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The Combination of Plyometric and Balance Training Improves Sprint and Shuttle Run Performances More Often Than Plyometric-Only Training With Children

Chaouachi, Anis1; Othman, Aymen Ben1; Hammami, Raouf1; Drinkwater, Eric J.2; Behm, David G.3

Journal of Strength and Conditioning Research: February 2014 - Volume 28 - Issue 2 - p 401–412
doi: 10.1519/JSC.0b013e3182987059
Original Research
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Chaouachi, A, Othman, AB, Hammami, R, Drinkwater, EJ, and Behm, DG. The combination of plyometric and balance training improves sprint and shuttle run performances more often than plyometric-only training with children. J Strength Cond Res 28(2): 401–412, 2014—Because balance is not fully developed in children and studies have shown functional improvements with balance only training studies, a combination of plyometric and balance activities might enhance static balance, dynamic balance, and power. The objective of this study was to compare the effectiveness of plyometric only (PLYO) with balance and plyometric (COMBINED) training on balance and power measures in children. Before and after an 8-week training period, testing assessed lower-body strength (1 repetition maximum leg press), power (horizontal and vertical jumps, triple hop for distance, reactive strength, and leg stiffness), running speed (10-m and 30-m sprint), static and dynamic balance (Standing Stork Test and Star Excursion Balance Test), and agility (shuttle run). Subjects were randomly divided into 2 training groups (PLYO [n = 14] and COMBINED [n = 14]) and a control group (n = 12). Results based on magnitude-based inferences and precision of estimation indicated that the COMBINED training group was considered likely to be superior to the PLYO group in leg stiffness (d = 0.69, 91% likely), 10-m sprint (d = 0.57, 84% likely), and shuttle run (d = 0.52, 80% likely). The difference between the groups was unclear in 8 of the 11 dependent variables. COMBINED training enhanced activities such as 10-m sprints and shuttle runs to a greater degree. COMBINED training could be an important consideration for reducing the high velocity impacts of PLYO training. This reduction in stretch-shortening cycle stress on neuromuscular system with the replacement of balance and landing exercises might help to alleviate the overtraining effects of excessive repetitive high load activities.

1Tunisian Research Laboratory “Sport Performance Optimization,” National Center of Medicine and Science, in Sports, Tunis, Tunisia;

2School of Human Movement Studies, Charles Sturt University, Bathurst, Australia; and

3School of Human Kinetics and Recreation, Memorial University of Newfoundland, St. John's, Canada

Address correspondence to David G. Behm, dbehm@mun.ca.

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Introduction

The training literature provides an abundance of information on programs to improve athletic power (18,40,46,57). Models for improving athletic performance most often emphasize strength, power, and speed exercises (40,57). However, the ability to jump, sprint, or be agile is multifactorial. Few articles include the importance of landing mechanics, balance, or posture (43). Hay (30) in his deterministic model of velocity lists body position as 1 factor among many others such as stance time, flight time, flight distance, velocity at touchdown, change in velocity, force exerted, and others. Thus, 1 training factor for power, sprinting, jumping, and agility that may be underemphasized is balance and stability.

A review of balance training studies (n = 647) by Behm and Colado (6) revealed an average 105% increase (effect size = 1.2) for balance and stability scores (i.e., center of pressure, Star Excursion Balance Test [SEBT], postural sway, Berg balance scale, and others) typically after 4–10 weeks of training. Kean et al. (38) reported that 5 weeks of static balance training with recreationally active adult subjects resulted in 33% increase in static balance and 9% improvement in vertical jump height. They attributed this improvement in jump height to a possible decrease in postural sway allowing the balance trained individual to direct their propulsive forces in a more vertical direction rather than having some of the forces moving in a more oblique or horizontal direction with a less balanced individual. In the review article by Behm and Colado (6), a summary of balance training studies (n = 85) that measured functional performances such as vertical jump, shuttle run time, squats, and other measures showed a mean improvement of 31.4%. Hence, just the improvement of balance without additional resistance training can improve activities involving explosive power. However, most training programs emphasize the ability to explosively move vertically or horizontally from the surface and far fewer studies include improving landing technique to ensure a strong balanced position from which to execute their next movement. The review by Behm and Colado (6) reported that on average (n = 179), force and power decreased by 29.3% (effect size = 2.15) when unstable. Hence, less than optimal balance whether performing repeated jumps, bounding, sprinting actions, or agility negatively affects performance.

Given that balance and coordination are not fully developed in children (56), exercises that require balance should be incorporated into youth resistance training programs because balance is essential for optimal performance and the prevention of athletic injuries (9,60). Improvements in static balance were shown to be significantly correlated (0.65) with maximum ice skating speed with hockey players from 15 to 17 years of age (11). A 4-week balance training program integrated into high school physical education lessons resulted in improved postural control, jumping height, and enhanced rate of force development. Lloyd et al. (43) suggested that “for a child to minimize injury and maximize plyometric performance, they must display correct landing mechanics.” As balance capabilities are immature in children (56), a training program including both explosive power and balance activities may accelerate their training adaptations.

Mechanisms underlying functional improvements with balance training may be related to enhanced coordination or motor control, which are less developed in children (9). Trunk stabilizers (i.e., transversus abdominis and multifidus) respond with anticipatory postural adjustments (APAs) to movements of the upper or lower limbs (31–33), so that exercises that help perfect APAs and appropriate motor coordination to deactivate these muscles on movement completion (co-contractions) would contribute to greater movement efficiency. Additionally, afferent feedback sensitivity can be improved with balance and motor skill training (12), resulting in quicker onset times of stabilizing muscles (2). Balance or instability training can promote co-contractions with shorter latency periods that allow for more rapid stiffening of joints (8,37) and hence stability (34). Although co-contractions would also contribute to force deficits by providing greater resistance to the intended motion, this effect is diminished by continued training (reduction in co-contractions) (15). The Canadian Society for Exercise Physiology position stand (7) on instability resistance training states that “Such training (instability/balance) may promote agonist–antagonist co-contractions with shorter latency periods, which allows for rapid stiffening and protection of joint complexes. Training programs must be structured so that athletes, non-athletes, and workers are prepared for the wide variety of postures and external forces encountered during physical tasks.” Although balance and stable landing training are not emphasized to as great a degree in the training literature, the few training studies and the associated physiological mechanisms suggest that it is a relatively neglected component of functional training.

Therefore, the objective of this study was to examine and compare with children; training programs that emphasized explosive power movements with a training program that combined explosive power movement exercises with landing exercises to improve dynamic balance. Based on the balance training literature, it was hypothesized that the training program involving both explosive movement and landing techniques would provide greater gains in balance measures and tests involving single and continuous explosive power movements.

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Methods

Experimental Approach to the Problem

As the objective of this study was to compare the effectiveness of plyometric only with balance (PLYO) and plyometric training on balance and power measures (COMBINED) in children, 42 adolescent boys (aged 12–15 years) volunteered. Before and after an 8-week training period, lower-body strength (1 repetition maximum [1RM] leg press), power (horizontal and vertical jumps, triple hop [THT] for distance, reactive strength, and leg stiffness), running speed (10-m and 30-m sprint), static and dynamic balance (Standing Stork Test and SEBT), and agility (shuttle run) were assessed. Children were randomly divided into 2 training groups (PLYO [n = 14] and COMBINED [n = 14]) and a control group (n = 12). The PLYO group trained using stretch-shortening cycle (SSC) movements such as jumps and hops with an emphasis on maximum jump height and short ground contact times. COMBINED group also used jumps and hops but maintained the position on landing to emphasize stability and balance on landing.

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Subjects

Forty-two healthy adolescent boys between 12 and 15 years of age, recruited from the same public school (Secondary School, Bou Arada, Tunisia), volunteered to participated in this study. Subject characteristics are included in Table 1. All participants were from similar socioeconomic status and had exactly the same daily school schedules. All of them participated in their normal physical education lessons twice per week. However, none were involved in any after-school activities or any formalized strength and conditioning training programs. Pubertal stage was determined by a self-assessment questionnaire in which subjects assessed their genital and pubic hair development according to the criteria of Tanner (4). Subjects ranged from stage 3–5 of the Tanner score. Before participation in this study, the subjects were given a letter that included written information about the study and a request for consent from the parents to allow their children to participate in the study. Parental and subject informed consent was obtained after thorough explanation of the objectives and scope of this project, the procedures, risks, and benefits of the study. The study was conducted according to the Declaration of Helsinki, and the protocol was fully approved by the Ethics Committee of the National Centre of Medicine and Science of Sports of Tunis (CNMSS) before the commencement of the assessments. Subjects and their parent/guardian were also informed that participation was voluntary and that they could withdraw from the study at any time. None had any history of musculoskeletal, neurological, or orthopedic disorder that might impair their ability to execute plyometric or balance training or to perform power tests.

Table 1

Table 1

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Procedures

One week before the commencement of the study, all the subjects participated in 2 orientation sessions to become familiar with the general environment, equipment, and the experimental procedures to minimize the learning effect during the course of the study. Each subject's height and body mass were collected using a wall-mounted stadiometer and an electronic scale, respectively. Afterward, subjects' performances were tested before and after the 8-week training period. The testing protocol included assessment of lower-body strength and power (1RM leg press, horizontal and vertical jumps, THT for distance, reactive strength, and leg stiffness), running speed (10-m and 30-m sprint), static and dynamic balance (Standing Stork Test and SEBT, respectively), and agility (shuttle run test). After the initial baseline testing session, subjects were randomly divided into 2 experimental groups (plyometric [PLYO, n = 14] and plyometrics and balance [COMBINED, n = 14]) and a control group (n = 12) without a training program. Groups were matched for age, maturation status, and physical characteristics. The plyometric only training (PLYO) group followed a structured plyometric training program only. The plyometric and balance training (COMBINED) group performed combined plyometric and balance training. The control group was limited to their regularly scheduled physical education class. In both groups, there were subjects in Tanner stage 3 (n = 6), 4 (n = 7), or 5 (n = 1). All participants' characteristics can be observed in Table 1.

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Independent Variable

Both experimental groups followed a training program of 8 weeks with a frequency of 3 sessions per week performed on nonconsecutive days during the spring of 2011. An 8-week program was implemented since the inclusion of testing before and after the training program would constitute the approximate duration of a semester and the children would not be able to be monitored for a duration greater than this period. The 8-week training programs for PLYO and COMBINED groups are detailed in Tables 2 and 3, respectively. Each training session was composed of 5 different exercises designed for the lower extremity with 1–2 sets of 8–15 repetitions. Progressive overload principles were incorporated into the program by increasing the number of foot contacts and varying the complexity of the exercises. For all rapid SSC plyometric exercises, subjects were instructed to give maximal efforts with minimal ground contact times. COMBINED group performed 1 balance exercise (i.e., single-legged squats on hemispherical dome) and 4 plyometric exercises in each session. In addition, the COMBINED group replaced 50% of the rapid SSC plyometric exercises with plyometric exercises that emphasized proper and balanced landing technique. These plyometric exercises that emphasized landings would hold the landing position for 3 seconds after each landing, eliminating the rapid SSC action. For example, in the Line jump, forward-deep hold drill, subjects alternated 1 jump and landing/takeoff segment with a short contact time (rapid SSC) with the next jump that would involve deep hold landing for 3 seconds. Thus, the COMBINED group would have approximately one-half of their training repetitions involving balanced 3-second landings and the other half of their training involving rapid SSC takeoffs. In contrast, the PLYO group would be 100% rapid SSC repetitions (short contact times). In terms of overall volume, the COMBINED group performed 40% rapid SSC and 60% balance/landing emphasis (4 of 5 exercises had half the repetitions with deep hold and half the repetitions with short contact time and the fifth exercise [squats] had all balance emphasis). The PLYO group used rapid SSC plyometric exercises for all sets and exercises.

Table 2

Table 2

Table 3

Table 3

Because most children do not have any history of traditional plyometrics, particular attention was paid to demonstration and execution, and verbal feedback in the initial stages was focused on correcting takeoff and landing mechanics. Four basic techniques were stressed: (a) correct posture (i.e., spine erect, shoulders back) and body alignment (i.e., chest over knees) throughout the jump; (b) jumping straight up for vertical jumps, with no excessive side-to-side or forward-backward movement; (c) soft landings including toe-to-heel rocking and bent knees; and (d) instant recoil preparation for the next jump. Phrases such as “shock absorber” and “recoil like a spring” were used as verbal and visualization cues for each phase of the jump (49).

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Testing Procedures

Both groups were tested before and after the 8-week training period for all variables. Each testing session was conducted over 3 separate days with the same test order. Testing was completed at the same time on each testing day for both pre- and post-tests, at the same indoor venue and by the same trained investigators. The participants were asked to wear the same clothing and footwear and to avoid strenuous activity during the 24 hours preceding each test session. Subjects were prohibited from consuming food, beverages, or any known stimulants (e.g., caffeine) that would possibly enhance or compromise alertness during the period of investigation. Water was permitted to be ingested ad libitum. Each player was instructed, and verbally encouraged, to give a maximal effort for each performance test. Performance testing was initiated after a standardized 15-minute warm-up, including submaximal running; dynamic stretching; low-intensity forward, sideways, and backward running; several acceleration runs; and jumping at a progressively increased intensity.

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Dependent Variables

Vertical Jump Tests

Three vertical jumps were used in this study: countermovement jump (CMJ), maximal hopping, and submaximal hopping. All these tests have been shown to be reliable and valid measures in pediatric populations (44). For each test, participants were instructed to keep their hands placed on their hips to minimize lateral and horizontal displacement during performance, to prevent any influence of arm movements on the vertical jumps, and to avoid coordination as a confounding variable in the assessment of the leg extensors' neuromuscular performance (17). Participants also had to leave the ground with the knees and ankles extended and land in the same position and location to minimize horizontal displacement and influence on flight time. All vertical jump tests were performed using an Ergojump system (ErgojumpP apparatus; Globus Italia, Codognè, Italy), which recorded jump height and contact and flight times. Each test was separated by a passive recovery period of at least 5 minutes.

The CMJ involved the participants lowering themselves from an upright standing position until approximating a knee angle of 90°, followed immediately by a vertical jump. Participants were encouraged to perform the eccentric phase of the jump as quickly as possible to maximize jump height. Three trials were performed with approximately 2 minutes of recovery, and the best result was used for analysis.

Maximal and submaximal hopping protocols were performed in the same manner as previously reported (44). The maximal hopping protocol involved participants performing 5 repeated bilateral maximal vertical hops in place on the contact mat. Participants were instructed to maximize jump height and minimize ground contact time (21). The first jump in each trial was discounted, whereas the remaining 4 hops were averaged for analysis of reactive strength index (43). Reactive strength index was calculated from the equation of Flanagan and Comyns (25).

Absolute leg stiffness was measured during submaximal hopping by modeling the vertical ground reaction force, based on the flight and contact time during hopping (21). This method has been validated by comparison with simultaneous force plate stiffness measurements (21). Leg stiffness was also normalized relative to leg length and body mass (48). Participants were asked to hop bilaterally for 20 consecutive hops at 2.0 Hz. An electronic metronome helped the subjects to maintain the required frequencies by means of an auditory signal. Leg stiffness (kilonewtons per meter) was calculated using measures of body mass, contact, and flight times based on the equation proposed by Dalleau et al. (21), which has been proven as a valid and reliable calculation in the pediatric population (44).

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Horizontal Jumping Tests

Each participant performed a series of horizontal jumps including a bilateral standing horizontal (long) jump (SLJ), and THT with the dominant leg. For the SLJ test, the participants stood stationary with the toes aligned level with the start line, were instructed to push off vigorously, and jumped forward as far as possible. Participants were allowed the use of a countermovement with arms and body swing. The distance jumped from the start line at takeoff to the position of the heel on landing was measured in centimeters using a metal tape measure. With the THT test, the tape measure was fixed to the ground, perpendicular to a starting line. Subjects were instructed to stand behind the starting line with their nondominant leg forward and the dominant leg off the ground. The leg used to kick a soccer ball identified the dominant leg. The subject performed 3 consecutive maximal hops forward on the same leg to reach the maximal horizontal distance. Arm swing was allowed. The investigator measured the distance hopped from the starting line to the point where the heel hit on the completion of the third and final hop. Both tests were repeated 3 times, and the maximum distance achieved during the 3 trials was recorded in centimeters and was used for analysis.

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Sprint

Running speed was evaluated using a maximal 30-m sprint (with a 10-m split time). Players were instructed to run as quickly as possible along the 30-m distance from a standing start. Time was automatically recorded using photocell gates (Brower Timing Systems, Salt Lake City, UT, USA; accuracy of 0.01 s) placed 0.4 m above the ground. The subjects commenced the sprint when ready from a standing start just behind the first timing gate. Stance for the start was consistent for each subject. Subjects performed 2 trials with at least 2 minutes of rest between them. The run with the lowest 30-m time was selected for analysis.

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Agility

Agility was evaluated with the 4 × 9-m shuttle run test (39). Subjects stood behind a starting line and started the electronic clock by passing through the first timing gate. At the end of the 9-m section, subjects were asked to step with 1 foot beyond a marker while reversing the running direction and sprinting back to the start where the same reversing of movement direction was required. After the fourth 9-m section, the subject passed through the second timing gate to stop the electronic clock. The best time of 2 consecutive trials was recorded for the statistical analysis.

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Balance Testing

Balance was assessed using the Stork stand balance protocol and the SEBT, respectively. To perform the Stork stand test (50), participants stood with their opposite foot against the inside of the supporting knee, and both hands on his hips. On the command, the subject raised the heel of their foot from the floor and attempted to maintain their balance as long as possible. The trial ended if the subject moved his hands from his hips, the ball of the dominant foot moved from its original position, or if the heel touched the floor. This test was carried out on the dominant leg acting as the standing leg. The test was timed (in seconds) using a stopwatch. The total time was recorded in seconds. The score was the best of 3 attempts.

The SEBT was performed as previously described by Bressel et al. (13). Briefly, the SEBT consisted of 8 lines of cloth measuring tape adhered to the floor with clear packing tape 45° apart from each other, in the shape of an asterisk. While testing, subjects stood on 1 leg in the middle of the star and reached as far as possible along each of the 8 directions with their toes of the opposite leg, holding their hands on their hips, and keeping the heel of the stance leg on the ground. Each distance was read from the center of the star to the mark. Distances were measured in centimeters and normalized by dividing by the subject's lower extremity length (anterior-superior iliac spine to distal end of the medial malleolus) and multiplying by 100. Three trials were performed in each direction and used for analysis. Testing was only performed for the dominant limb.

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Strength

Lower-body strength was assessed on the leg press with a 1RM test as reported by Faigenbaum et al. (24). Before attempting a 1RM, subjects performed 3 submaximal sets of 1–6 repetitions with a light to moderate load. Subjects then performed a series of single repetitions with increasing loads. If the weight was lifted with the proper form, it was increased by approximately 1–2 kg, and the subject attempted another repetition. The increments in weight were dependent on the effort required for the lift and became progressively smaller as the subject reached 1RM. Failure was defined as a lift falling short of the full range of motion on at least 2 attempts spaced at least 2 minutes apart. The 1RM was typically determined within about 6–8 trials. Throughout all testing procedures, an instructor-to-subject ratio of 1:1 was maintained, and uniform verbal encouragement was offered to all subjects. Test-retest reliability was high for all tests (typical error of measurement [TEM] range from 0.3% to 3.2%).

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Statistical Analyses

To avoid the shortcomings of research based in null hypothesis significance testing, magnitude-based inferences and precision of estimation were used (36). Measures were log transformed before analysis to reduce the nonuniformity of error. Magnitude-based inferences were determined for the mean changes elicited in each group (control, PLYO, and COMBINED) and on the interaction effects between the intervention trials. The interaction effect between conditions was calculated from the mean difference between pre- and post-training for each group. Precision of estimates are indicated with mean difference ± 95% confidence limits, which defines the range representing the uncertainty in the true value of the (unknown) population mean.

Qualitative descriptors of standardized effects were assessed using these criteria: trivial < 0.2, small 0.2–0.5, moderate 0.5–0.8, and large > 0.8 (19). We defined effects as clinically meaningful when the likelihood that the observed effect size (d) exceeded 0.20 was greater than 75% while effects with 95% confidence limits overlapping the thresholds for small positive and negative effects (exceeding 0.2 of the SD on both sides of the null) were defined as unclear (42). TEM was calculated to measure day-to-day reliability of the test measures.

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Results

Within Group Changes

Improvements in all dependent variables were moderate to large in the COMBINED group (d = 0.55–1.72) and were all likely to be clinically meaningful (>88% likely). Similar results were true for the PLYO group (d = 0.59 to 1.21, >94% likely) except for leg stiffness (d = 0.30, 64% likely) and 10-m sprint (d = 0.02, 61% likely to be trivial), which were considered unclear. The effect of being in the control group over the training period was unclear (<75% likelihood) in all dependent variables except for 30-m sprint and reactive strength (>75% likely of being trivial) and THT (d = 0.38, 82% likely to improve) (Table 4).

Table 4-a

Table 4-a

Table 4-b

Table 4-b

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Between Group Differences

COMBINED training was considered likely to be superior to PLYO training in leg stiffness (d = 0.69, 91% likely), 10-m sprint (d = 0.57, 84% likely), and shuttle run (d = 0.52, 80% likely). In 8 of the 11 dependent variables, the difference between COMBINED vs. PLYO training was unclear (<75% likely to be clinically different). COMBINED training was considered likely to be meaningfully better than control in all dependent variables (d = 0.57–1.42, >84% likely). PLYO training was considered likely to be meaningfully better than control in all dependent variables (d = 0.34–1.25, >77% likely) except leg stiffness (d = 0.34, 64% likely to be better) and 10-m sprint (d = 0.02, 39% likely to be trivial) (Table 5).

Table 5-a

Table 5-a

Table 5-b

Table 5-b

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Discussion

The most important findings of the present study were that with an 8-week training program for adolescents (a) combination of balance and plyometric (COMBINED) training induced training responses that were substantially greater for leg stiffness, 10-m sprint, and shuttle run, than plyometric alone (PLYO) and (b) PLYO training improved balance, jump, and squat strength measures to a similar degree as the COMBINED training group. Thus, with the 12- to 15-year-old adolescent boys, an 8-week COMBINED training improved some functional measures more often than PLYO training; however, the COMBINED program did not achieve substantially greater improvements in balance.

In accordance with the concept of training specificity (10), it might be expected that a program that included balance activities would improve balance scores to a greater extent than a program without specific balance training exercises. A number of studies have demonstrated improved balance-related test scores with balance training programs (27–29,38,39,51). Behm and Colado (6) reviewed 18 balance training studies encompassing 647 subjects and summarized that these studies had induced an average 105% improvement in balance-related measures.

However, balance can also be improved with strength training programs. Brooks et al. (14) showed improvements in APAs after an 8-week Pilates exercise program targeting trunk muscles. Seven (39) and 7 (59) weeks of traditional resistance training improved balance scores by 44.8% and 12.4%, respectively. Holviala et al. (35) reported 35.8% and 23.4% improvements in dynamic balance in middle-aged and older women, respectively, after 21 weeks of heavy resistance training. In elderly individuals, electromyostimulation training over 4 (1) and 6 (54) weeks, respectively, improved postural sway/control; however, voluntary strength training was more effective than electromyostimulation in the study by Paillard et al. (54). Strength training would be expected to enhance the ability and speed of postural muscles to return to a more stable position (59) after a balance perturbation.

Similar to the present study, 7 weeks of plyometric training produced similar improvements in dynamic balance measures (decreases in the SD of the center of pressure during hop landings) as a dynamic stabilization and balance training group (52). Plyometrics are a dynamic form of resistance training involving a rapid SSC and can involve both vertical and horizontal displacements of the center of gravity (41). Although specific landing and balance exercises were not included with the PLYO training in the present study, the dynamic nature of the exercises would place a training stress on postural control or equilibrium. Similarly, a number of different types of athletes without specific balance training programs exhibit better balance than the average population because of the dynamic nature of their sports. For example, triathletes have been reported to be more stable and less dependent on vision for posture than controls (53). Gymnasts are reported to be more efficient at integrating and reweighting proprioceptive inputs (61). Because plyometric exercises can provide a spectrum of balance challenges, specific balance or landing exercise may not be necessary for all individuals or activities.

However, the COMBINED training group did show advantages for leg stiffness, 10-m sprint, and shuttle run performances. Sprinting and shuttle runs involve consecutive or repeated unilateral landing and propulsion phases (22). The repetitive unilateral hip flexion and extension movements can place considerable destabilizing torques on the trunk and pelvis (58). While sprinting, the center of gravity pivots over the support foot (sequentially moving outside to inside to outside the limits of the center of gravity) primarily in the sagittal plane (anteroposterior) when transitioning from foot contact to takeoff. In addition, with unilateral foot contacts, the pelvis on the swing leg phase side will drop unless stabilized by the trunk muscles. Behm et al. (5) demonstrated the high intensity of trunk muscle activation induced from running to stabilize the pelvis needed to ensure optimal running performance in both trained (triathletes) and untrained runners. Hence, running provides instability or balance training stresses because of the anteroposterior translation of the body, angular momentum of the limbs, and the vertical or oblique displacements of the pelvis. Shuttle runs with the changes in direction would further displace the center of gravity in all 3 planes of movement. Hence, the rapid perturbations to and oscillations of the center of gravity while sprinting and performing shuttle runs can be considered a far more demanding physiological test of balance than static balance tests.

PLYO training was not clearly different than combined training for CMJ, 1RM leg press, reactive strength, THT, and SLJ. All these measures involved bilateral actions (greater base of support can provide less balance challenges than unilateral actions), and thus the balance benefits from PLYO training were as equal to the task as the COMBINED training. Whereas PLYO and COMBINED training provides similar adaptations for static balance and bilateral tests, the increased emphasis on balance and stable landings in the combined training program can provide an advantage for the more complex, high speed, unilateral, repetitive dynamic tasks that may tax postural control to a greater degree.

Leg stiffness also increased to a greater degree with the COMBINED training group. Carpenter et al. (16) indicated that a stiffening strategy was adopted when individuals were presented with a threat of instability. Increased antagonist activity can contribute to increased joint stiffness (37). Antagonist activity is greater when uncertainty exists in the task (47), which may occur during the flight phase of a drop jump. The role of the antagonist would be to control mechanical impedance (opposition to a disruptive force) (34) and optimally position the limbs and body on surface contact (23). Furthermore, co-contractions are prevalent for joint protection (3).

Balance and perturbation training can significantly improve neuromuscular control by promoting APAs (26). Anticipatory postural adjustments are not unique to the spine and have been shown in peripheral joints as well (20). Repeated exposures to balance and stability challenges result in proactive (45) or feedforward (55) adjustments that would contract (and stiffen) appropriate muscles before landing. Furthermore, the sensitivity of afferent feedback pathways can be improved with balance and motor skill training (12) resulting in quicker onset times of stabilizing muscles (2). Thus, the increased leg stiffness with the COMBINED training group could also have contributed to the greater 10-m sprint and shuttle run performances. Furthermore, the APA or feedforward adjustments could also contribute to injury prevention. For example, balance training has been reported to reduce the incidence of ankle sprains with volleyball players (60).

In conclusion, an 8-week training program for 12- to 15-year-old adolescent boys demonstrated that PLYO training was as effective as COMBINED training for static balance tests and that COMBINED training produced better results for 10-m sprint and shuttle run times, and leg stiffness. The inherent balance challenges associated with plyometric training are sufficient to adequately improve static balance measures and bilateral activities that commence with an adequate base of support (i.e., CMJ, SLJ, squats), whereas the greater stability or balance challenges of consecutive unilateral activities such as high speed sprinting and change of direction running (shuttle) may be enhanced with the addition of balance training to plyometric exercises. The increased leg stiffness scores found with the combined training may also have contributed to the greater improvement in sprint and shuttle run performance.

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Practical Applications

As balance is less well developed in youth, coaches and athletes should consider adding or replacing some of their jumping, hopping, bounding, and other similar training activities with balance activities. The COMBINED training group maintained their position on landing and thus attempted to absorb the stress of landing to a greater extent than the rapid SSC movements of the PLYO group. Such COMBINED training could also be an important consideration for reducing the high velocity impacts and eccentric to concentric transitions of PLYO training. This reduction in SSC stress on muscle and connective tissue with the replacement of balance and landing exercises might help to alleviate the overtraining effects of excessive repetitive high load activities.

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Keywords:

youth; stretch-shortening cycle; jumps; stability; agility; magnitude-based inferences

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