It is recommended that athletes preparing for explosive activities such as sprinting and jumping include plyometric drills in their training. During a plyometric drill, also known as a stretch-shortening cycle (SSC) drill, the movement begins with the muscle stretching and then contracting concentrically. This type of exercise uses the elastic and reactive properties of the neuromuscular system for maximum force production.
For producing more force, the muscle must be stretched first in order to store elastic energy and then release the energy to increase force when it shortens. However, the length variance of a stretched muscle affects the performance of the SSC movement. Rack and Westbury (18) proposed the idea of short-range elastic stiffness that appears at the initial stage with a high level of stiffness when implementing a stretched muscle movement. But, as the muscle is stretched beyond this range, the number of cross-bridges between actin and myosin will be reduced and so will elastic stiffness. Thus, the variation in stretch amplitude is a key element in performing SSC movements.
Not only the mechanism of muscle contraction but also the neuromuscular system determines the force and power of the SSC movement (12). From the neuromuscular aspect of the stretched muscle, the spindle is stimulated with a strong activation of the stretch reflex increasing the electromyographic (EMG) amplitude (5,12), but when the muscle is stretched beyond a certain limit, the activation is inhibited (6,8). Thus, the stretch amplitude is an important factor for SSC movements influencing its performance (1,3,20).
When investigating the neuromuscular activation of the SSC movement, the EMG signal was analyzed by the biphase data-processing method in which the stance phase between landing and take off of the movement was divided into biphases: biconcentric and bieccentric. Dietz et al. (5) found higher EMG activation during the bieccentric phase in fast SSC movement and attributed this phenomenon to the joining of the stretch reflex (9). As for the biconcentric phase, if the jump fits the SSC mechanism, the stretched muscle stores more energy and generates more pretension. When the muscle shortens, it does not have to contract as strongly as when there is no help from released energy, so the reduction of EMG activation could be expected (10).
The variation of the stretch amplitude induces different SSC characteristics because of the excitation and inhibition of the stretch reflex (6,8). With the short-stretch movement, the excitation of the stretch reflex might occur during the bieccentric phase and continue to the initial stage of the biconcentric phase and increase the EMG activation (10). With the long-stretch movement, the inhibited reactive mechanism might be initiated for preventing the overstretch of muscles at the end stage of the bieccentric phase and continue to the beginning stage of the biconcentric phase and reduce the EMG activation. The mixture of activation of voluntary and reflex during biconcentric phase might not reflect a real situation of an SSC mechanism using the biphase data treating method. It is necessary to add a phase between the eccentric and concentric phases, namely, the coupling phase, the duration of which is determined by the length of the stretched muscle (2). After the coupling phase was classified, it should make the concentric phase of the triphases a more voluntary phase with the exclusive influence of reflexes as possible and reflect a more accurate neuromuscular mechanism of SSC movement. Therefore, the coupling phase should not be mixed with eccentric and concentric phases and should be separated as a complete phase when studying the neuromuscular activation of the SSC mechanism at the topic of stretch amplitude (17,22).
The trieccentric phase begins when the athlete touches the ground and lasts until the stretch stimulus is initiated. The advantages of a correct eccentric include an increase in the muscle spindle activity by prestretching the muscle (13). The second phase is the tricoupling phase between the yielding trieccentric contraction and the initiation of a triconcentric force. During this phase, the muscle must switch from overcoming work to imparting the necessary amount of acceleration in the required direction. The final period of the SSC exercise is the triconcentric phase. The response phase is the summation of the trieccentric and tricoupling phases, and it is often referred to as the payoff phase because of the enhanced triconcentric contraction (4). This study investigated the effects of different stretch amplitudes on performance and EMG activities during the drop jumps (DJs) using different data-processing methods, namely, the biphase and the triphase data treating methods, examining which one is more efficient for identifying the different activation patterns of the short-stretch and long-stretch DJs.
Experimental Approach to the Problem
In order to understand the EMG activity produced by SSC movement between the two different stretch amplitudes, the experiment data were processed by bi- and triphase methods. With the biphase data-processing method, the ground support time of DJ was divided into two phases: bieccentric and biconcentric. In the triphase data-processing method, the coupling phase was added between the eccentric and concentric phases, which emphasizes the stretch amplitude of stretched muscles.
For the short-stretch DJs, the muscle is not stretched as long as those of long-stretch jumps. Therefore, we hypothesized that short-stretch DJs induce more EMG amplitude by the continuing of stretch reflex because of a shorter muscle stretch and this excitation would continue to the initial stage of concentric phase using the biphase method. On the other hand, with the long-stretch DJ, for preventing the overstretch of muscles, the inhibition would be activated and therefore the EMG amplitude would be less. Also, this inhibition would continue to the initial stage of the concentric phase using the biphase method. After the coupling phase was introduced, it should make the concentric phase become a more voluntary phase that should exclude the influence of reflexes and reflect a more actual situation for identifying the different activation patterns of the short-stretch and long-stretch DJs.
Eleven men (age: 23.18 ± 2.64 years; height: 173.0 ± 4.03 cm; mass: 64.31 ± 5.97 kg) voluntarily participated in this study. The subjects consisted of the best sprinters, elite jumpers, and a volleyball player in Taiwan. Although they were not all at the power or the peak power phases of their training period, they were the third (junior) and fourth grade (senior) physical education students and well trained. All experimental procedures were approved by the local ethical committee, and the purpose of this study was clearly explained to each subject who was then required to sign an informed consent form.
Equipment and Devices
A Biovision EMG System and an electrical goniometer (Biovision, Wehrheim, Germany) were used to record the EMG signals from the rectus femoris of each subject's right leg and knee at angular displacement. The skin was abraded and cleaned, and bipolar electrodes were fastened over the belly of the muscle with a center-to-center distance of 3 cm. A reference electrode was placed on the kneecap. Signals were analog processed with a double differential amplifier (bandwidth = 10-700 Hz, input impedance = 1012Ω, common mode rejection ratio = 120 dB at 60 Hz, and gain adjustable for 1,000, 2,500, and 5,000).
AMTI force plate (AMTI, Watertown, MA) was used to record the ground reaction force (GRF) while the subjects performed the DJ and the squat jump (SJ). The consecutive EMG signals, knee angular displacement, and GFR were simultaneously collected and preliminarily analyzed with a data acquisition system (Biovision) and DasyLab (version 6.0) software (DATALOG GmbH, Moenchengladbach, Germany) at 1,000-Hz sampling frequency.
All subjects performed the SJ and DJ on the force platform. For the SJ, which was performed as reference for EMG normalization, the subject was asked to begin in a squatting position with the knee at an angle of approximately 90°, hold that position for 1-2 seconds, and then jump into the air with maximum effort. For the DJs, the landing heights were 20 and 40 cm. The subject was instructed to start in an upright position on a 20- or 40-cm height of the board. The intensity for two moderate heights was not too low to induce stretch reflex or too high to result in unstable EMG activation for the subjects. The subjects were asked to drop on the force plate; when touching the surface, they had to squat down immediately with short-stretch or long-stretch displacements and then jump into the air as quickly as possible. The amplitudes of the short-stretch DJ were restricted to less than 75° and those of the long-stretch DJ were more than 85°. The subject's arms were positioned akimbo in order to prevent them from swinging during the jump. Three trials were conducted with 5-minute rests between trials or the subjects would be asked to jump when they were ready and prepared to do it. (See Figure 1 for the experiment setup).
Data Recording and Analysis
The signals of the ground reaction and the angular displacement were low pass filtered at 10 Hz. The raw EMG signal was band pass filtered at 10-500 HZ and then full-wave rectified. The average amplitude values of the muscle group were calculated for each phase of the stance period of the DJs. To normalize the EMG signals, the EMG average amplitude of the propulsion phase of the SJ was adopted as the normalization reference value. All EMG readings were normalized as a ratio of the reference value.
The biphase method divides the stance phase of a movement into two phases: bieccentric and biconcentric contraction phases. The bieccentric phase was defined as the time interval from the beginning of the touchdown on the surface of the force plate to the lowest point of the knee angle. Following the bieccentric phase, the biconcentric phase was defined from the lowest point of the knee angle to the take off of the DJ (Figure 2).
For the triphase method (17,22), a tricoupling phase was put between the trieccentric and the triconcentric phase. The tricoupling durations were calculated by using 5% threshold levels above the absolute minimum knee angle (Figure 3) indicated by Aura and Komi (2). The trieccentric phase was defined as the time interval between the beginning of the touch down on the force plate and the beginning of the tricoupling phase. The triconcentric phase was from the end of the tricoupling phase to the take off of the DJ (Figure 3).
In this study, the independent variables were short- and long-stretch DJs as well as the biphase and triphase data-processing methods. The dependent variables included the average EMG amplitude, GRF, and angular-displacement variables. Descriptive statistics were used to understand the features of all measured parameters. Repeated t-tests were executed to test the difference between short- and long-stretch DJs. The data were analyzed using the Statistical Package for Social Sciences (SPSS) software program, version 10.0 (SPSS, Inc., Chicago, IL). The alpha levels of all statistic tests were set at 0.05.
Duration and Stretch Amplitude of Divided Phases
When performing DJs, the subjects were told to jump with a knee angle less than 75° for the short-stretch DJ and more than 85° for the long-stretch DJ. The average of angular displacement of short-stretch DJs of all heights was 70.4 ± 4.9° and that of long-stretch DJs was 90.9 ± 4.7°. The stance duration between landing and take off of short-stretch DJs of all heights was 416 ± 41 milliseconds and that of long-stretch DJs was 576 ± 43 milliseconds. The durations of the eccentric and concentric phases of the two stretched DJs are shown in Table 1.
The Forces and Angular Velocity Induced by Short-Stretch and Long-Stretch Drop Jumps
The passive force, the eccentric end's force, and concentric average force of the short-stretch DJ were larger than those of the long-stretch DJ from the height of 20 and 40 cm. The small amplitude jump also produced faster eccentric angular velocity (Table 2).
Comparison of the Electromyographic Amplitude Between the Short-Stretch and Long-Stretch Drop Jumps Using the Biphase and Triphase Methods
The bieccentric EMG amplitudes of short-stretch DJs were significantly larger than those of long-stretch DJs (P < 0.05), but the EMG amplitudes of the biconcentric phase showed no difference between the two DJs of different stretch amplitudes (Table 3 and Figures 2 and 4). The EMG amplitudes of the trieccentric and tricoupling phase of the short-stretch DJs were significantly larger than those of the long stretch DJs (P < 0.05), whereas the triconcentric EMG amplitudes of the short-stretch DJs were significantly smaller than those of the long-stretch DJs (P < 0.05) (Table 4 and Figures 3 and 5).
The Forces Induced by Short- and Long-stretch Drop Jumps
The work intensity and force of the short-stretch DJs were larger than those of the long-stretch DJs because of larger forces in the passive phase and at the end of the prestretch. There were two explanations for this phenomenon: (a) Long-stretch DJs spent more time to get more cushion with the same load (same mass DJs from the same height), so it required less action intensity than short-stretch DJs. (b). The long-stretch DJs were stretched beyond the short-range elastic stiffness causing more cross-bridge detachment (7), so the force would diminish more than those of the short-stretch DJs. For the SSC movement, the aim of the eccentric phase was to maximize the force developed at the end of the prestretch that determined the magnitude of the concentric average force. Because of the larger force developed at the end of the prestretch, the higher concentric average force developed from the short-stretch DJs was to be expected. Under the same mass of subjects, larger eccentric force induced by short-stretch DJs helped to enhance faster eccentric angular velocity. These findings are consistent with the findings of other research (1,3,20). Asmussen and Bonde-Petersen (1) and Thys et al. (20) found that high mechanical efficiency was associated with a small angular displacement during an SSC movement. Bosco et al. (3) demonstrated that small amplitude movement made better use of elastic energy, inducing greater stretching speed, greater force of the eccentric end, and a shorter coupling time than large-amplitude movements.
Comparison of the Electromyographic Amplitude of Short- and Long-stretch Drop Jumps Using the Biphase Method
The higher neuromuscular activity of short-stretch DJs during the bieccentric phase seemed to be determined by the higher activation of the stretch reflex activity (5,11). On the other hand, the EMG amplitudes of the biconcentric phase showed no difference between the two DJs of different stretch amplitudes. The result was not in accordance with the predescribed results that, comparing to long-stretch DJs, short-stretch DJs, induced more force and stored more energy during the eccentric phase and released the energy to enhance force during the concentric phase, thus not having to contract as strongly and leading to a reduction of neuromuscular activation and smaller EMG amplitude (10). The short-stretch DJs should have induced a lower EMG amplitude than the long-stretch DJs at the biconcentric phase. However, the results of the biphase did not support our hypotheses (Figures 2 and 4). This might be due to the stretch reflex activated during the bieccentric phase continued to the initial stage of the biconcentric phase (10) for the short-stretch DJs and thus increasing the EMG amplitude in the biconcentric phase. For the long-stretch DJs, the inhibition initiated at the end of the bieccentric phase continued to the initial stage of the biconcentric phase causing a decrease in the EMG amplitude of the biconcentric phase. Based on the above reasons, it resulted in no difference in the EMG amplitude of the biconcentric phase between these two DJs (Figure 2), which deviated from the real mechanism. If the phase between the end stage of the prestretch and the initial biconcentric stage had not been specified, the variation of the reactive properties would have been mixed and the comparison of EMG activation would not have been adequate.
Comparison of the Electromyographic Amplitude Between Short- and Long-stretch Drop Jumps Using the Triphase Method
Although the stretch amplitude was emphasized, the biphase data-processing method failed in specifying muscle length and indicating the particular characteristics of the SSC movement. In order to complement the shortcoming, the triphase method was used: trieccentric, tricoupling, and triconcentric phases (Figure 3). The higher muscle activation during the trieccentric phase might contribute to the excitation of the stretch reflex (5), because of (a) the electrical activity of the muscle increasing sharply 40 milliseconds after ground contact, (b) the peak level muscle EMG in the eccentric phase of the SSC movement was higher than the activity during maximum voluntary contraction, and (c) after partial blocking of the Ia afferent by ischemia, the increase in EMG at 40 milliseconds after ground contact was markedly diminished (5). So, the increase in the trieccentric EMG amplitude reflected the spinal stretch reflex of motor neurons and had a higher activation. According to Meinder et al. (15), the jump with the faster stretch velocity is associated with greater facilitation of spindle and higher EMG amplitude. The DJs of small amplitudes in this study induced a jump with faster angular velocities than the large amplitude ones, so the higher EMG amplitudes were expected (8,12,14,16) (Figures 3 and 5).
The durations of the coupling phase of short-stretch DJs were shorter than those of long-stretch DJs, and shorter coupling phases were associated with high stretching velocity and high force of the eccentric end (2). During this period, stretch amplitude played a key role in determining the magnitude of the EMG contributing to the proprioceptors. Stimulation of these receptors could cause facilitation, inhibition, and modulation of the neuromuscular system. When the muscle was stretched, the spindle was stimulated and Ia motor neuron activated. As the muscle was further stretched, the stimulation of the Golgi tendon was initiated and the effect of inhibition took over. When implementing the short-stretch DJ, the stretch reflex was excited during the bieccentric phase and continued to the initial stage of the biconcentric phase that belonged to the tricoupling phase, and therefore the EMG amplitude increased (Figures 2 and 3). However, the lower muscle activation during the tricoupling phase might be due to the autogenetic inhibition of the stretch reflex because the protection mechanism was switched to protect the long-stretched muscle (6,8,15,19,21) (Figure 3).
After eliminating the reactive properties of the tricoupling phase of the short- and long-stretch DJs, the triconcentric phase got pure voluntary contraction, demonstrating the real situation of the concentric voluntary EMG activity. It also corresponded to the findings that the short-stretch muscle would increase the capacity of elastic energy, which would induce more force and store more elastic energy during the trieccentric as well tricoupling phases and released them during the triconcentric phase (2,3). With the help of the released energy, the muscle did not have to contract as strongly as when there was no energy released. Therefore, the average EMG amplitudes of the short-stretch DJs were lower than those of the long-stretch DJs during the triconcentric phase (Figure 5c).
The triphase data processing was a better method to evaluate the facilitation of the stretch reflex, autogenetic inhibition of reactive property, and voluntary contraction happening in the trieccentric, tricoupling, and triconcentric phases (Figure 5). The higher average amplitude EMG of the trieccentric phase and the tricoupling phase demonstrated that the short-stretch DJ activated higher stretch reflex activity or inhibited less. On the other hand, the lower average amplitude EMG of the triconcentric phase indicated that the short-stretch DJ had a better elasticity energy capacity. Therefore, this way of data processing is recommended for emphasizing the stretch amplitude of the DJ, after adding a coupling phase between the eccentric and concentric phases.
The eccentric and concentric forces of the short-stretch DJs were greater than those of the long-stretch DJs, indicating that high mechanical loading was associated with a small angular displacement during an SSC movement. On the other hand, the triphase of the EMG data-processing method showed that the short-stretch DJs had a higher activation and a better elasticity energy capacity than those of the long-stretch DJs. This means that the stimulation of short-stretch DJs for the neuromuscular system is higher than those of long-stretch DJs, and from the point of view of stimulation, adaptation, the neuromuscular system ought to be adapted differently. Therefore, the short-stretch DJs are recommended in the training for the SSC movement.
The authors gratefully acknowledge the subjects who participated in this study and the laboratory assistants in Sport Biomechanics Laboratory at the Chinese Culture University, Taipei, Taiwan.
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Keywords:© 2008 National Strength and Conditioning Association
stretch-shortening cycle; short-stretch drop jump; long-stretch drop jump; biphase; triphase; electromyographic amplitude