Neuromuscular adaptations to plyometric training: depth jump vs. countermovement jump on sand
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- Mirzaei, B., Norasteh, A.A. & Asadi, A. Sport Sci Health (2013) 9: 145. doi:10.1007/s11332-013-0161-x
The aim of the current investigation was to examine the effects of depth jump and countermovement jump training on neuromuscular adaptations using vertical jump and electromyography activity in the muscles. Twenty-seven healthy males (age 20.4 ± 0.3 years; weight 69.8 ± 6.1 kg; height 177.3 ± 6.2 cm) were recruited to participant in the present study and were randomly divided into three groups: depth jump (DJ), countermovement jump (CMJ) and control group. The experimental groups performed either DJ or CMJ training twice weekly for 6 weeks. The training program included five sets of 20 repetitions DJ (from the height of a 45-cm box) or CMJ exercise onto 20-cm dry sand. The electromyography activities in the vastus medialis (VM), and rectus femoris (RF) muscles, and vertical jump (VJ) were measured a week pre and post 6 weeks of training. The results showed significant increases in the integrated electromyography for the VM and RF following DJ and CMJ training on sand, also the DJ and CMJ training on sand induced significant improvement in VJ performance (P < 0.05). In conclusion, the DJ and CMJ training on sand improved electrical activities in the muscle and jump performance, and it can be recommended that, coaches and athletes design plyometrics on sand, because these types of training on sand can be an effective method for improving neuromuscular adaptations.
KeywordsNeuromuscular Exercise performance Plyometric training Jumping
Plyometrics are training modes used by athletes in all types of sports to increase strength and explosiveness . Plyometrics consists of a rapid stretching of a muscle (eccentric phase) immediately followed by a concentric or shortening action of the same muscle and connective tissue . Success in many sports and sporting events depends heavily upon the athlete’s explosive leg power and muscular strength. In jumping, throwing, track and field events and other activities, the athlete must be able to use strength as quickly and forcefully as possible. This display comes in the form of speed-strength or power . Researchers have shown that plyometric training can improve muscular power [4–6]. Muscle power depends on the amount of nerve stimulation and the number of active motor units. To evaluate the power production mechanism, muscle activities will be studied and compared through direct measurement techniques. Inner-muscular neural adaptations consist of using motor units, the amount of stimulation and inter-muscular harmony. A qualitative procedure that can be used with the existing methods and can make needed quantitative measurements is the electromyography (EMG) .
It appears that plyometric training on land improved muscle activation and motor unit recruitment during depth jump and maximum voluntary isometric contraction (MVIC) [7–10]. Others have recommended that these exercises (e.g., plyometric exercise) be done on sand surface. Plyometric training on sand may increase motor unit recruitment because of the absorptive qualities of sand are likely to increase contraction time and allow the leg extensor muscles to build up active state and force prior to shortening. This can enable subjects to produce more work and force development on sand, than on the land . Unfortunately, no study examined the effects of intense plyometric training on sand on neuromuscular adaptation using MVIC and muscle power vertical jump test, and the effects of plyometric training especially depth jump and countermovement jump on MVIC, and vertical jump performance are unknown. Thus, the aim of this study was to investigate the neuromuscular adaptation to plyometric depth jump (DJ) and countermovement jump (CMJ) training on sand (compliance) surface. Our hypothesis is that performing DJ and CMJ on the compliance surface will produce changes in the EMG pattern and increase in vertical jump performance. This study may contribute to understanding of the neuromuscular adaptations induced by DJ and CMJ training on sand surface.
Materials and methods
This study was designed to address the question of how two different plyometric training programs (DJ and CMJ) on sand affect neuromuscular adaptations, after a 6-week plyometric training program. To do this, we compared the effects of 6 weeks of plyometric treatment in three groups of subjects with a different type of plyometric training sessions. Some tests were executed before and after the plyometric treatment (changes in electrical activity in the muscles and jump performance). This design enabled us to examine the impact of sand surface on neuromuscular adaptations. Subjects totally visited the laboratory in three sessions. One week prior to starting the training period, the participants were recruited to laboratory for the measurement of age, body mass and height. During this session, each participant was instructed in the proper form and technique of exercise and testing (familiarization session, day 1). At least 48 h before performing plyometric training program, the subjects were recruited again to laboratory for the assessment of neuromuscular adaption tests (day 2; pre-test). Forty-eight hours after completing training program, participants came back to laboratory and performed neuromuscular adaption tests (day 3; post-test). The subjects were instructed to avoid any strenuous physical activity during the duration of the experiment and to maintain their dietary habits for the whole duration of the study.
Twenty-seven healthy male subjects volunteered to participate in this study and were randomly assigned to two treatment groups performed twice weekly: depth jump (n = 9; age 20.5 ± 0.7 year; weight 70.6 ± 5 kg; height 180.6 ± 7 cm) and countermovement jump (n = 9; age 20.6 ± 0.7 year; weight 69.5 ± 7.8 kg; height 176.5 ± 4.2 cm). A control group of nine subjects (age 20 ± 0.3 year; weight 69 ± 5.2 kg; height 174.8 ± 3 cm) did not train and were tested before and after a 6-week period. The subjects were healthy, free of lower body injuries and they had no medical or orthopedic problems. Subjects were carefully informed about the experiment procedures and possible risk and benefits associated with participation in the study and signed an informed consent document before the investigation. The Institutional Review Board of the University approved the research protocol.
Neuromuscular adaptation measurement
The EMG activity was acquired for the vastus medialis (VM) and rectus femoris (RF) muscles for the subjects’ right legs using 8-channel electromyography equipment (Muscle Tester ME 3000P8, Mega Electronic Ltd, Finland), consisting of signal conditioner with a band pass filter with cut-off frequencies at 20–500 Hz, and amplifier gain of 2,000×, and a common mode rejection ratio >120 dB. Pre-amplified bipolar superficial electrodes of Ag/AgCl (Skintact®).
After skin preparation (shaving, gently scrubbing, and cleaning with alcohol), electrodes were placed over the muscle belly along the longitudinal axis of the muscle fibers, with 2 cm inter-electrode distance. Then, subjects performed a 5-min warm-up on a stationary bicycle at a self-selected pace, and some regular stretching of lower-extremity muscles. After setting up the instrumentation, a MVIC was measured for in sitting position. During the MVIC, the subjects’ right legs were fixed at 90° of knee flexion and subjects were verbally encouraged to extend their knees as hard as possible for a five-second bout. The better of two maximal effort isometric contractions per muscle were used for statistical analyses. There was a 1-min rest period between trials [12, 13]. The EMG signal was full wave rectified and integrated (IEMG in μV).
The electrodes of VM were located 20 % of distance from the anterior superior iliac spine to the midpoint of the medial joint line. The RF electrodes were placed halfway between the greater trochanter and medial epicondyle of the femur. A common reference electrode was placed over the proximal tibia [12–14].
Vertical jump test
The participants performed VJ test (Power Systems, Knoxville, Tennessee, TN 22550, USA) after 10 min of rest following EMG measurement. This test involves measuring the difference between a person’s standing reach and the height recorded from a jump and reach. The difference between the standing height and the jump height is the vertical jump value. Subjects were instructed to perform two-foot vertical jump and peak vertical jump value was recorded in cm . The test-retest reliability of this test was 0.95.
Plyometric training on sand
The plyometric training programs included 2 days a week for 6 weeks . In this study, we chose 6 weeks plyometric training period because of previous studies reported that neural and muscular adaptation can occur during these times following power and plyometric training [8, 16]. Each training session lasted 35 min, including 10 min warm-up (e.g., jogging, stretching and ballistic exercises), 20 min training (DJ or CMJ), and 5 min cool-down (e.g., jogging and stretching exercises). Subjects performed 5 sets of 20 repetitions  of DJ or CMJ with an 8-s interval between jumps. There were 2-min rest between sets and 72-h recovery between training sessions. Subjects performed DJ (from the height of a 45-cm box) or CMJ onto a 0.2-m-deep dry sand surface . Subjects in plyometric groups (DJ and CMJ) were instructed to perform exercises in each training session with maximal effort. Participants in DJ group began by standing on a 45-cm box and were instructed to land with one foot as they stepped down from the box and land with two feet on the sand. After sand contact, immediately subjects were instructed to explode off the sand by jumping as quickly and as high as possible. In contrast, subjects in the CMJ group performed CMJ training when they stood on sand and then flexed their knees approximately 90° and jumped as high as possible (16). During the training, all subjects were under direct supervision and were instructed on how to perform each exercise. During the intervention of 6 weeks, DJ, CMJ and CG groups continued their normal daily activities, and were not allowed to perform any other training (such as resistance training and/or plyometric training) that would impact the results.
All data are presented as mean ± SD. A two-way analysis of variance with repeated measures was used to determine significant differences among groups. In the event of a significant F ratio, Tukey post-hoc tests were used for pairwise comparisons. A criterion α level of P ≤ 0.05 was used to determine statistical significance. All statistical analyses were performed through the use of a statistical software package (SPSS®, Version 16.0, SPSS., Chicago, IL, USA).
The novel approach of this study was to investigate the effects of DJ and CMJ training on sand on neuromuscular adaptations. The finding of the present study showed that 6 weeks of DJ and CMJ training increased motor unit recruitment during MVIC in the VM, and RF muscles. Also, CMJ training showed significant differences in the VM and RF compared to CG. These findings are not in line with Mehdipour et al.  study. They examined the effects of 6 weeks plyometric training on RF muscle activity and did not find significant difference. Some of the reasons for such a variety of results may be noted as the difference in type and intensity of exercise. These findings are in line with Rezaimanesh et al.  and Hakkinen et al.  who reported significant changes in the motor unit recruitment and rate of force development for the lower body muscle. It appears that plyometric exercises add much force and tension to muscle cords. Performing such activities or tolerating extreme force and tension may lead to needed physiological or biological changes in muscle cords and other parts of the contraction system and can also cause muscle EMG changes to rise [6, 12]. The results indicated that plyometric training on sand increased EMG activities. The absorptive qualities of sand are also likely to increase contraction time, thus allowing the leg extensor muscles to build up an active state and force prior to shortening [19, 20]. As sand, is mobile and uneven in nature it may be important to consider the role of postural muscles in relation to the co-ordination required for jumping [17, 18]. The compliance of sand surface made it hard for the ankle to push along the vertical axis of the movement of the body and as a result it slipped behind in an attempt to maximize population. As a result, the body tries to balance and equalize this movement and move the hip to larger extension . Perhaps above mechanisms of sand plyometric become effective on the contraction elements and the muscle physiology changes. Also, changes in the IEMG following plyometric training on sand can increase the firing rate and motor unit recruitment. The CMJ training indicated significant changes than CG. The reason of this result can be mechanical characteristic of exercise. Knee flexion during DJ was lesser than CMJ (30° vs. 90°), and maybe muscle fiber and motor unit did not use properly [1, 16]. Overall, plyometric training (DJ and CMJ) on sand can lead to enhanced rate of force development and motor unit recruitment, and consequently, leg extensor muscle activity increased using IEMG.
The current study indicated that 6 weeks of DJ and CMJ training on sand induced positive effects on VJ, but no significant differences between two modalities of training. These results are in agreement with Thomas et al.  who reported gains in jumping ability after a 6-week of DJ or CMJ training on firm surface in youth soccer players. They also did not find any significant differences between DJ and CMJ. Gehri et al.  examined the effects of 12 weeks of DJ and CMJ training on jumping and energy production. They found significant increases in VJ for both training groups. None of the training methods improved utilization of elastic energy. It appears that high intensity plyometrics (e.g., DJ and CMJ) can improve jumping ability in men and women [13, 15]. Previous studies recommended that plyometric training drills should be performed on the firm surface, because compliance of surface like sand may reduce elastic energy following plyometric training such as DJ or CMJ . In this study, we not only found significant improvements in jumping abilities, but also found 15 % increases in VJ for DJ and 13.5 % for CMJ training after 6 weeks of training on sand. A possible explanation for the jump enhancement in the present study could be the rate of force development, stiffness and power enhancement [9, 18]. Improved muscle performance (VJ) due to a plyometric training program like DJ and CMJ may be in part to increase “motor unit functioning” . It has been suggested that DJ and CMJ training are more effective in improving jump performance in stretch shortening cycle jumps because it enhances the ability of individuals to use the elastic and neural benefits of the stretch shortening cycle [1, 15].
In summary, the results of this study are very encouraging the benefits of DJ and CMJ plyometric training on sand for improving electrical activities and vertical jump. It is recommended that, coaches design plyometrics on sand for athletes or individuals, because these types of training on sand can be an effective technique for improving neuromuscular adaptations.
The authors would like to thank all the participants for their cooperation in this study.
Conflict of interest