
ISOh
Table 4. Mean ( S E ) Tpeak and RFDav, values for COMt (N=12) and COMh (N=9) recorded during performance of volitional explosive plantarflexions (exPFs). *Indicates significant difference from Pre value (p< 0.05).
COMh COMt COMh exPFs Pre 164.9 13.3 2.2 0.2 169.0 11.2 2.2 0.2 Tpeak RFDavg (ms) SE ( N d s ) SE Tpeak RFDavg (ms) SE ( N d s ) SE
Table 5. Mean RFD (Nm) of exPFs divided into discrete time intervals (ms) for COMt (N=12). *Indicates
significant difference from Pre value at the (p < 0.05) COMt exPFs 0-30 0-60 0-90 0-120 0-150 Pre 11.6 32.8 50.8 61.5 66.2 15s 14.7 42.2* 65.1* 79.0 85.4 60s 15.7 42.3* 63.3* 75.3 80.5 120s 13.9 39.5 61.2 75.3 82.8 180s 13.8 38.9 60.2 73.8 80.5 240s 12.6 34.8 54.7 68.7 76.1 300s 13.2 35.8 55.0 67.9 74.9 360s 13.8 36.9 56.4 68.9 74.9 420s 13.7 35.1 53.4 66.2 73.6 480s 12.7 35.7 56.0 70.7 79.7 540s 14.2 37.1 57.5 71.4 78.6 600s 13.1 36.1 57.5 72.6 81.2
Table 6. Mean RFD (Nm) of exPFs divided into discrete
time intervals (ms) for COMh (N=9). *Indicates significant difference from Pre value at the (p < 0.05) COMh
exPFs 0-30 0-60 0-90 0-120 0-150
Appendix B Informed Consent
Effect of contractile history on neuromuscular output
You are being invited to participate in a study entitled Effect of contractile history on
neuromuscular output that is being conducted by Matt Hodgson who is a graduate student in the school of Physical Education at the University of Victoria and you may contact him if you have
further questions by either phone (385-0422) or email (hod~sonm(duvic.ca). As a graduate
student, I am required to conduct research as part of the requirements for a degree in Master's of Science. It is being conducted under the co-supervision of Dr. David Docherty and Dr. Paul
Zehr.You may contact my supervisors at 721 -8375/nochert~(a~uvic.ca or 721-
8379 'pzehr@uvic.ca.
The purpose of this research project is to understand how the previous contractile history of muscle influences volitional force production. An additional research objective is to examine the correlation between volitional force production and other measures of neuromuscular output in an attempt to discern underlying mechanisms contributing to possible alterations in force production following muscle contractions.
Research of this type is important because it will help clarify the effect of contractile history induced via volitional activation, on various measures of neuromuscular output. Moreover, this research will provide a valid index of force production with other concurrent measures of neuromuscular output which may reveal the physiological mechanism(s) mediating alterations in volitional force production post-contractile conditioning. From an applied perspective, this will provide a principled basis for the development of strategies which are effective in optimizing force production. This information is relevant to individuals in a clinical motor-rehabilitation setting and individuals training for sport.
You are being asked to participate in this study because you have no history of neurologic or orthopaedic disorders and are free from a lower leg injury. An additional reason although not a necessary requirement for your participation is that you have at least two years experience with heavy lower body resistance training and/or you compete at the inter-university level or above in a sport requiring repeated explosive plantarflexion- i.e, volleyball, basketball, running etc.
If you agree to voluntarily participate in thls research, your participation will include one
experimental test session, which is approximately 2 hours in length. During this session, specified measures of neuromuscular output will be measured before and after a standard conditioning protocol which consists of a series of maximal voluntary isometric plantarflexions performed while seated in an experimental apparatus. Electrical activity of the test muscle will be measured with electrodes placed on the skin over the nerve that innervates the test muscle. Brief pulses of electrical stimulation will be derived to this nerve via these electrodes. This stimulation is used to evoke reflexes and/or direct motor responses measured as changes in muscle activity. The sensation of stimulation will be brief (-21100 of a second) and will feel like strong tingling in the back of the lower leg. The stimulation should evoke a strong sensation but should not be painful. You may feel what the stimulation is like and decide if you wish to continue. You will receive an explanation of the testing procedures both verbally and visually prior to the initiation of the testing session.
Participation in this study may cause some inconvenience to you, including remaining seated in the experimental apparatus without significant postural adjustment or movement for
approximately one hour. Additionally, you may be further inconvenienced by the physical discomfort associated with repeated electrical stimulation of the test muscles. Further
inconvenience may result due to time commitment (3 separate 1 -hour sessions over a period of 2-3
weeks) required for your participation in this study.
There are some potential risks to you by participating in this research. The main risk is accidental shock because electrical equipment is used and connected to you. To prevent or to deal with risks the following steps will be taken. To minimize the risk of accidental shock the equipment is
connected such that the participants are electrically isolated. That is, there is no direct electrical contact between the wall socket into which the equipment is plugged and the electrodes placed on your body for recording muscle activity or stimulating nerves. Because of this, the risk of accidental very low. Moreover, should accidental shock occur, the equipment is furnished with a safety mechanism whereby the amplitude of electrical current transmitted to the participant would be insufficient to cause physical harm or result in systemic shock.
The potential benefits of your participation in this research include improving the state of knowledge concerning how volitional production may be affected by a series of preceding maximal voluntary contractions, and further, how certain features of the neuromuscular system may play a causal role in such alterations of force production.
Your participation in this research must be completely voluntary. If you do decide to participate, you may withdraw at any time without any consequences or any explanation. If you do withdraw from the study your data will not be used in the study.
In terms of protecting your anonymity, data will be stored and protected by assigning a code number to the data sheet rather than a name. Only the principal investigator and the supervising professors will have access to the data. However, your anonymity is partial as the researcher and individuals involved in data collection will know who participated. At no time during the public disclosure of the data (e.g. at conferences, in publications) will individual subject be identified. Your confidentiality and the confidentiality of the data will be strictly protected. Your raw data and electronic files (recorded onto a CD) will be stored in a personal locked file cabinet for a
minimum of 5 years. Upon completion of the retention period, the documents will be shredded and
the CD destroyed.
It is anticipated that the results of this study will be shared with others in the following ways: presentation at scholarly meetings and conferences, in academic journal publications and a Master's level thesis paper.
In addition to being able to contact the researcher (and, if applicable, the supervisors) at the above phone numbers, you may verify the ethical approval of this study, or raise any concerns you might have, by contacting the Associate Vice-president, Research at the University of Victoria (250-472- 4362).
Your signature below indicates that you understand the above conditions of participation in this study and that you have had the opportunity to have your questions answered by the researchers.
Name of Participant Signature Date
Appendix C Literature Review
