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Surface Electromyographic Assessment of the Effect of Dynamic Activity and Dynamic Activity with Static Stretching of the Gastrocnemius on Vertical Jump Performance

Wallmann, Harvey W; Mercer, John A; Landers, Merrill R

Journal of Strength and Conditioning Research: May 2008 - Volume 22 - Issue 3 - p 787-793
doi: 10.1519/JSC.0b013e3181660e27
Original Research
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The purpose of this study was to investigate the effects of dynamic activity and dynamic activity/static stretching of the gastrocnemius muscle on vertical jump (VJ) performance. Additionally, muscle activity was recorded using electromyography. Thirteen healthy adults (7 men and 6 women) with a mean age of 26 ± 4 years served as subjects. The average jump height and muscle activity from 3 separate maximal VJ attempts were performed at the start of each session to be used as baseline measures using the Kistler force plate and the Noraxon telemetry EMG unit. Subjects then performed 1 of 2 protocols: dynamic activity only or dynamic activity with static stretching. Each protocol was followed by 3 maximal VJ trials. Average VJ height was analyzed using a 2 (time: pre, post) × 2 (prejump protocol: dynamic activity, dynamic activity + stretching) analysis of variance with repeated measures on both factors. A paired-samples t-test was used to compare the intraday difference scores for EMG activity between the 2 conditions. Jump height was not influenced by the interaction of pre-post and protocol (p = 0.0146. There was no difference for the main effects of time (p = 0.274) and pre-jump protocol (p = 0.595). Gastrocnemius muscle activity was likewise not different for the 2 prejump protocols (p = 0.413). The results from this study imply that the use of static stretching in combination with dynamic activity of the gastrocnemius muscle does not appear to have an adverse affect on VJ height performance. The practical importance concerns the warm-up routine that coaches and athletes employ; that is, they may want to consider including an aerobic component when statically stretching the gastrocnemius immediately prior to a vertical jumping event.

Departments of Physical Therapy and Kinesiology, University of Nevada, Las Vegas, Nevada

Address correspondence to Harvey W. Wallmann, harvey.wallmann@unlv.edu.

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Introduction

Athletes, ranging from amateur to professional, commonly include static stretching as part of their warm-up routine since it is thought that it may increase tissue temperature, prepare muscles for activity with a decreased chance of injury, and enhance performance (5,10). In contrast to popular thinking, some research has demonstrated that preperformance static stretching can adversely effect vertical jump (VJ) performance (2,7,8,15,25,27). Moreover, other studies have reported little to no effect (17,24). Furthermore, only a few studies have investigated the benefits in performance when combining a stretching protocol with dynamic activity in a warm-up session (18,19,23). Currently, there is very little empirical evidence of the possible effects of combining dynamic activity with static stretching immediately prior to performance.

Previous research has demonstrated that static stretching can have adverse effects on force production and musculotendinous stiffness of the stretched muscle groups (6,9,15,21,23,28). Authors of these studies have concluded that, after stretching, a muscle becomes more compliant, thus adversely affecting maximal force production (9,15,23,28).

Young and Elliot (28) compared the acute effects of 2 stretching protocols and isometric maximal voluntary contraction (MVC) on explosive force production and jumping performance of the squat jump (SJ) and the drop jump (DJ). They reported a significant reduction in the DJ performance and a nonsignificant decrease in concentric explosive muscle performance (SJ) from static stretching compared to the other conditions. Cornwell et al. (7) examined the effects of statically stretching the hip and knee extensors on the SJ and countermovement jump (CMJ) heights and reported a significant decrease in jump heights for both after stretching.

Kokkonen et al. (15) demonstrated that there was a substantial decrease in 1 repetition maximum (1RM) performance for seated knee extension and prone knee flexion after an acute bout of 5 different static stretching activities. A study by Fowles et al. (9) suggested that after prolonged stretching (13 stretches with 5-second rest intervals for a total of 30 minutes) of the gastrocnemius muscle, there were long-lasting effects of a decrease in muscle strength for up to an hour after stretching.

Muscles must have a high degree of stiffness to effectively use elastic energy required for activities such as the VJ (1). Conversely, an increase in muscle compliance may result in an adverse effect on jumping performance. Knudson et al. (14) investigated several kinematic variables during the VJ and reported that, although there were no significant differences between groups, over half of the subjects decreased their jumping performance by an average of 7.5% following stretching. Wallmann et al. (25) reported that static stretching of the gastrocnemius immediately prior to vertical jumping adversely affected jump height by an average of 5.6%.

The results of these studies suggest that the immediate effects of stretching may be detrimental to performances that require high force production such as vertical jumping, resulting in potentially lower jump height values. Young and Behm (27) compared the effects of running, static stretching of the leg extensors, and practice jumps on explosive force production and jumping performance. They reported that submaximal running and practice jumps had a positive effect and that static stretching had an adverse influence on explosive force and jumping performance. They suggested that an alternative for static stretching be considered in warm-ups prior to performance of power activities.

Despite the current literature, the effect of dynamic activity on VJ when combined with static stretching during the warm-up routine remains unclear. In an effort to examine the effects of different modes of stretching within a pre-exercise warm-up, Little and Williams (17) tested 18 professional soccer players for CMJ in addition to other performance variables. The different warm-up protocols consisted of static stretching, dynamic stretching, or no stretching. They reported no difference among warm-ups for the VJ.

It has been demonstrated that gastrocnemius muscle activity remains depressed in muscles after a bout of acute static stretching ranging from 9 to 90 seconds primarily due to muscle relaxation as it is being stretched (2,19,20). In a study comparing fine-wire electromyography (EMG) firing patterns during static stretching of the gastrocnemius muscles, Mohr et al. (19) reported EMG activity to be low, remaining constant with time. This was supported by Behm et al. (2), who reported that integrated EMG (iEMG) activity recorded during a maximal voluntary contraction was lower following a bout of quadriceps stretching. Cornwell et al. (6), however, reported no change in muscle activity of the triceps surae after stretching, and Wallmann et al. (25) reported that gastrocnemius muscle activity was 17.9% greater for the VJ when muscle activity during poststatic stretch jumping was compared with prestatic stretch jumping.

This previous research was necessary to determine the isolated effects of static stretching on performance (18,19,23,27). However, these studies did not investigate the use of dynamic activity or dynamic activity combined with static stretching as might be typically done when performing a warm-up session, nor did they investigate muscle activity during jumping after performing stretching protocols. Additionally, there is a lack of research investigating the acute effects of stretching on a performance activity, such as jumping, for a specific muscle group. In light of previous knowledge, information derived from empirically measuring an outcome such as VJ height might help to further elucidate the mechanisms involved in stretching and dynamic activity. It may be that the combination of dynamic activity and static stretching can offset the detrimental effects of static stretching alone.

Therefore, the purpose of this study was to investigate the effects of combining dynamic activity with static stretching as compared to dynamic activity alone of the gastrocnemius muscle on maximal VJ performance. Secondarily, we examined the muscle activity of the gastrocnemius muscle to understand any possible mechanisms for change in jump height performance.

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Methods

Experimental Approach to the Problem

Previous research has shown us that static stretching alone can have an adverse impact on VJ height (7,25,27,28). Consequently, in this study, we wanted to examine the effects of dynamic activity with static stretching by comparing 2 prejump protocols prior to performing a VJ: dynamic activity plus static stretching versus dynamic activity alone.

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Subjects

Thirteen healthy, untrained, physically active adults (7 men and 6 women) with a mean age of 26 ± 4 years volunteered for this study. Prior to the study, an information session was held in which subjects were asked to complete a background questionnaire to ensure that they met the criteria of no lower extremity injuries or any bone and/or joint disorders within the past year. Subjects were given instructions as to how to properly perform the jumping task and were asked to demonstrate this task to the researchers prior to engaging in the study. The university's institutional review board approved the study, and each subject gave both written and oral consent before participating in the experiment.

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Procedures

Equipment

All subjects used a CMJ style during each vertical jump. A force platform (Type 9281B; Kistler Instrument Corp., Amherst, NY) was used to record ground reaction forces and flight time during a VJ task. Data were collected using the Bioware Version 3.21 software. VJ test reliability in our study was based on between-day baseline measures using the same rater for all subjects: intraclass correlation coefficient (ICC) = 0.990 (95% confidence interval (CI): 0.967-0.997). Intraday consistency reliability for both groups was likewise high: ICC = 0.970 (95% CI: 0.900-0.991) for dynamic activity and stretching, and 0.989 (95% CI: 0.965-0.997) for dynamic activity only.

Muscle activity was recorded with a telemetry EMG system (Noraxon USA Inc., Scottsdale, AZ) using 2 electrodes placed 2 cm apart on the surface of the left lateral gastrocnemius muscle one third of the distance down the leg between the head of the fibula and the calcaneus and a common ground (13). Prior to electrode placement, the area of skin with hair was shaved and cleaned with alcohol. Ground reaction force and EMG data were each sampled for 6 seconds at a rate of 1000 Hz with data sets synchronized using a square wave simultaneously sent to each data acquisition system. Gastrocnemius EMG reliability for between-day baseline measurements was low: ICC = 0.562 (95% CI: −0.435 to 0.866). However, test reliability for consistency on intraday, intrarater gastrocnemius EMG was much higher: ICC = 0.886 (95% CI: 0.628-0.965) for dynamic activity and stretching and 0.940 (95% CI: 0.804-0.982) for dynamic activity only.

Data were analyzed between the time the CMJ started and the take-off time using custom laboratory software. The onset time of the CMJ was identified as the time that the ground reaction force dropped below 20 N of body weight. Take-off time was identified as the moment the vertical ground reaction force was less than 20 N during the jump phase. Force data were used to calculate the acceleration of the center of mass and acceleration was integrated to yield velocity. Jump height was calculated by identifying velocity at the moment of take-off.

Average EMG (EMGavg) was calculated during the jump phase by first extracting data between start of the movement and takeoff. Any zero offset of the data set was removed and EMGavg was then calculated from the full-wave rectified signal.

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Experimental Protocol

The study was conducted over a period of 2 nonconsecutive days with all subjects performing 1 of 2 different conditions each day. Subjects were instructed to avoid excessive physical activity prior to testing and to continue their normal routine between test days. Prior to testing on each day, VJ performance was assessed following a baseline protocol as well as after completing of 1 of 2 different prejump protocols. Three maximal effort VJ trials were performed for each condition. The order of the daily assignment of the prejump protocol was randomly assigned to each subject. Prior to performing the VJ, the subjects were reinstructed in the proper jumping technique. This involved performing a 2-legged vertical CMJ with both feet on the force platform with a maximal effort while the hands were placed on the hips. Since it has been shown that the contribution of the arms to the VJ performance can be 10% or more, subjects were asked to place their hands on their hips in an effort to reduce this potential contribution (11,16).

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Baseline Protocol

Baseline VJ performance was assessed each test day. Upon arrival, subjects were instrumented with EMG surface electrodes. Each electrode was outlined on the skin with a marker in order to have consistent electrode placement between days. After electrode placement, the subjects performed a baseline activity by walking on a treadmill at 3.0 mph (1.34 m·s−1) for 5 minutes. Following the baseline activity, EMG data were collected for 10 seconds of quiet stance followed by 3 baseline countermovement vertical jumps completed with maximal effort. After the jumps, the subjects rested 15 minutes prior to performing the pre-jump protocol.

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Prejump Protocols

Two prejump protocols were completed on separate test days. (a) Dynamic activity only: continuous hopping with both legs together for 1.5 minutes at a pace of about 60-100 jumps per minute. Muscle activity was recorded during the first and last 10 seconds of the warm-up period. (b) Dynamic activity and static stretching: completion of the protocol as outlined in (a) for 1.5 minutes followed by static stretching for 1.5 minutes. Moderate stretch of the gastrocnemius muscle of both legs was performed while standing in full knee extension using a slant board. The muscles of both legs were stretched for 30 seconds 3 times for a total of 1.5 minutes. Stretching was supervised by the investigators with the holding point of each static stretch subjectively selected by each subject as the point prior to discomfort. Muscle activity was recorded during the first and last 10 seconds of this protocol.

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Testing

During each testing session, the subjects completed 3 maximal effort VJ trials within 30 seconds following completion of the prejump protocol.

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Statistical Analyses

VJ height as well as muscle activity (EMGavg) were each averaged across the 3 trials. The dependent variable of VJ height was analyzed using a 2 (time: pre, post) × 2 (prejump protocol: dynamic activity with stretching versus dynamic activity) analysis of variance with repeated measures on both factors (SPSS, version 14.0) for jump height. If an interaction existed, then appropriate simple main effects testing were conducted. If no interaction existed, then we used appropriate main effects testing. Postmeasurement EMG data were normalized to premeasurement EMG (baseline) data for dynamic activity with stretching versus dynamic activity alone. The percentage change of the post-EMG scores compared to baseline scores was analyzed using paired samples t-tests. The α level for all analyses was set at 0.05.

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Results

Jump height was not influenced by the interaction of time (pre-post) and condition (dynamic activity with stretching versus dynamic activity) (F1,12 = 0.418, p = 0.146) (Figure 1). Jump height was not influenced by the main effect for time (pre and post), (p = 0.274) or the main effect for condition (p = 0.5950). There was no significant difference between the dynamic activity with stretching (mean decrease 12.64%, SD ± 34.41%) and dynamic activity alone (mean decrease 2.38%, SD ± 19.14%) conditions for percentage EMG difference scores (t12 = 0.849, p = 0.412) (Figure 2).

Figure 1

Figure 1

Figure 2

Figure 2

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Discussion

Previous research has demonstrated that preperformance static stretching can have an adverse effect on VJ performance (7,25,27). Since these previous studies only investigated the effect of static stretching alone on VJ performance, we wanted to ascertain whether these findings would be different combining dynamic activity with static stretching as might be used in a warm-up routine. Therefore, a primary objective of this study was to determine the influence of the combination of dynamic activity with static stretching compared to dynamic activity alone of the gastrocnemius muscle on VJ performance. We observed that the VJ height was not adversely influenced when static stretching of the gastrocnemius was combined with a dynamic activity or when dynamic activity only was performed. Thus, it appears that preperformance static stretching in conjunction with a dynamic activity does not adversely affect VJ performance.

A second goal of this study was to investigate the gastrocnemius muscle activity as a way to explain the mechanism of influence (if any) of dynamic activity on VJ height performance. It has been hypothesized that the adverse impact of static stretching on jump height performance is due to changes in muscle compliance and/or muscle activation. Obtaining EMG data was necessary in determining the activity in a muscle and may help to discern the role that potential muscle activity depression has on performance following static stretching. We observed that the muscle activity in this study was not influenced when static stretching of the gastrocnemius was combined with a dynamic activity or when dynamic activity only was performed.

The jump height values that were observed in this study were consistent with other studies examining the effects of a warm-up routine that included static stretching on VJ performance for untrained physically active subjects (26-29 cm) (7,25,27). Other studies using trained subjects revealed much higher jump height values (35-48 cm) (4,17,24,28).

The current study is somewhat unique in that we combined dynamic activity with static stretching versus dynamic activity alone. The goal, therefore, was to investigate whether this combination affected VJ performance, not to determine whether dynamic activity alone had a positive effect on VJ height. Additionally, there is a lack of research investigating the acute effects of stretching on a performance activity, such as jumping, for a specific muscle group.

Previous research has shown that VJ height can be adversely influenced when static stretching is the only exercise used in a warm-up routine (7,25,27). While investigating the effects of static stretching of the gastrocnemius muscle on maximal VJ performance, Wallmann et al. (25) reported decreased VJ height values for 13 untrained physically active men and women. Cornwell et al. (7) also found a decrease in VJ height after statically stretching the hip and knee extensors of 10 male untrained physically active subjects. Additionally, Young and Behm (27) showed that static stretching had a negative influence on VJ performance.

In contrast to the current literature, when static stretching is included with some type of aerobic activity, the adverse effects of static stretching on VJ are not necessarily observed (17,24). However, the research designs in these studies differed from what was performed in the current study, as these studies did not combine dynamic activity immediately followed by static stretching. Little and Williams (17) investigated the effects of static stretching, dynamic warm-up, or no stretching at all on VJ height. Aside from the stretching protocols used for 6 muscle groups, they implemented a warm-up that consisted of several minutes of jogging (4 minutes) and agility exercises (4 minutes), followed by a 2-minute rest, prior to assessing VJ. They reported no differences in VJ heights among the 3 groups.

It is difficult to directly compare the results of the present study with that of Little and Williams (17) since in that study, several muscle groups were stretched, whereas in this study, only one muscle group was stretched. It is not clear what length of time the potential adverse effects of static stretching has on jump performance. Knudson et al. (14) investigated several kinematic variables during the vertical jump and reported that even though there were no differences between groups, over half of the subjects decreased their jumping performance by 7.5% following stretching. Although Knudson et al. (14) examined the effects of stretching on the kinematics of the vertical jump, they used a variety of stretches. It might be that by incorporating a number of different stretches, the effect of any one stretch on performance is masked.

Unick et al. (24) examined the acute effects of static, ballistic, and no stretching on VJ performance in trained women. After a warm-up jog of 5 minutes and a 30-second rest, subjects performed 1 of the 3 stretching protocols. This was immediately followed by a 4-minute walk, after which all subjects performed both CMJs and drop jumps. They then rested 15 minutes, and VJ tests were again performed. Due to the elapsed time from stretching to jumping, the effects of the stretching protocols may be questioned, since we really do not have evidence as to the time frame of how long these effects last. Additionally, the walk incorporated immediately prior to the jumps may not constitute a dynamic activity (speed of the walk was not given).

In contrast, the current study assessed only the gastrocnemius and used 2 protocols with the emphasis being on the immediate effects of stretching on VJ performance rather than a prolonged rest session after stretching. One protocol used a dynamic activity (hopping), which was followed by assessing VJ after a 30-second rest. The other protocol involved a dynamic activity (hopping) immediately followed by static stretching, after which VJ was assessed given a 30-second rest. These 2 protocols were preceded by an initial baseline activity (warm-up walk) of 5 minutes and a 15-minute rest. These results suggest that the inclusion of static stretching after a dynamic activity does not adversely affect VJ performance. Since the dynamic activity was performed prior to stretching in this study, the additional muscle activity may have aided in decreasing overall muscle-tendon compliance (and enhancing stiffness), thereby not adversely affecting VJ performance.

No changes in muscle activity were observed in this study with either protocol. This finding is consistent with Cornwell et al. (6), who examined the effects of a warm-up routine that included static stretching on CMJ VJ performance. They reported a significant decrease in single-joint CMJ performance following a short bout of stretching; however, there was no change seen in iEMG activity of the triceps surae after stretching. They concluded that a decrease in jump performance could not be solely attributed to a decrease in muscle stiffness due to the relatively small decrease in active stiffness compared to the larger decrease in jump height.

In contrast, Wallmann et al. (25) reported increased muscle activity during the CMJ VJ following static stretching. They suggested that the subsequent lower jump height could be that the inherent properties of the muscle may have compensated for the possible increased compliance.

Other studies reported decreased muscle activity (2,23), but did not report VJ performance. Behm et al. (2) reported that iEMG activity recorded during an MVC was depressed following a bout of stretching of the quadriceps. They concluded that a decrease in MVC force after 20 minutes of static stretching of the quadriceps was more likely caused by muscle inactivation than changes in muscle elasticity. Rosenbaum and Hennig (23) reported depressed muscle activity following stretching of the gastrocnemius during an Achilles tendon tap reflexive response. Accordingly, there was a decrease in active force production elicited by the tendon tap as well post-stretching.

Combining dynamic activity with static stretching may negate the potential adverse effects of static stretching alone. Moreover, it seems that just performing dynamic activity results in the same outcome as dynamic activity and static stretching.

A limitation of the present study is that we did not attempt to elucidate through our study design what mechanism or mechanisms are responsible for the findings. However, drawing on previous research, one can speculate as to the potential mechanisms involved. For example, Wallmann et al. (25) reported increased muscle activity immediately post-stretching of the gastrocnemius muscle. They suggested that the stretching may have resulted in increased compliance of the musculotendinous unit resulting in the need for the muscle to increase its activity to overcome the purported tendon slack. This notion seems to be consistent with Wilson et al. (26), who suggested that a stiff musculotendinous unit might transmit force more effectively than a compliant unit. Extrapolating to the present study, dynamic activity preceded stretching. By incorporating activity first, musculotendinous unit stiffness may have been enhanced, thus more effectively transmitting force and resulting in decreased muscle activity as observed.

Another limitation may involve the procedure itself. Several factors are important when determining successful performance of the VJ. For example, it was expected that subjects gave a maximal effort for each jump. It is possible that the subjects may not have felt prepared or warmed up enough after the stretching to give a maximal effort. There also exists the possibility of individual differences in stretching intensity. Although instructed to stretch just prior to the point of discomfort, some may have stretched to the point of pain and discomfort, thus eliciting a nociceptive response, which may have inhibited the neural pathways responsible for activation of the muscle, resulting in limited force production (20).

Additionally, coordination, the timing of segmental movement, may also have played an essential role in achieving maximal VJ performance. The transfer of mechanical energy from the proximal to the distal segments in the VJ involves many muscle groups. We chose to investigate the effect of stretching the gastrocnemius muscle only based on its essential and unique role during jumping (3). Previous studies have demonstrated that during the push-off phase of a CMJ, a large plantar flexion moment is required (3,12,22). Jacobs et al. (12) observed that the gastrocnemius produced 25%, the rectus femoris 21%, and hamstrings 7% of the power during a VJ. In addition, Pandy and Zajac (22) noted that the gastrocnemius increased jumping height by as much as 25%.

Contrary to previous research involving static stretching only, the use of dynamic activity in combination with static stretching does not appear to have an adverse effect on VJ height performance. Furthermore, there appears to be no added benefit of dynamic activity in combination with static stretching compared to just dynamic activity prior to maximal VJ performance. Although gender differences were not considered in this study due to the small sample size, further research should examine the effects of different types of stretching on gender, VJ performance, and whether decreases in performance (if observed) are time dependent.

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Practical Applications

Although static stretching alone, prior to VJ performance, has been shown to result in suboptimal jump height values, the results of this study provide evidence that the short-term effects of static stretching in combination with dynamic activity does not appear to have an adverse effect on VJ performance. Coaches and trainers should consider advising athletes to perform a warm-up routine without stretching or to include a dynamic activity when statically stretching the gastrocnemius muscle immediately prior to performing a VJ. If these results were to hold true, further research should investigate other events requiring maximal force such as throwing.

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Keywords:

muscle activity; jump height; compliance; warm-up

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