Postactivation potentiation during voluntary

230

Appl. Physiol. Nutr. Metab. Downloaded from www.nrcresearchpress.com by EDITH COWAN UNIVERSITY (M) on 03/22/15 For personal use only.

ARTICLE Postactivation potentiation during voluntary contractions after continued knee extensor task-specific practice Laurent B. Seitz, Gabriel S. Trajano, Fabien Dal Maso, G. Gregory Haff, and Anthony J. Blazevich

Abstract: The purposes of this study were to determine whether performing dynamic conditioning activities (CAs) contributes to postactivation potentiation (PAP); to examine the potential confounding effects of CAs with different velocity, total contraction duration, and total work characteristics; and to gain a greater understanding of potential peripheral and central mechanisms underlying PAP. Voluntary (isokinetic knee extensions at 180°·s−1) and electrically evoked torques and electromyogram (EMG) data were captured before and 1, 4, 7, 10, and 13 min after 5 different dynamic CAs (4 knee extensions at 60°·s−1, 4 and 12 at 180°·s−1, and 4 and 20 at 300°·s−1), after the participants had completed a full warm-up including extensive task-specific practice to the point where maximal voluntary contractile capacity was achieved. Even after maximal voluntary contractile capacity had been achieved, the imposition of CAs of longer total contraction duration (6 s) and a minimum total work of ⬃750–900 J elicited significant increases in both voluntary (for 7 min; up to 5.9%) and twitch (for 4 min; up to 13.5%) torques (i.e., PAP), regardless of the velocity of the CA. No changes in EMG:M-wave were detected after any CA. A dynamic voluntary CA can contribute to improved voluntary and electrically evoked torques even when maximal voluntary contractile capacity has previously been achieved. Furthermore, a minimum CA contraction duration and minimum total work appear important to increase torque production, although movement velocity appears unimportant. Changes in peripheral function but not central drive may have contributed to the observed PAP under the present conditions. Key words: conditioning activity, torque, knee extension, electrical stimulation, velocity. Résumé : Cette étude avait pour objectifs de déterminer les effets de stimulus conditionnant dynamiques possédant différentes vitesses et durées de contraction sur la potentialisation musculaire (« PAP »), et d’améliorer la compréhension des mécanismes périphériques et centraux sous-jacents a` la PAP. Le moment de force maximal volontaire (extensions iso-cinétiques du genou a` 180°·s−1) et involontaire (stimulations électriques supra-maximales) et les activités électromyographiques (EMG) des extenseurs du genou ont été enregistrés avant et 1, 4, 7, 10 et 13 min après cinq stimuli conditionnant: 4 répétitions a` 60°·s−1, 4 et 12 répétitions a` 180°·s−1, et 4 et 20 répétitions a` 300°·s−1. Les participants avaient réalisé au préalable un échauffement incluant une pratique spécifique et extensive de la tâche jusqu’a` ce que la capacité de contraction musculaire volontaire soit atteinte. Les stimuli conditionnant possédant des temps de contraction plus longs (6 secondes) et produisant 750–900 J au minimum induisent des augmentations significatives du moment de force maximal volontaire (pendant 7 min, jusqu’a` 5,9 %) and twitch et involontaire (pendant 4 min, jusqu’a` 13,5 %). En revanche, la vitesse de contraction n’a aucun effet sur la PAP. Aucun changement de l’EMG:l’onde M n’a été observé après chacun des stimuli conditionnant. Un stimulus conditionnant dynamique contribue a` la PAP même lorsque la capacité de contraction musculaire volontaire a été précédemment atteinte. Un temps minimum de contraction et d’énergie développée semblent être des facteurs déterminants, alors que l’importance de la vitesse de contractions semble être limitée. Des changements au niveau périphérique semblent contribuer a` la PAP alors que l’influence des mécanismes centraux semble être limitée. Mots-clés : stimulus conditionnant, moment de force, extension du genou, stimulation électrique, vitesse.

Introduction There is significant practical interest in the idea that the performance of maximal, or near maximal, voluntary muscle contractions (i.e., a voluntary conditioning activity (CA)) might result in an increase in force production in a subsequent contraction. This postcontraction increase in force production is commonly termed postactivation potentiation (PAP); voluntary PAP is defined as the increase in muscular force production during a voluntary contraction, whereas twitch PAP refers to the increase in muscular force production during an electrically elicited (twitch) contraction. There is substantial evidence demonstrating an increase in voluntary muscle force production after a CA (see reviews by Hodgson et al. 2005 and Tillin and Bishop 2009). However, voluntary PAP responses

elicited by voluntary CAs have often been explored in subjects who did not perform a warm-up before baseline measurements were taken (Gossen and Sale 2000; Hamada et al. 2000; Jubeau et al. 2010; Miyamoto et al. 2011a, 2011b) or who performed only standardized warm-up activities consisting of cycling or running, light (Duthie et al. 2002; French et al. 2003; Jo et al. 2010) or dynamic (Turki et al. 2011) stretching, and submaximal repetitions of the performance test. The findings from such studies are important from a practical standpoint because they show that voluntary PAP can be elicited after a voluntary CA, particularly when extensive warm-ups are not possible (or feasible). However, they do not allow speculation as to whether improvements in performance might be possible in exercise, sporting, and some clinical contexts where extensive preexercise routines are completed before exercise. It is also not possible

Received 5 September 2014. Accepted 3 November 2014. L.B. Seitz, G.S. Trajano, G.G. Haff, and A.J. Blazevich. Centre for Exercise and Sports Sciences Research (CESSR), School of Exercise and Health Sciences, Edith Cowan University, Joondalup, WA 6027, Australia. F. Dal Maso. Laboratory of Simulation and Motion Modelling, Kinesiology Department, University of Montreal, QC H3C 3J7, Canada. Corresponding author: Laurent B. Seitz (e-mail: [email protected]). Appl. Physiol. Nutr. Metab. 40: 230–237 (2015) dx.doi.org/10.1139/apnm-2014-0377

Published at www.nrcresearchpress.com/apnm on 6 November 2014.

Appl. Physiol. Nutr. Metab. Downloaded from www.nrcresearchpress.com by EDITH COWAN UNIVERSITY (M) on 03/22/15 For personal use only.

Seitz et al.

to determine whether the increase in muscle performance observed after a PAP-inducing CA is a consequence of acute neuromuscular alterations relating to the CA itself or whether it simply reflects warm-up and/or familiarization effects (MacIntosh et al. 2012). A definitive answer to this question would be important to those who commonly complete a full warm-up routine prior to participation in sports or exercise and who seek an additional strategy to enhance muscle performance. To address this question, a complete warm-up can be performed until continued task-specific practice results in no further torque enhancement, and then a voluntary CA with different force–time or movement pattern characteristics can be imposed to determine whether it enhances subsequent torque production. A considerable body of evidence supports the possibility of extra muscle force development even after maximal muscle contractile capacity (i.e., maximal torque production) has been attained (Collins et al. 2001; Frigon et al. 2011). For example, variable-frequency muscle or nerve stimulation techniques such as triangular-frequency stimulation ramps (greater force is produced at the same stimulation frequency on the descending slope of a triangular-frequency-ramped contraction; Binder-Macleod and Clamann 1989), top hat stimulations (e.g., 20-80-20 Hz frequency trains, where greater force is produced at the same stimulation frequency in the second 20-Hz train; Frigon et al. 2011), and staircase stimulations (constant low-frequency stimulation progressively increases force production; Rack and Westbury 1969) have been shown to increase force generation above that elicited using standard stimulation parameters. However, it remains to be determined whether incidences of “extra torque” can occur under voluntary conditions (i.e., PAP) following voluntary contractions (i.e., voluntary CAs) with altered force–time characteristics. Previous studies of the effects of voluntary CAs on voluntary force/torque production have revealed significant increases (French et al. 2003; Miyamoto et al. 2011b; Seitz et al. 2014a, 2014b), no changes (Gossen and Sale 2000; Gourgoulis et al. 2003), or decreases (Duthie et al. 2002; Chiu et al. 2003) in muscle function (i.e., jump and sprint performance, isokinetic peak velocity, and isokinetic torque production). Therefore, there is some contention as to whether PAP can be reliably elicited during voluntary contractions, even when extensive task practice is not undertaken. Based on previous evidence, it is likely that a voluntary CA that elicits PAP after a complete warm-up would have particular contraction velocity, duration (i.e., time under tension), and/or total work characteristics (Chaouachi et al. 2011). It was previously demonstrated that a fast CA (300°·s−1 knee extension) benefited subsequent performance in fast (300°·s−1) but not slow (60°·s−1) tests, whilst a slow CA had no effect on either fast or slow performance (Chaouachi et al. 2011). These findings are indicative of a velocity-specific effect; however, some caution in interpretation is required, since baseline muscle performance may not have been stable prior to the CA being undertaken, as a complete task-specific warm-up was not included. Also, the fast and slow CAs had different contraction durations (3 and 15 s, respectively) and probably resulted in different total work being performed. Therefore, the relative influence of voluntary CA velocity on the development of voluntary PAP could not be clearly determined. Altering parameters such as the velocity of a voluntary CA clearly alters the contraction duration in addition to the total work performed, and these effects have not been separated in previous studies. Although it has been previously demonstrated that the contraction duration of an isometric CA influences voluntary PAP (Vandervoort et al. 1983), it has yet to be determined whether this occurs using dynamic CAs. The contraction duration and total work of dynamic CAs may influence voluntary PAP because the performance of CAs of shorter contraction duration or of too little work might prevent the triggering of the mechanism(s) responsible for voluntary PAP, whilst longer contraction durations or too much work might induce excessive fatigue and therefore mask potentiation (MacIntosh and Rassier 2002). The testing of dynamic CAs with different contraction duration and velocity characteristics would allow their effects to be examined

231

simultaneously with the effect of total work. Nonetheless, given the above, it could be hypothesized that dynamic CAs that elicit voluntary PAP after a complete task-specific warm-up, thus replicating the phenomena observed during some electrical stimulation protocols, might have specific contraction velocity, duration, and total work characteristics. One difficulty with determining the optimum conditions under which PAP is evoked is that the mechanisms contributing to the effect are still poorly understood. There is a large body of published research indicating that an improved excitation–contraction (E-C) coupling efficiency may be an important mechanism underlying PAP (Tillin and Bishop 2009). However, it is important to note that the time course of twitch PAP, which lasts up to 5–6 min and peaks immediately after completion of a CA, appears to be different to the time course of voluntary PAP, which persists for up to 18.5 min and peaks at 7–10 min (Wilson et al. 2013). These results suggest that the mechanisms underlying twitch and voluntary PAP may be at least partly divergent. Also, it is not known whether changes in the E-C coupling efficiency can be elicited when a complete task-specific warm-up is imposed before the CA. At the central level, increases in electromyogram (EMG) activity (Hough et al. 2009) and recruitment of higher-order (type II) motor units (Gullich and Schmidtbleicher 1996; Trimble and Harp 1998; Folland et al. 2008) are thought to contribute to voluntary PAP. However, these results may be at least partly influenced by inherent methodological constraints associated with EMG activity and H-reflex measurements. Therefore, such findings must be interpreted with caution. Nevertheless, the results of Fukutani et al. (2013) suggest that changes in central drive may not be a major influence on voluntary PAP, although a complete warm-up was not provided in that study. The inclusion of a complete task-specific warm-up may provide important information regarding the potential influences of central drive on voluntary PAP under these specific conditions. Given the above, the purposes of the present study were (i) to determine whether performing dynamic CAs could contribute to improved maximal voluntary knee extensor torque production in individuals who had performed a complete warm-up including extensive task-specific practice to the point where no further improvement in voluntary torque production could be achieved by continued task practice; (ii) to examine the influence of dynamic CA velocity, total contraction duration, and total work characteristics on changes in torque production; and (iii) to gain a greater understanding of potential peripheral and central mechanisms underlying PAP. It was hypothesized that performing a knee extension CA would increase both voluntary and electrically evoked torque production, that the velocity of the CA would have little impact on changes in torque production as long as the total contraction duration and total work of the CA exceeded a particular threshold, and that PAP would be associated with changes at both the peripheral (twitch torque and M-wave) and central (EMG amplitude normalized to M-wave amplitude; EMG:M) levels.

Materials and methods Participants Seventeen resistance-trained men (mean ± SD; age, 25.4 ± 3.9 years; height, 1.82 ± 0.4 m; body mass, 84.3 ± 10.5 kg) volunteered for the study. All volunteers had been involved in a lower-body resistancetraining program for power and/or muscle strength for at least 6 months. They were required to abstain from taking any stimulants or depressants prior to testing; this included abstention from caffeine for at least 6 h and alcohol for at least 24 h before testing. The experimental procedures were approved by the Edith Cowan University Human Research Ethics Committee (project code: 7818) and were in agreement with the principles of the Declaration of Helsinki. Informed consent was obtained from each participant. Published by NRC Research Press

Appl. Physiol. Nutr. Metab. Downloaded from www.nrcresearchpress.com by EDITH COWAN UNIVERSITY (M) on 03/22/15 For personal use only.

232

Study design and overview The participants visited the laboratory on 6 separate occasions, separated by 3–4 days, at the same time of day on each occasion. The purpose of the first visit was to familiarize the participants with the isokinetic knee extension and electrical stimulation procedures. During subsequent visits, the participants completed an experimental procedure consisting of (i) determination of the intensity required to evoke the maximum M-wave amplitude, (ii) a complete warm-up including extensive task-specific practice, and (iii) a test protocol performed before and 1, 4, 7, 10, and 13 min after completion of 1 of 5 knee-extension CAs. The participants were seated on a dynamometer (Biodex System 3 Pro, Biodex Medical System, Shirley, N.Y., USA) during the electrical twitch stimulations and voluntary contractions, with their dominant (strongest) thigh strapped to the dynamometer’s chair and the ankle fixed to the dynamometer lever arm. The lateral femoral epicondyle was aligned to the axis of rotation of the dynamometer, and the knee and hip joints were flexed at 90° and 85°, respectively. Nerve stimulation procedure (twitch torque) On arrival at the laboratory, the participants cycled on a Monark cycle ergometer for 5 min at 60 rpm with 1 kg resistance. They were then seated on the dynamometer chair and the stimulation intensity required to evoke the maximal M-wave amplitude was determined by delivering single 0.2-ms square-wave pulses to the femoral nerve using a constant current stimulator (DS7H, Digitimer Ltd., Welwyn Garden City, UK). The cathode electrode (Kendall Medi-Trace 200 series electrode, 10 mm diameter) was positioned immediately beside the femoral artery in the inguinal region and the anode electrode was positioned over the greater trochanter. The cathode was moved above the inguinal ligament to find the location that produced the greatest M-wave amplitude at a low stimulation intensity. The maximal intensity was then determined by increasing the current until a further increase in stimulus intensity failed to elicit a greater M-wave amplitude. To ensure that a supramaximal current was used during the twitch stimulation, an intensity of 120% of maximal M-wave amplitude was used. The peakto-peak amplitude of the M-wave was calculated from the vastus lateralis (VL) EMG data recorded during the evoked twitches. Measurement of muscle activity (EMG) After careful preparation of the skin by shaving, lightly abrading, and cleaning with alcohol to reduce the impedance below 5 k⍀, the surface EMG signals from VL, vastus medialis, and rectus femoris were recorded at a 4-kHz analogue-digital conversion rate using a Bagnoli-8 Main Unit EMG system (DelSys Inc., Mass., USA) and band-pass filtered (10–500 Hz) using LabChart software (PowerLab system, version 6.1.3, ADInstruments, NSW, Australia). The interelectrode distance was 1 cm and the reference electrode was positioned on the patella. Surface EMG was also recorded from VL using a pseudo-monopolar configuration (sample rate 4 kHz) using the BioAmp EMG system (PowerLab system, ADInstruments, NSW, Australia) to obtain M-wave data. The active electrode was positioned over the VL, the dispersive electrode was positioned over the quadriceps tendon proximal to the superior border of the patella, and the reference electrode was placed on the tibia. This configuration, which is similar to that used previously for the triceps surae (Pinniger et al. 2001; Trajano et al. 2013), gave larger and more reliable responses in pilot testing. Muscle activity was expressed as root mean square EMG amplitude (100-ms averaging windows) and normalized to the M-wave amplitude to obtain EMG:M. Complete task-specific warm-up procedure Following determination of the stimulation parameters, the participants performed 2 isokinetic knee extensions at 180°·s−1 at

Appl. Physiol. Nutr. Metab. Vol. 40, 2015

20%, 40%, 60%, and 80% of their perceived maximal torque at 45-s intervals (8 knee extensions in total). Isokinetic knee extensions at 100% of maximum were then performed “as fast and as hard as possible” every minute until peak torque production in 3 consecutive contractions differed by less than 2%. This was used to ensure that the warm-up was complete and that further contractions had no additional effect on voluntary torque production. A 90-s rest period, chosen based on pilot data showing that this rest interval did not affect torque production, was imposed prior to starting the test protocol. Test protocol The test protocol was completed 90 s before (pretest) and 1, 4, 7, 10, and 13 min after (post-test) each CA. For each test, 1 supramaximal stimulus was delivered at rest to the femoral nerve with the knee angle set at 60° (0° = full knee extension) 15 s before 2 maximal dynamic knee extensions at 180°·s−1, which were separated by a 15-s rest period. An angular velocity of 180°·s−1 was chosen based on research demonstrating that the magnitude of PAP is greater during tests at fast (i.e., 180°·s−1) versus slow (30°·s−1) joint angular velocities (Fukutani et al. 2013). For the dynamic knee extensions, the participants received verbal encouragement to extend their knee “as fast and as hard as possible” throughout the whole range of motion. The range of motion was set to 90° to 0° (0° = full knee extension) and participants were asked to move through the entire range of motion; a repetition was completed when the lever arm was stopped at the mechanical stop position of the dynamometer. The participants were asked to relax their leg before extending their knee, so the knee angle was at 90° before the knee extension was initiated. The leg was passively returned to the starting position after the shortening contraction. The knee extension resulting in the highest voluntary peak torque at each time point (i.e., pretest and 1, 4, 7, 10, and 13 min after each CA) was selected for further analysis. Voluntary PAP was calculated as follows: % Voluntary PAP ⫽ [(␶vol,post-CA ⫺ ␶vol,pre-CA)/ ␶vol,pre-CA] × 100 where ␶vol,post-CA and ␶vol,pre-CA are the voluntary peak torques measured during the load range phase (i.e., the period during which velocity was maintained at the predetermined velocity; Brown et al. 1995) of the test protocol after and before the CA, respectively. Twitch PAP was calculated as follows: % Twitch PAP ⫽ [(␶tw,post-CA ⫺ ␶tw,pre-CA)/ ␶tw,pre-CA] × 100 where ␶tw,post-CA is the elicited peak twitch torque measured at rest during the test protocol after the CA and ␶tw,pre-CA is the elicited peak twitch torque measured before the CA. Voluntary peak torque was achieved during the isovelocity phase only. Conditioning activity protocols CAs involved the performance of a series of maximal dynamic contractions as outlined in Table 1. Knee extensions were performed 10 s apart. The effect of the CA at 1 knee extension velocity was examined in each of 5 randomized sessions. Two CAs (i.e., CA180/4: 4 knee extensions at 180°·s−1; CA300/4: 4 knee extensions at 300°·s−1) of shorter contraction duration were performed to test the hypothesis that the contraction duration of a CA is an important factor influencing PAP. CA180/4 also served as the control condition, since it included only a small number of extra repetitions of the test contraction. Calculation of total contraction time and total work Total contraction time was calculated as (ROM/vCA) × n, where ROM is the range of motion of the knee extensions performed Published by NRC Research Press

Seitz et al.

233

Appl. Physiol. Nutr. Metab. Downloaded from www.nrcresearchpress.com by EDITH COWAN UNIVERSITY (M) on 03/22/15 For personal use only.

Table 1. Conditioning activities (CAs) used in the experimental protocol. Conditioning activity

Repetitions and angular velocity

Total contraction time (s)

CA60/4 CA180/12 CA300/20 CA180/4 CA300/4

4 repetitions at 60°·s–1 12 repetitions at 180°·s–1 20 repetitions at 300°·s–1 4 repetitions at 180°·s–1 4 repetitions at 300°·s–1

6 6 6 2 1.2

Fig. 2. Maximal voluntary torques produced during the last 3 knee extensions during warm-up (warm-up 1, warm-up 2, warm-up 3) and the knee extension during pretest. CA, conditioning activity.

Fig. 1. Percent difference between the highest and lowest voluntary torques produced during the last 3 knee extensions during warm-up and the knee extension before each conditioning activity (CA).

Fig. 3. Time course of (A) voluntary and (B) twitch torques after the 5 conditioning activities (CA).

during the CA (i.e., 90°), vCA is the velocity (°·s−1) of the CA, and n is the number of knee extensions performed during the CA. Total work was calculated throughout the whole range of motion as follows: [兺(␶mean,KE)/n] × ␪ where ␶mean,KE is the average torque of each knee extension repetition performed during the CA, n is the number of knee extensions performed during the CA, and ␪ is the total angular displacement (in radians) of the knee extension repetitions performed during the CA. This calculation includes the acceleration and deceleration phases of the movement and provides a better representation (in comparison to the work during load range only) of what would happen during traditional resistance exercises, which contain both acceleration and deceleration phases (Wilson et al. 1991) and are commonly used to induce PAP. Statistical analyses One-way repeated measures ANOVAs were performed to compare (i) the voluntary torque produced during the last 3 knee extensions of the task-specific warm-up and during the pretest knee extension to determine whether the task-specific warm-up was complete and (ii) the work done during the different CAs. Separate 2-way (time × condition) repeated measures ANOVAs were performed to compare changes in all variables before and 1, 4, 7, 10, and 13 min after each CA. Pairwise comparisons with Bonferroni corrections were performed when significant interaction effects were detected. Pearson’s correlation analyses were used to assess the relationship between total work and the maximal voluntary PAP response. For all statistical analyses, the level of significance was set at an ␣ level of 0.05.

Results Complete task-specific warm-up On average, the participants required 4.6 ± 1.1 knee extensions at their maximal capacity to produce less than 2% difference in voluntary torque in 3 consecutive knee extensions. There were no differences for any condition between the highest and lowest torques produced during the last 3 knee extensions during warm-up and the knee extension before each CA (Fig. 1). Similarly, no differences were observed between the maximal voluntary torques produced during the last 3 knee extensions in warm-up and the knee extension during pretest in any condition (Fig. 2). The lack of a statistical difference in voluntary torque production among these contractions indicates that maximal voluntary contractile capacity was achieved before undertaking the CA and that the task-specific warm-up was complete, since no further improvement in voluntary torque production could be achieved by further practice using a 45-s rest interval. Published by NRC Research Press

234

Appl. Physiol. Nutr. Metab. Vol. 40, 2015

Appl. Physiol. Nutr. Metab. Downloaded from www.nrcresearchpress.com by EDITH COWAN UNIVERSITY (M) on 03/22/15 For personal use only.

Fig. 4. Maximum voluntary postactivation potentiation (PAP) response (%) for (A) all participants after each conditioning activity (CA) (n = 85; *, significantly different from CA180/4 and CA300/4) and (B) all participants after conditioning activities of different contraction durations (*, significantly different from 2 and 1.2 s). (C) Correlation between maximum voluntary PAP response (%) and total work done during the CAs. (D) Total work done after each conditioning activity (*, significantly different; †, significantly different from CA60/4, CA180/12, and CA300/20).

Voluntary PAP Figure 3A shows the significant interaction (time × condition) effect for voluntary PAP (p < 0.04). Post hoc analyses revealed a significant increase in voluntary torque from 4 to 7 min for CA60/4 (4 knee extensions at 60°·s−1; 4.8% ± 3.0% and 3.0% ± 2.8% at 4 and 7 min, respectively), CA180/12 (12 knee extensions at 180°·s−1; 5.9% ± 4.1% and 4.0% ± 3.1% at 4 and 7 min, respectively), and CA300/20 (20 knee extensions at 300°·s−1; 3.6% ± 2.4% and 2.1% ± 1.7% at 4 and 7 min, respectively), whilst no significant difference from the pretest value was found at 10 and 13 min. There was no change in voluntary torque after CA180/4 (control) and CA300/4 (p > 0.1). Twitch PAP Similarly, Fig. 3B shows the significant interaction (time × condition) effect for twitch PAP (p < 0.05). Post hoc analyses revealed a significant increase in twitch torque from 1 to 4 min for CA60/4 (13.5% ± 6.6% and 4.9% ± 4.5% at 1 and 4 min, respectively), CA180/12 (13.4% ± 7.4% and 4.7% ± 4.3% at 1 and 4 min, respectively), and CA300/20 (10.6% ± 6.2% and 2.6% ± 2.4% at 1 and 4 min, respectively), whilst no significant difference from the pretest value was

found at 7, 10, and 13 min. There was no change in twitch torque following CA180/4 and CA300/4. Contraction velocity As shown in Figs. 3A and 4A, CA60/4, CA180/12, and CA300/20 induced similar (p > 0.05) changes in maximal voluntary torque post-CA. Likewise, changes in post-CA maximal twitch torque were similar (p > 0.05) after CA60/4, CA180/12, and CA300/20. These results suggest that the CA contraction velocity was not an important factor for both voluntary and electrically evoked PAP. Contraction duration Figure 4B shows that CA60/4, CA180/12, and CA300/20, the CAs with the longest contraction duration (i.e., 6 s), elicited significant maximal voluntary and twitch PAP responses (p < 0.05). There were no statistically significant differences in maximal voluntary or twitch PAP responses among these 3 CAs. In contrast, CA180/4 and CA300/4, the CAs with the shortest contraction durations (2 and 1.2 s, respectively), did not elicit voluntary or twitch PAP (p > 0.05). Published by NRC Research Press

Seitz et al.

235

Appl. Physiol. Nutr. Metab. Downloaded from www.nrcresearchpress.com by EDITH COWAN UNIVERSITY (M) on 03/22/15 For personal use only.

Fig. 5. Example of data obtained from 1 participant before (left) and 4 min after (right) a conditioning activity. EMG, electromyogram; MVC, maximum voluntary contraction; RMS, root mean square.

Total work As shown in Fig. 4C, no correlation (r = 0.18; p = 0.32; r2 = 0.03) was found between the total work done during the CAs that did not induce PAP (CA180/4 and CA300/4; left side) and the PAP response elicited by these activities. Similarly, no correlation (r = –0.08; p = 0.59; r2 = 0.01) was found between the total work done during the CAs that induced PAP (CA60/4, CA180/12, and CA300/20; right side) and the PAP response elicited by these activities. Note, however, that a correlation existed for all data (r = 0.42; p = 0.0001; r2 = 0.18), indicating at least some importance of total work; however, the heteroscedastic nature of the data invalidates the use of standard correlation procedures. At least a minimum amount of work appeared to be required for PAP to be induced (i.e., ⬃750– 900 J in the present experiments; Fig. 4C). As shown in Fig. 4D, significant differences (p = 0.0001) were observed in the total work produced during CA60/4, CA180/12, and CA300/20, although these CAs induced similar PAP responses. In addition, the total work produced during CA180/4 and CA300/4 was significantly (p = 0.001) less than that produced during CA60/4, CA180/12, and CA20/300. Electromyogram and M-wave amplitudes When compared with the pretest values, no significant interaction effect was found for EMG:M or M-wave amplitude at any time point for any CA, indicating a lack of change in central drive and muscle excitability following the CAs (p > 0.05) (raw data for 1 participant are shown in Fig. 5).

Discussion The acute effects of dynamic (isokinetic) CAs of different contraction velocity, duration, and total work characteristics on subsequent torque production were examined in the knee extensors after individuals performed a complete warm-up including extensive task-specific practice to the point where no further improvement in voluntary torque production could be achieved with

repeated test practice (i.e., maximal voluntary contractile capacity was achieved). The main findings were as follows: (i) performance of dynamic CAs could improve subsequent voluntary and electrically evoked torques, even when maximal voluntary contractile capacity was achieved after completing a full warm-up; (ii) dynamic CAs of longer contraction duration (i.e., time under tension) elicited significant increases in torque production regardless of the movement velocity and total work done during a CA, although a minimum of work appears to be required for the PAP to be elicited; and (iii) no change in M-wave amplitude or EMG:M was observed after the CAs. Consistent with previous published research (Babault et al. 2008; Chaouachi et al. 2011; Fukutani et al. 2013), the present investigation demonstrates that the performance of isokinetic CAs can improve both voluntary and electrically evoked torques. Because participants completed a full task-specific warm-up in which additional practice resulted in no further effect on voluntary torque production (i.e., maximal voluntary contractile capacity was achieved under these conditions), the present data demonstrate that increases in voluntary and electrically evoked torque production following a CA with force–time characteristics different to the performance test may result specifically from acute physiological changes in response to the CA, rather than being either a warm-up or familiarization effect. Our findings indicate that the participants’ maximal voluntary muscle contractile capacities were achieved before the CA was performed, since maximal torque in the final 3 contractions in the warm-up differed by