Effects of 6-Week Plyometric Training on Vertical Jump Performance and Muscle Activation of Lower Extremity Muscles

Author: Kerim SOZBIR*(1)

(1)Kerim SOZBIR is an Assistant Professor and lectures Human Anatomy and Physiology, Speed Training, and Track and Field in the Department of Coaching Education at the University of Abant Izzet Baysal in TURKEY. He is also Head Coach of University Table Tennis, and Track and Field Teams.

*Corresponding Author:
Kerim SOZBIR, Ph.D
Department of Coaching Education, School of Physical Education and Sports,
University of Abant Izzet Baysal, Golkoy Kampusu, 14030, Bolu/TURKEY
Office Phone: +903742541000 (ext. 2027)
Fax: +90 374 2534636
Email: sozbir_k@ibu.edu.tr

ABSTRACT
The purpose of this study was to determine the effects of 6-week plyometric training on vertical jump performance and electromyography (EMG) activities of vastus lateralis (VL), vastus medialis (VM), and gastrocnemius (GAS) muscles during countermovement jump (CMJ). Twenty-four highly physically active physical education students were randomly assigned either to a plyometric (PLY) group or a control group. The experimental group performed plyometric exercises 2 times a week for 6 weeks, whereas the control group participated only in their lectures. The results revealed that there were no significant changes in either vertical jump height or EMG activities of selected muscles for the control group (p greater than 0.05). However, after 6 weeks of plyometric training, significant improvements (p less than 0.05) were observed in EMG activities of VL (13.25%), VM (9.60%), and GAS (13.93%) muscles, and no significant increase (p greater than 0.05) was found in CMJ (2.77%) in the PLY group. In conclusion, the findings of the this study suggest that 6 weeks of PLY training, in addition to the regular academic program, induced significant improvements in EMG activities of lower extremity muscles but no significant increases in vertical jump height. Accordingly, PLY exercises are recommended as part of a regular academic program in order to increase important components of athletic performance for physical education students.

KEYWORDS: Stretch-shortening cycle, electromyography, root mean square, countermovement jump, knee extensor muscles.

INTRODUCTION
Eccentric muscle contractions are rapidly followed by concentric contractions in many sport skills in team or individual sports. When the muscles are stretched during an eccentric contraction, they store elastic energy, and this energy, accompanied by a rapid concentric contraction, produces more power than an independent concentric contraction (43). Plyometric (PLY) exercise involves stretching the muscles immediately before making rapid concentric contractions (2). This combined action is commonly called a stretch-shortening cycle (SSC) (14, 38, 55) and depends on using passively elastic energy in the muscle and the active role of stretch reflex (32).

PLY exercises such as jumping, hopping, skipping, and bounding are executed with the goal of increasing dynamic muscular performance (29, 55). Therefore, PLY has been widely used for increasing dynamic athletic performance such as vertical jump ability (24, 31, 33, 35, 37, 41), speed (1, 14), agility (11, 47), and muscle activation of lower extremities (3, 8, 9).

Vertical jumping is a fundamental component of many sports and also may be predictive of performance in other sports in which it is not the primary component. The effects of PLY on vertical jump performance have been widely researched. Particularly, some authors have reported significant increases in vertical jump height after PLY training (3, 4, 12, 24, 31, 33, 48), whereas others have reported no significant effects (15, 19, 27, 38, 43, 45). The findings of PLY training may be different depending on the various participants’ characteristics, such as fitness level (2, 4, 15, 33, 45), gender (12, 45), age (27, 48), or sport disciplines (13, 27, 35, 38). The authors have studied a wide range of participants, and this might have provided conflicting results.

Several studies suggest that PLY, either alone or in combination with other typical training modalities such as weight training, elicit numerous positive changes in neural and musculoskeletal systems, muscle function, and athletic performance (3, 9, 15, 16, 44, 51). Sözbir et al. (51) have revealed that 6 weeks of combined PLY and stretching exercise intervention increased root mean square (RMS) electromyography (EMG) of vastus lateralis (VL) and vastus medialis (VM) muscles during countermovement jump (CMJ). Similar results could be observed by Toumi et al. (54), who have revealed that the squat jump was performed with similar RMS values of lower extremity muscles before and after the training period for the weight and combined (weight + jump) training groups. However, only combined training produced a significant increases in RMS values of VL and VM muscles in the transition and positive (transition+concentric) phases during the CMJ. On the other hand, no significant differences in RMS values were observed during the eccentric and concentric phases of the CMJ. EMG activities of different lower extremity muscles have also been investigated. Chimera et al. (15) have examined that significant change in EMG activity after 6 weeks of PLY training included increased adductor muscle-group amplitude during the preparatory phase of landing, but no changes in the EMG activity of quadriceps and hamstrings muscles were observed. Another previous study has observed no changes in plantar flexor muscle activity during pre-landing and eccentric phases of vertical jumps following PLY. However, they reported a significant increase in plantar flexor muscle activity during the concentric phase of squat, CMJ, and drop jumps (34).

Regarding neuromuscular adaptation to PLY, the results generally show positive increases in lower extremity strength, power, and SSC muscle function in healthy individuals (39). Arabatzi et al. (3) have reported that PLY increases jumping height because of a greater muscle activation level and a higher mechanical output of the contractile machinery. Furthermore, Chmielewski et al. (16) have revealed that the SSC enhances the ability of the neural and musculotendinous systems to produce maximal force in the shortest amount of time. Although several studies have reported vertical jump performance gains following PLY training, only a few studies focused on the possible neuromuscular mechanisms. There is limited and incompatible evidence on neuromuscular adaptation of lower extremity muscles during vertical jump. Therefore, the purpose of this study was to examine the effects of 6-week PLY training on vertical jump height and EMG activity of VL, VM, and gastrocnemius (GAS) muscles during CMJ.

METHODS
Participants
Twenty-four highly physically active students who were studying in the 2011-2012 academic year at the school of physical education and sports (18-23 years), volunteered to participate in this study. The participants were randomly assigned to PLY group (n= 12) and control group (n= 12). After randomization, the two groups did not differ significantly (p greater than 0.05) in any of the anthropometric characteristics and dependent variables. All participants were highly physically active and experienced in performing various jumps through participation in various sport activities (i.e., track and field, gymnastics, soccer, badminton, basketball) through their regular academic program but were not involved in any structured physical training regime. The participants were informed of the purpose of the study, completed a medical history form, and signed an informed consent form.

Procedures
The experiment consisted of two test sessions (pre- and posttest) and PLY training. Pre- and posttests were performed in the neurophysiology laboratory at the medicine department, within two days and the same time (16:30-18:30) before and after 6-week PLY training. Participants were verbally encouraged to perform all jumps at maximal effort for both pre- and posttests measurements. All measurements began with a generic warm-up consisting of 10 bodyweight squats, 10 forward lunges each side, and 3 minutes of dynamic stretching of relevant lower extremity muscles (53). All participants were familiarized with the PLY exercises and test procedures at least one week before the beginning of the experiment.

Anthropometric Measurements
All the measurements were made following the guidelines outlined by the International Society for the Advancement of Kinanthropometry (30) by the same experienced technician.

Body mass was measured with a Seca scale (Seca 700, Medical Scales and Measuring Systems, Hamburg-GERMANY; precision of 0.1 kg) and with the participant wearing underwear only and barefoot. Height was measured with a stadiometer incorporated into the scale (Seca 700, Medical Scales and Measuring Systems, Hamburg-GERMANY; precision of 0.1 cm). The participant was instructed to take and hold a full breath. Stretched height with the head at the Frankfort plane was recorded.

Skinfold thickness was measured on the right side of the body to the nearest 0.1 mm with a skinfold caliper (Holtain Ltd, Crymych, UK) at following sites:
• Triceps, a vertical fold halfway between the acromion process and the superior head of the radius, in the posterior aspect of the arm.
• Biceps, at the same level as the triceps skinfold and directly above the centre of the cubital fossa.
• Subscapular, about 20 mm below the inferior angle of the scapula and 458 to the lateral side of the body.
• Iliac crest, immediately above the marked iliocrestale (the point on the most lateral aspect of the iliac tubercle, which is on the iliac crest). The line of this skinfold generally runs slightly downward posterior–anterior, as determined by natural fold lines of the skin.

All anthropometric variables were measured in the same order, and the average of the three measurements was used in the analyses (22). Body density was calculated using the formula of Durnin and Womersley (21), and the percentage of body fat was then calculated by Siri’s equation (49).

Vertical Jump Measurements
The CMJ was used to assess vertical jump performance. The participants were instructed to keep their hands placed on their hips during CMJ. The takeoff was performed in a continuous movement with no pause between the downward and upward phases. Flying times were measured by jump mat (Bosco Ergojump, FINLAND) and vertical jump performance was determined by the vertical takeoff velocity (V0) of the center of gravity, which was calculated from the flight time (tf) according to the equation V0 = 1/2 tf X g (g = 9.81 m·s-2). Jump height (H) was then calculated using H= V02/2g (54). After the placement of the EMG electrodes, each participant performed three jumping trials, and the best performance was recorded for statistical analyses (1, 12).

Electromyography
EMG of VL, VM, and GAS muscles of the dominant leg was recorded during CMJ performance. Dominant limb was determined by asking which leg the subject would prefer as the kicking limb when kicking a soccer ball (25). Thorough skin preparation for all recording electrodes included removal of body hair and dead epithelial cells with a razor, slight abrading with sandpaper, and cleansing of the designated areas with isopropyl alcohol (46). Bipolar surface electrodes (silver cup) were placed along the longitudinal axes and muscle belly of the selected muscle (46, 54) at an interelectrode distance of 20 mm for VL (7), VM (52), and GAS (20). Hermens et al. (26) reported that researchers used between 10-50 mm for electrode distance in order to measure different types of muscles, but the largely preferred distance was 20 mm for bipolar electrode configurations by many researchers.

Therefore, 20 mm was preferred in this study. The reference electrode was placed over the bony part of the right wrist (26). The wires connecting the electrodes were well secured with tape to avoid artifacts from lower limb movements.

Signal Processing
The EMG signals was sampled at a frequency of 1 KHz and stored on a computer using a data acquisition system (Nicolet Viking Select Electrodiagnostic Sistemi-Nicolet Biomedical, Madison, WI, USA), and EMG activity was quantified as the RMS values. Raw EMG data were RMS with a time averaging period of 200 milliseconds to produce a linear envelope for each muscle activity pattern. The EMG system bandwidth was 10–500 Hz (7, 52) with an overall gain of 1000 (10). The amplitudes were calculated when the participant was in the eccentric phase of knee of CMJ (6). EMG was normalized with a maximal voluntary isometric contraction (10). Each participant performed three jumping trials, and the best RMS value was recorded for statistical analyses.

Plyometric Training Program
The experimental group performed PLY exercises on Tuesdays and Fridays for six weeks (31, 38, 43) and also participated in their lectures. The control group was instructed to maintain their regular daily activities (i.e., academic schedule) and to avoid any additional strenuous physical activity during the study. Each PLY training session commenced with a 5-min run of low intensity, followed by five minutes of stretching exercises for lower extremity. During each session, the subjects were instructed to perform jumps as quickly as possible with minimum ground contact time. The feet during jumps were set slightly outward, and the jumps were performed on a synthetic surface. None of the participants reported current injuries of the spine or the lower extremities, and no injuries occurred during the experiment. Participants who failed to attend a training session had 24 hours to make it up. Any participants who missed more than 2 sessions during the 6 weeks were excluded from the analysis.

The study adopted a 6-week PLY training program that had been used in previous studies (43, 45) (Table 1). Training volume ranged from 90 foot contacts to 140 foot contacts per session, and the intensity of the exercises increased throughout the course of the training program. Participants were instructed to perform exercises to their maximal ability. Participants were given a brief description and demonstration of each exercise before completing each training session. All training sessions were supervised by the author and two experienced track and field coaches who are experts of PLY to ensure participants’ safety, to verbally encourage them to perform with maximal effort, and to ensure that jumps were executed with proper technique.

Table 1

Statistical Analyses
The variables were expressed as means and standard deviations (±SD). The normal distribution was assessed with the Shapiro-Wilk test, and homogeneity of variance was determined with Levene’s test procedures. The independent sample t-test was used to identify the differences between PLY training and control groups for dependent variables (CMJ, EMG activities of VL, VM and GAS muscles), and paired t-test was used to identify the differences between the pre- and posttests for the dependent variables for each group. All data analysis was performed by means of the IBM-SPSS statistical software 20.0 for Windows (SPSS, Inc., Chicago, IL). The level of statistical significance was set at p less than 0.05.

RESULTS
There were no significant differences in the anthropometric characteristics and dependent variables (CMJ, EMG activities of VL, VM, and GAS muscles) between the groups prior to training (p greater than 0.05) (Table 2).

Table 2

According to paired t-test results, there were significant increases in EMG activities of VL, VM, and GAS muscles in the PLY group (p less than 0.05). However, no significant differences were observed in the PLY group for CMJ and in the control group for any variable from pre- to posttest (p greater than 0.05) (Table 3).

Table 3

The percentage change from pre-test to post-test in EMG activities of VL (13.25% and 4.58%), VM (9.60% and 0.08%), and GAS (13.93% and 2.34%) were significant (p less than 0.05) differences between the PLY and control groups, respectively (Figure 1). However, there were no significant differences in percentage change of CMJ (2.77% and 0.28%) between the PLY and control groups, respectively (Figure 2).

Figure 1

Figure 2

DISCUSSION
The present study was designed to elucidate the effects of 6-week PLY training on vertical jump performance and EMG activities of VL, VM, and GAS muscles in physically highly active students during CMJ. The main findings of this study indicated that PLY training significantly increased EMG activities of VL, VM, and GAS but did not significantly improve vertical jump height.

CMJ Performance
The results of this study showed no significant improvement in CMJ (2.77%) after 6-week PLY training. Contrary to the widely held belief that PLY training increases physical performance, numerous studies have demonstrated that PLY training actually increases performance in activities that require strength, speed, agility and power (3, 9, 11, 33, 37, 41, 47). Numerous studies on PLY training have reported significant increases in vertical jump height (4, 12, 18, 23, 33, 37, 41, 47, 51) ranging from 4.30 to 34.67%, and even 69.9% improvement was reported (48), that could be attributed to the enhanced muscle coordination (17) and muscle power after PLY training (47). In contrast, a number of authors have reported no significant positive effects in PLY on vertical jump performance (19, 31, 38, 43, 45), and some even have reported negative effects (36, 45).

The training protocol in this study was used in previous studies with slightly different sets and repetitions (43, 45, 50). However, the only similar results were obtained by Miller et al. (43) and Pleog et al. (45). Miller et al. (43) found 2.44% and 5.38% no significant improvements in vertical jump after 6-week chest-deep and waist-deep aquatic after PLY training, respectively. Moreover, Ploeg et al. (45) reported that vertical jump height did not significantly increase after 6-week doubled-volume aquatic PLY (3.11%) and aquatic PLY (0.66%), and there was no significant decrease after land-based PLY training (-2.63%). In contrast, Sozbir et al. (50) demonstrated that similar PLY training protocol yielded significant gains in CMJ of collegiate female contemporary dancers by 7.17%. These discrepancies may reflect differences in the fitness level of the participants (trained contemporary dancer vs. sedentary and untrained). However, previous authors, who were Dodd and Alvar (19), Makaruk and Sacewicz (38), King and Cipriani (31) and Chimera et al. (15) found 1.91%, 2.44%, 1.13%, and 5.59%, that is, no significant increases in vertical jump after different training volumes of PLY training, respectively. These percentage changes are concurring with the finding of present study (2.77%).

The results of this study are consistent with some studies (19, 38, 43, 45) and in contrast to others (4, 12, 18, 23, 33, 37, 41, 47, 50). The majority of the studies found significantly positive changes in vertical jump performance. Mackała and Fostiak (37) found 10.2% improvement in CMJ in well-trained male sprinters. Arabatzi et al. (3) and Franco-Márquez et al. (23) investigated effects of combined (weight+PLY) training and found 15.12% and 9.04% gains in CMJ after PLY training, respectively. Effects of 8-week PLY training on vertical jump have also been examined in several studies (11-13, 24). Chaouachi et al. (11) compared effectiveness of PLY and combined balance and PLY in healthy adolescent boys. They found 11.67% and 14.09% improvements in CMJ in PLY and combined training groups. Therefore, they suggested that combined training could be an important consideration for reducing the high velocity impacts of PLY training. Moreover, Chelly et al. (13) revealed that an 8-week biweekly course of lower- and upper-limb PLY training would enhance characteristics important to competition for top-level adolescent handball players. They found 9.52% and 12.82% increases in CMJ and squat jump performance in the normal season period.

Numerous studies have shown that PLY training increases vertical jump performance in both children and young adults, regardless of their previous athletic experience, sex, and training status. Matavulj et al. (40) stated that no specific strength level is required to begin such programs. On the contrary, de Villarreal et al. (17) suggested that higher enhancements after PLY training can be observed in athletes competing at the international level compared with those gains reached in athletes at the regional level in their research. Also, Miller et al. (43) recommended that trained participants be used for future studies because PLY training requires appropriate technical ability as well as optimum levels of muscle strength and joint coordination. The participants who participated in this study were neither international-level athletes nor untrained. Therefore, participants with low fitness levels or less experienced individuals are expected to benefit less from PLY training (28).

The discrepancy between the results in this study and those from previous studies might be attributed to several reasons: differences in the length of the training program and by the higher training loads and volumes used in the studies; less experience, the specificity of the training and the athletic ability; the speed of movement rather than the resistance or load was more important and positively affected the jump performance of physically highly active students.

EMG Activation
After 6-week PLY training, the present study showed 13.25%, 9.60%, and 13.93% significant improvements in EMG activities of VL, VM, and GAS muscles after 6-week PLY training, respectively. The results of this study are consistent with previous studies that illustrated significant increases in EMG activities of lower extremity muscles’ after PLY training (3, 5, 9, 15, 16, 44, 51). Asadi (5) reported significant increases in the EMG activities for VL, VM, and rectus femoris muscles following depth jump and CMJ training in healthy male collegiate students. Mehdipour et al. (42) found that 6 weeks of PLY training significantly improved EMG activity of GAS, but no significant difference was observed for EMG activity of the rectus femoris. Similarly, Toumi et al. (54) demonstrated no significant differences in RMS values for the biceps femoris during CMJ. No significant differences in rectus femoris and biceps femoris muscles may be explained by differences in muscle length changes during loading. The rate and magnitude of loading modulate the stretch reflex output, with faster rates and higher magnitudes of loading contributing to an increased stretch reflex, and the stretch reflex can augment muscle activity in the loading phase of a PLY exercise (16).

The main findings of this study demonstrated that a 6-week PLY training significantly increased EMG activities of VL, VM, and GAS, whereas, no significant increase was observed for vertical jump height in the PLY group (2.77%=0.98 cm). Similar results were obtained by Chimera et al. (15) and Behrens et al. (9). The authors have shown that 6-week PLY training significantly improved EMG activities of lower extremity muscles, but no significant increases were observed regarding vertical jump height, 5.59% (1 cm) and 5% (1.8 cm), respectively. In contrast, Sözbir et al. (51) have examined the effects of two different combined training programs (6-week of PNF stretching+PLY and static stretching+PLY) on RMS-EMG of VL and VM muscles during CMJ. They found significant increases for VL (13.55% and 20.50%), VM (18.95% and 18.17%), and CMJ (13.55% and 20.50%) in PNF+PLY and static+PLY training groups, respectively. Moreover, Toumi et al. (54) have also revealed a significant increase in RMS values for the VL and VM in the transition and positive (transition+concentric) phases during CMJ and also found a significant increase in the CMJ performance (13.2%). In another study, Asadi (5) reported significant increases both in the EMG activities for lower extremity muscles’ and vertical jump height following PLY training. The inconsistent results might be due to the different exercises performed during the training and the dissimilar durations of the training periods.

6 weeks of PLY training induced significant increases in the EMG activity of lower extremity, which could result from an increased sensitivity of the muscle spindle via enhanced α-γ co-activation to enhance the stretch reflexes (54). Thus, the change that occurred in the knee joint stiffness during the negative phase could be effected by the result of a change in muscle activation. Previous authors suggested that enhanced EMG activities of lower extremity muscles’ during eccentric phase of CMJ might provide an advantage by allowing for the greater storage and release of elastic energy (24, 54). The mechanisms for improved EMG activities after the training may involve an increase in α-motor neuron firing frequency and/or recruitment, neural drive to the agonist muscles, improved intramuscular coordination, and changes in the mechanical characteristics of the muscle-tendon complex during SSC (9, 12, 17, 55). These neuromuscular changes together may improve the ability to store and release elastic energy during CMJ performance following PLY training (12, 39). Nevertheless, no significant increase was observed for vertical jump height in the PLY group in this study. It might be related to the lower experience level of participants and insufficient control of the performance of PLY exercises. Another potential explanation is the different muscles’ activation, that is, lower back muscles and hip flexor muscles are rapidly contracting and generating power during CMJ performance.

CONCLUSIONS
In conclusion, the findings of the present study suggest that 6-week PLY training, in addition to the regular academic program, induced significant improvements in EMG activities of lower extremity muscles but no significant increases in vertical jump height of highly active physical education students. Previous studies indicated that PLY training elicits significant changes in vertical jump performance by numerous positive changes in neuromuscular adaptations and muscle functions (17, 39). However, this study suggests that improvement of vertical jump performance may not be related to only a change in EMG activities of lower extremity muscles. CMJ performance is also highly affected by training experience and appropriate technical ability as well as optimum levels of muscle strength and joint coordination.

APPLICATIONS IN SPORT
Many trainers and strength coaches struggle to improve athletic performance of their players in sports which jumping task are a constant element of different sports’ movement (i.e., track and field, volleyball, soccer and basketball). Therefore, PLY training is effectively used in order to improve vertical jump and neuromuscular performance in many training regime for wide-range of participants who are from untrained to well-trained. This study showed that PLY training can be used to enhance neuromuscular functions during dynamic contractions but this improvement does not necessarily cause gains in the vertical jump height. However, PLY exercises can be nevertheless suggested to reach the same jump height with shorter time in many sport actions by increases biomechanical technique and neuromuscular adaptation during high impact activities such cutting and landing. Trainers and strength coaches also need to be focused on improvement of training experience and proper jumping ability in vertical and horizontal planes by precise instructions of performance of PLY exercises.

ACKNOWLEDGMENTS
The author is grateful to Prof. Dr. R. Gul Tiryaki SONMEZ from Department of Health Sciences, Lehman College, The City University of New York for reviewing the manuscript. Further, the author thanks to national track and field coaches who are Sinem FAZLA and Ümit DEMİRCİ for assistance during PLY training. No sources of funding were used to assist in the preparation of this study. The author declares that he does not have any conflict of interest in accordance with the journal policy and disclose no professional relationships with companies or manufacturers who will benefit from the results of the present study.

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