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Original Research

Bilateral Neuromuscular and Force Differences During a Plyometric Task

Ball, Nick B; Scurr, Joanna C

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Journal of Strength and Conditioning Research: August 2009 - Volume 23 - Issue 5 - p 1433-1441
doi: 10.1519/JSC.0b013e3181a4e97f
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Abstract

Introduction

Plyometrics is a method of developing explosive power with exercises such as bounding, hopping, and jumping used to parallel the movement patterns of sprinting and jumping (17). Plyometrics have been shown to improve performance as part of a periodized training program for athletic development (20). Plyometric exercises work through use of the stretch shortening cycle (SSC) and the elastic components found within muscles by training the neuromuscular apparatus to switch rapidly from force-resisting to force-producing actions (22). The SSC enables production of higher force and power outputs compared with concentric muscle actions alone (6). After 40 ms of ground contact during a lower body plyometric exercise, the isometric loading phase begins whereby energy is stored and the switch from eccentric to concentric occurs (29). This switch is called the amortization phase and its minimization is a primary aim of plyometric exercise to increase power production in a short time period (22). To assist in the use of stored energy, the neuromuscular system prepares for the impending load by activating muscles before contact (24). This preactivation serves to stiffen the joint to prevent collapse immediately after contact and assists in contributing to an efficient pushoff phase (8).

The drop jump is a popular plyometric exercise and involves stepping off a box to prestretch the ankle plantar flexors, landing on the floor with both feet, and then immediately jumping causing rapid shortening of these muscles (27). The minimization of ground contact time is related to the muscles' capability to use the stored elastic energy as a result of a reduced amortization phase. Drop jumps can be modified to focus on the amortization phase of particular muscle groups (14) with the use of the SSC during drop jumps being shown to be a factor of knee range of motion (25). This study uses the bounce method (6), whereby minimal knee flexion increases the use of the triceps surae and reduces the force absorbed on impact (14). Different phases of the drop jump have been proposed by many authors to establish muscular behavior during the task with precontact (100 ms before contact), initial contact (35-40 ms postcontact), isometric loading phase (commences 40 ms after contact), and propulsion the most common (11,14,15,29).

Previous electromyographic (EMG) research on drop landings has shown precontact muscle activity to be greater than postcontact muscle activity in the gastrocnemius muscle as they are recruited to stabilize the ankle in preparation for impact (9). In contrast, the soleus has been shown to have greater activity postcontact when performing a drop landing (29) as a result of its role in energy transference as a monoarticular muscle; however, these studies merely looked at the landing without a subsequent jump. More recent research into drop jump exercises, however, has shown that both soleus and medial gastrocnemius absolute EMG activity is greater during the stretch reflex phase of the jump compared with the ground contact (21).

Previous drop jump research has examined the effects of various drop heights (2,6,9,11,29), jump techniques (6,14), and the implementation of drop jump training within a training program (20). No research has been performed bilaterally on drop jump exercises with regard to comparing neuromuscular contribution and peak forces from each limb. Bilateral comparison is required because there is a tendency to touch the feet down at different times during a drop jump (time differential); therefore, one limb may store more energy than the other and this could have implications for long-term training effects (7). A comparison of single-leg versus double-leg jumps on a leg press machine showed significantly greater EMG for the soleus and medial gastrocnemius from single-leg jumps compared with double-leg jumps, indicating the potential neuromuscular response from nonsequential foot placement (13).

During training, preferential use of muscles on one side may result in changes in muscle fiber composition with a higher prevalence of slow twitch fibers in the nondominant limb as a result of a reduced stimulus (10). For muscles such as the gastrocnemii, which are predominantly fast twitch fibers, this could result in reduced firing rates of motor units, thus causing a performance decrement. The type of muscle action, the velocity of muscle action (19), and the joint position can contribute to strength deficits of between 3 and 25% between limbs (3). Many studies have assessed bilateral deficit through neuromuscular assessment of unilateral and bilateral exercises (5); however, bilateral comparisons of the triceps surae during a bilateral exercise have not been addressed.

The purpose of this study was to compare the neuromuscular contribution between left and right triceps surae in the precontact, initial, and postcontact phases of a bilateral drop jump exercise to assess the immediate implications of nonsimultaneous foot placement. Based on previous research, it is hypothesized that: (a) a bilateral difference will be shown between the left and right triceps surae normalized EMG activity during the drop jump; (b) normalized EMG activity after contact will be significantly greater than the pre- and initial contact phases for all muscles; (c) there will be a significant relationship between both duration of contact and time differential at contact with normalized EMG activity; (d) there will be a significant difference between left and right peak force during the drop jump; and (e) peak force and normalized EMG will decrease with an increase in contact duration.

Methods

Approach to the Problem

Using a single group design, subjects performed six drop jumps from 0.4 m. Electromyography data were recorded from the triceps surae of the left and right leg throughout the duration of the jumps with force platforms were used to assess the difference in the onset of contact, contact duration, and force between left and right legs. The force data were synchronized with the EMG data and were used to assess the bilateral differences of the recorded variables in different phases of the drop jump in addition to determining the effect of differences in contact onset on the corresponding EMG activity.

Subjects

Full institutional ethical approval was obtained from the institutional ethics committee. Sixteen recreationally active male participants (age: 25 ± 4.7 years; stature: 1.79 ± 0.05 m; body mass: 76.9 ± 8.5 kg) gave written informed consent to participate in this study. Stature was recorded using a stadiometer (Leicester, Hamburg, Germany) and mass was recorded using calibrated weighing scales (SECA, Hamburg Germany). All subjects had a minimum of 1 year's resistance training experience. Jumping was familiar within all their sports played; however, none had undertaken a structured plyometric training program involving drop jumps before the study. At least 48 hours before the experimentation, the participants were familiarized with correct drop jump technique under the guidance of a UKSCA Accredited Strength and Conditioning Coach.

Procedures

After a standardized warmup, including a general warmup of 5 minutes followed by a series of dynamic stretches, experimental sessions consisted of one set of 6 repetitions of the drop jump (7) with a minimum of 2 minutes recovery between jumps (28). The drop jump box was set at 0.40 m in accordance with guidelines for optimal drop height (5). All jumps from each individual took place in a single testing session. Synchronized EMG and force variables were recorded throughout the duration of the jump.

Drop Jump Technique

Participants were instructed to start and terminate the landing movement in a standing position, to touch down with both feet, and to lean forward with the body at takeoff rather than jumping off the box. Participants were instructed to initiate the jump with the right leg on each repetition. The arms were placed on the hips throughout the jump (Figure 1). On landing on the force platforms (one for each leg), the participants were instructed to jump maximally for height while minimizing contact time. This minimization of contact time discouraged knee flexion and encouraged use of the triceps surae (14).

Figure 1
Figure 1:
Drop jump technique.

Equipment

The electrical muscle activity of the soleus, (SOL) medial gastrocnemius (MG), and lateral gastrocnemius (LG) of both legs was recorded using active (Ag/AgCl) bipolar preamplified disc electrodes (Biometrics, Gwent, UK) (gain × 1000; input impedance >10 kΩ; CMRR >96 dB; bandpass, 10-1000 Hz; noise <5 μV) with a 1-cm separation distance with hypoallergenic adhesive tape (3M, Bracknell, UK) applied directly onto its casing. EMG signals were amplified (1,000×) and sampled at 1,000 Hz. Electrodes were placed on the muscles in accordance with SENIAM guidelines for application, location, and orientation. A superficial reference electrode (Biometrics R300) was placed on the pisiform bone on the wrist. The data log is fitted with a high-pass third-order filter (18 dB/Octave) to remove DC offsets resulting from membrane potential and a low-pass filter for frequencies >450 Hz. The electrodes also contain an eighth-order elliptical filter (-60 dB at 550 Hz). The electrodes were then attached to a Biometrics 8-channel Datalog (P3X8) electromyography system (Biometrics), which was attached to the lower back region of the participant. The effects of crosstalk were assessed through seated manual muscle testing using an unloaded heel raise with the knee flexed at 90° to isolate soleus muscle activity, which was subsequently compared with medial and lateral gastrocnemius activity for any crossactivation. Two force platforms (9281C) (Kistler, Winterthur, Switzerland; 60 × 40 cm, natural frequency 1,000 Hz) were used to measure the resultant force, bilateral contact time initiation, and duration of the drop jumps contacts for each leg.

Normalization

Force platform normalization involved weighing the participant complete with the Datalog pack attached (0.5 kg) on each platform before testing. All force platform equipment had been calibrated according to the manufacturer's guidelines. To elicit reference EMG activity for the subsequent normalizations, participants performed 3 maximal 20-m sprints from a standing start 1 hour before the drop jump testing. Sprint normalization was used as a result of its ability to provide a standardized and reproducible reference EMG value and its ability to elicit large EMG amplitudes (4). Electromyographic activity from all muscles was recorded throughout the duration of the sprints. The peak EMG from each stride within each sprint trial was recorded and the mean of the peak root mean square EMG activity was then used as a normalization value for corresponding EMG activity from the drop jump.

Signal Processing

Electromyography and force platform signals were amplified and then passed to separate channels of an analog to digital converter (Kistler). The force platforms were set to record for 5 seconds initiated by using an external trigger. The trigger also activated the synchronized EMG Datalog system.

Force Platform Processing

All force platform data were processed using Bioware software (Version 3, Kistler, Winterthur, Switzerland). Data was converted from Newtons to body weight (BW) to allow direct comparisons between participants. Start and finish times of each force trace for each leg were recorded to give the duration of contact. This study used 3 time phases: precontact, contact40ms, and contactpost40ms with “contact” referring to the instantaneous point of foot touchdown. Contact duration was the sum of contact40ms and contactpost40ms.

Electromyography Processing

The raw EMG signals (measured in millivolts) recorded from the drop jump were visually checked for artefacts and removed if necessary. A root mean square (RMS) filter was applied at a window length of 20 ms. Peak RMS EMG amplitudes were recorded from the precontact and contact phase (comprising of contact40ms and contactpost40ms) of each drop jump. The mean of these peak values was then used for analysis. Timing of the EMG phases was derived from the force platform data. Precontact phase EMG data were defined as 100 ms before contact in accordance with previous research (11). Contact duration was derived from the start and end times from the force data. Contact40ms was the 0- to 40-ms period after contact. Contactpost40ms was the remaining time in contact with the force platform.

Statistical Analysis

All statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS for Windows, version 14; SPSS Inc, Chicago, IL). All data were shown to be normally distributed using Shapiro-Wilk test of normality (p > 0.05) and showed homogenous variance using Levene's statistic. The normalized EMG data from the drop jump trials were statistically compared between legs (left and right) and between phases (pre, contact40ms, and contactpost40ms) using 3 (2 × 3) repeated-measures multivariate analysis of variance (leg × phase) for each muscle followed by a post hoc test with Bonferroni adjustment. Within-leg muscle comparisons were assessed using a 3 × 3 repeated-measures multivariate analysis of variance (muscle × phase) followed by a post hoc test with Bonferroni adjustment. Based on the Bonferroni and alpha level of p = 0.017 was set.

Peak resultant ground reaction forces and contact durations were compared between legs using a one-way repeated-measures analysis of variance with an alpha level of p = 0.05 with curvilinear trend lines used to assess relationships. Pearson's correlations (r) were used to ascertain relationships between normalized EMG activity and left and right contact time with linear trend lines used to indicate the relationship. Based on the number in this study, correlation values <0.5 were considered weak, between 0.5 and 0.6 were considered average, 0.6 and 0.8 were considered good with >0.8 being high.

Results

Electromyographic Activity During the Drop Jump

Figure 2 indicates that during the contactpost40ms phase, only approximately 70% relative activation of the triceps surae was demonstrated. This suggests that the exercise was in fact submaximal for this muscle group.

Figure 2
Figure 2:
Relative mean peak electromyographic values normalized to a sprint muscle action andSDs for each muscle in the pre-, contact40ms, and contactpost40ms phases of a drop jump (N = 16) (LS, left soleus; LMG, left medial gastrocnemius; LLG, left lateral gastrocnemius; RS, right soleus; RMG, right medial gastrocnemius; RLG, right lateral gastrocnemius).

Between-Leg Comparison

Significantly greater EMG activity in all the left triceps surae compared with the right were shown in the precontact (SOL, MG, LG: p = 0.0001) phase and the MG for the contact40ms phase. Significantly greater EMG activity for the right SOL and LG compared with the left were shown in the contact40ms phase (p = 0.0001) of the drop jump. Once the participant had been in contact with the ground for 40 ms, the EMG activity between left and right triceps surae muscles showed no significant differences (p > 0.17) indicating a similar muscular demand being placed on both limbs in this phase.

Within-Leg Comparison

The soleus EMG activity of both legs was significantly lower than the respective medial gastrocnemius (right and left: p = 0.01) and lateral gastrocnemius (right and left: p = 0.0001) in the precontact phase. However, no significant differences in muscle activity within the triceps surae of each leg were shown in both phases after contact.

Between Phase

Contactpost40ms muscle activity was significantly greater than both precontact (p < 0.17) and contact40ms (p < 0.05) muscle activity for all muscles tested in both legs, indicating that greater muscular demands are evident in the phase when propulsion occurs. Only the soleus and lateral gastrocnemius of both legs were significantly greater in the contact40ms phase compared with the precontact phase, indicating a similar muscular demand placed on the medial gastrocnemius in precontact and contact40ms.

Temporal Effects

Figures 3 and 4 assess the effect of the time differential in foot placement between left and right both before and after contact on the collated mean normalized EMG activity from all the triceps surae muscles. Each participant's left and right triceps surae activity was plotted against their average time differential in foot placement in 6 drop jumps with a negative time differential indicating a right foot contact first and a positive time differential indicating left foot contact first. The hypothetical line is for the right triceps surae and aids the reader by providing a reference point. This line would indicate that for participants who touched down with the right foot first, greater EMG activity occurred in the right triceps surae with a corresponding decrease in left triceps surae EMG activity for the left foot, which would have contacted second.

Figure 3
Figure 3:
The effects of nonsimultaneous foot placement on the normalized electromyographic activity from the triceps surae muscles during the precontact phase of a drop jump (n = 16).
Figure 4
Figure 4:
The effects of nonsimultaneous foot placement on the normalized electromyographic activity from the triceps surae muscles during the contactpost40ms contact phase of a drop jump (n = 16).

Figure 3 shows the majority of participants had minimal time differentials at the point of contact. In precontact EMG, there is a slight bias toward increased right triceps surae EMG; however, the lack of correlation indicates that the leg that touches down first is not preactivated more than the leg that touches down second. Figure 4 indicates that once in contact with the ground (comprising contact40ms and contactpost40ms), EMG activity is still unaffected by any differences that may occur between limbs at touchdown.

Duration and Electromyography

Figures 5 and 6 show left and right triceps surae as a function of length of time in the left and right normalized triceps surae EMG activity as a function of the contact time duration. All muscles showed increased EMG as contact duration increased. The data did show that greater normalized EMG for each muscle (apart from the right lateral gastrocnemius) in each leg occurred in contact durations between 0.38 and 0.41 second compared with participants who were in contact for durations outside this time epoch. Contact durations below 0.25 second do show a marked decrease in neuromuscular activation compared with longer contact durations.

Figure 5
Figure 5:
Relationship between normalized electromyographic activity of the left triceps surae muscles and contact duration during a drop jump (n = 16).
Figure 6
Figure 6:
Relationship between normalized electromyographic activity of the right triceps surae muscles and contact duration during a drop jump (n = 16).

Force Data

No significant differences (t = −0.81[14]; p = 0.432) were shown between the left and right peak resultant forces in the drop jump (Figure 7). A significant inverse curvilinear relationship was found between peak resultant force and contact duration for both left (r = 0.65) and right leg (r = 0.66) with an increased contact duration resulting in a decrease in resultant force (Figure 8). A weak relationship was found between peak resultant force and nonsimultaneous foot contact (r = 0.5).

Figure 7
Figure 7:
Overall peak resultant forces from the left and right leg during the whole of ground contact in drop jumps (N = 16).
Figure 8
Figure 8:
Relationship between peak resultant force and contact duration during a drop jump exercise (N = 16).

Discussion

Significant differences between left and right triceps surae normalized EMG activity were found before and directly after contact; however, differences were not evident after 40 ms of contact when loading and propulsion occur (Figure 2). Furthermore, within-leg differences were only present in the precontact phase of the drop jump. No differences were found between peak force in the left and right leg (Figure 7) and an inverse relationship between contact duration and force was shown for both legs (r = 0.65) indicating an increased duration reduces the peak force imparted (Figure 8). However, no relationship between contact duration and normalized EMG was found, although EMG did increase as contact duration increased, suggesting that mechanisms other than neuromuscular contribution of the triceps surae are being used (Figures 5-6). The EMG activity was not significantly affected by which foot touched down first (p > 0.05) (Figures 3-4), although a slight trend toward increased EMG in precontact for the foot that first initiated contact was shown (Figure 3).

Bilateral Differences

In comparisons between bilateral muscles, the normalized EMG activity indicated a disparity between left and right legs for the precontact and contact40ms phases; therefore, hypothesis 1 is accepted for these phases but rejected for the contactpost40ms phase. The disparity between contralateral muscles in the initial phases of the drop jump agrees with previous findings that showed a neuromuscular bilateral deficit in plantar flexion actions as a function of knee position and reflex excitability (18). This is the first study to report bilateral differences between the same muscles when a reflex response (such as the stretch shortening cycle) is initiated as opposed to using a voluntary effort muscle action. An increased bilateral deficit has been shown as a function of movement velocity, in which rate of force development has also been shown to differ between limbs, which may have significant effects on long-term adaptations to even slight contact time differentials (31). The similarity in neuromuscular response between bilateral triceps surae during the contactpost40ms phase shown in the present study (Figure 2) could be expected as a result of the paucity of unilateral loaded exercises in everyday life in addition to being a function of foot placement timing. The triceps surae muscles are primarily used for walking or standing, which are automatic movements; thus, the contralateral muscles may get the same stimulus and loads placed on them at separate times.

Drop Jump Phase Differences

The EMG values were significantly greater (p < 0.01) in the contactpost40ms phase compared with the precontact and contact40ms phases for all muscles accepting hypothesis 2. After 40 ms of contact, the amortization phase is shown to occur (29), which includes the isometric loading of the muscle and a switch from muscle lengthening to muscle shortening takes place primarily as a result of the stretch reflex system (22). This initial release of energy through the stretch reflex coupled with the initial shortening of the muscle fibers is the suggested origin of the greater EMG values found in the present study compared with previous research that has only looked at landings without subsequent propulsion (9). Peak EMG values for the soleus and gastrocnemius have previously been shown to occur as the muscle changes from lengthening to shortening (11).

An increase in muscular activation for the soleus and lateral gastrocnemius from precontact to contact40ms, may be the result of increased fascicle lengthening as has been shown for the knee extensors (16). The possible fascicle lengthening in the present study may occur at contact whereby plantar flexors will lengthen to control deceleration of ankle rotation along with further storage of energy to regulate ankle stiffness in preparation for subsequent work. For the soleus muscle, the findings from this study agree with Santello and McDonagh (29) who showed that normalized EMG activity for the soleus was greater during the period of onset of ankle rotation when contact is initiated (contact40ms in the current study) compared with preactivation; emphasizing the soleus' importance in controlling ankle stiffness alongside the other plantar flexors and the tibialis anterior. As a result of the nature of the drop jump performed in this study (ie, bounce jump technique), minimal knee flexion and brief ground contact time before propulsion was emphasized. An increased contact time would suggest reduced ankle stiffness and cause increased dorsiflexion, which as a result of increased heel contact, results in the absorption of more force. Gottlieb (12) suggested that preactivation increases the sensitivity of the muscle spindles to enhance stretch reflexes that could also assist in the enhancement of muscle stiffness (1). Therefore, the 0- to 40-ms period after contact seems an important energy transition period in which brief contact time and minimal knee flexion is desired. As shown in the present study, the relative neuromuscular activity was either maintained (left soleus) or enhanced (left medial gastrocnemius, left lateral gastrocnemius, right soleus, right medial gastrocnemius, right lateral gastrocnemius) from the precontact phase to the contact40ms phase. This may have been for the purposes of enhanced ankle stiffness to control joint rotation velocity and allow further storage of elastic energy in preparation for the release of the energy during the propulsion of the body.

The increased gastrocnemius activation in both legs compared with the soleus is expected as a result of the relatively higher composition of fast twitch fibres of the gastrocnemii muscles (26). Furthermore, as a result of the minimal knee bend used in the technique of the drop jump, the gastrocnemii muscles would have maintained their optimal muscle length to exert maximal force (32) and place greater emphasis on the triceps surae compared with the knee extensors to generate force production for the subsequent jump. With greater knee bend, the gastrocnemius becomes more slack, which in turn lowers the force generated through the calcaneal tendon (30). Significantly greater ankle moments, power, and work in the pre- and postcontact phases of a bounce jump have been shown in comparison to jumps in which greater knee bend is allowed (14).

Time Differential at Contact

When correlating the EMG data of the right plantar flexors to participants who touched down with their right legs, first a slight trend was shown for precontact; however, this was not significant (Figure 3). Therefore, hypothesis 3 is rejected. This indicates that if there is nonsimultaneous ground contact, precontact EMG activity may be increased in the foot touching down first; however, it cannot be confirmed in the present study.

Bilateral Force Comparisons

This study showed no significant difference between both the duration of foot contact and the peak resultant force between legs; therefore, hypothesis 4 is accepted, agreeing with previous research in other dynamic activities (23). The results from this study showed a combined mean peak resultant force from both force platforms of 4.7 BW, which is in line with results found from previous studies (29,30). Higher resultant force values have been shown in previous studies that use greater drop heights or to instructing subjects to perform a stiffer landing (34).

Effects of Contact Duration on Force and Neuromuscular Factors

Contact durations in the present study ranged from 0.2-0.5 seconds, which are in line with previous nonathletes' drop jump contact durations (32). Shorter contact duration coupled with a stiffer landing is considered necessary for better athletic performance in stretch-shortening cyclic movements such as sprinting or repeated jumping (15); indeed, shorter contact times are considered to be an indicator of high leg stiffness values (2,7). Longer contact durations have also been shown to have a higher contribution of power from the knee and are largely the result of increased knee flexion, which increases the absorption of the force applied at contact (14,15,33). However, this study indicated that increased mean peak EMG for all triceps surae as contact duration increased was coupled with a decrease in force; therefore, hypothesis 5 is rejected. The relationship between contact duration and EMG activity seems to indicate that there was an optimal contact period with which higher neuromuscular activity occurred (Figures 5-6), although the correlation between contact and EMG is recognized as being weak. Contact durations >0.25 second did elicit a greater neuromuscular response compared with durations below this time epoch. Contact durations below this time epoch resulted in reduced relative muscular activation, possibly as a result of reduced ankle stiffness.

As expected, the peak force increased steadily in conjunction with decreases in contact time, which agrees with previous work (33). This further adds to the importance of contact time in drop jump exercises and the consideration of its minimization. The increased EMG resulting from increased contact duration may be the result of the increased loading time available requiring greater muscular demands to absorb the force; however, the decreased force with increased contact duration (Figure 8) may be a result of increased force dissipation (30).

Practical Applications

Differences between left and right triceps surae muscle activation are engaged in the early phases of a drop jump task; however, the disparity is readdressed during the loading/propulsion phase of the jump after 40 ms of ground contact. Differences in contact time between legs were present; however, they were not significant enough to cause neuromuscular differences in the plantar flexor muscles. However, as a result of the known load differences between single-leg and double-leg plyometric exercises, the importance of simultaneous foot placement in intense exercises such as the drop jump is imperative to make sure that both limbs are getting equal stimulus. This study has shown no bilateral deficit in EMG for the triceps surae is present in a 2-footed landing in drop jumps from 0.4 m; however, the longitudinal effects of the slight time differential present is unknown and thus initiation of drops jumps on alternate legs each repetition is advised.

Acknowledgments

The author acknowledge the University of Portsmouth Sports Science Department for funding the project. It is recognized that the results of the present study do not constitute endorsement of the product by the NSCA.

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Keywords:

electromyography; drop jump; normalization; triceps surae

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