the effects of 4-weeks of plyometric training on ...

THE EFFECTS OF 4-WEEKS OF PLYOMETRIC TRAINING ON REACTIVE STRENGTH INDEX AND LEG STIFFNESS IN MALE YOUTHS RHODRI S. LLOYD,1 JON L. OLIVER,2 MICHAEL G. HUGHES,2

AND

CRAIG A. WILLIAMS3

1

Faculty of Applied Sciences, University of Gloucestershire, Longlevens, Gloucester, United Kingdom; 2School of Sport, Cardiff Metropolitan University (UWIC), Cardiff, United Kingdom; and 3Children’s Health and Exercise Research Center, School of Sport and Health Sciences, University of Exeter, Exeter, Devon, United Kingdom

ABSTRACT

INTRODUCTION

Lloyd, RS, Oliver, JL, Hughes, MG, and Williams, CA. The effects of 4-weeks of plyometric training on reactive strength index and leg stiffness in male youths. J Strength Cond Res 26(10): 2812–2819, 2012—Although previous pediatric research has reported performance improvements in muscular power, agility, speed, and rate-of-force development after exposure to plyometric training, the effects on reactive strength index (RSI) and leg stiffness remain unclear. One hundred and twenty-nine boys from 3 different age groups (9, 12, and 15 years) participated and were divided into either an experimental (EXP) or control (CON) group within their respective age groups. The EXP groups followed a twice-weekly, 4-week plyometric training program, whereas the CON groups participated in their normal physical education lessons. Preintervention and postintervention measures were collected for RSI (during maximal hopping) and leg stiffness (during submaximal hopping). Both 12- and 15-year-old EXP groups made significant improvements in both absolute and relative leg stiffness (p , 0.05). The 9-year-old EXP group and CON groups for all ages did not make significant changes in leg stiffness. The 12-year-old EXP cohort also made significant improvements in RSI (p , 0.05). Both 15- and 9-year-old EXP cohorts, and CON groups for all ages, failed to show any significant improvements in RSI. The study concludes that improvements in RSI and leg stiffness after a 4-week plyometric training program are age dependent during childhood.

KEY WORDS stretch-shortening cycle, long-term athlete development, pediatrics, hopping

Address correspondence to Dr. Rhodri S. Lloyd, [email protected]. 26(10)/2812–2819 Journal of Strength and Conditioning Research ! 2012 National Strength and Conditioning Association

2812

the

P

lyometric ability is governed by the capacity to effectively use the stretch-shortening cycle (SSC) (33). This unique, cyclical muscle action involves preactivity of the leg extensors before ground contact, a fast eccentric action, and a rapid transition between the eccentric and subsequent concentric phase (13). Despite previous concerns regarding the risk of injury that plyometrics pose to young athletes, there is now consensus among researchers that plyometrics are a safe and effective training strategy to develop physical abilities in young athletes (4). Specifically, research shows that children can improve; rebound jump height (25), agility and power (32), vertical jump performance (5), running velocity (15), and rateof-force development (22) after plyometric training interventions. Additionally, research studies suggest potential health benefits from plyometric training, notably increases in peak bone mass in adolescent girls (36). Although the aforementioned studies demonstrate the effectiveness of plyometric training, to our knowledge, the effects of plyometric training on measures of leg stiffness and reactive strength index (RSI) in youth remains unclear. Recent research has indicated that different test protocols measure different expressions of SSC function, highlighting that basic vertical jumps are not representative of intermediate- or fast-SSC owing to the absence of any rebounding component with the ground (21). It is recommended that calculations of leg stiffness and RSI during cyclical rebounding provide the most accurate model of naturally occurring locomotive activities (21). Leg stiffness distinguishes the ratio between peak ground reaction forces and peak center of mass displacement (24), and previous studies have highlighted its association with maximal running velocity (2), stride cadence (6), and running economy (12). More recently, research has examined the development of leg stiffness in male youth (20) and investigated the developmental relationship between leg stiffness and peak power during vertical jumping (14,35). Alternatively, the RSI is a measure that can be used to quantify SSC function and has previously been shown to increase with age in youth populations (20). The RSI is the

TM

Journal of Strength and Conditioning Research

Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited. Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.

the

TM

Journal of Strength and Conditioning Research ratio between jump height and time spent in contact with the ground developing the requisite forces to jump and represents an individual’s ability to change quickly from an eccentric to concentric muscle action (7). Given the intricate sequencing of different muscular contractions during an SSC action and the disparities in motor coordination development in children and adolescents owing to biological variation, potential adaptations in response to plyometric training in youth populations may be explained by some form of age-dependent neuromuscular developments. This is highlighted by previous research that shows that there is greater variability in performance of SSC tasks in children than in older adolescents (8,9). The inability of the premotor cortex in youth to accurately maintain postural control and accommodate for rapid corrections during SSC movements by innervating the appropriate motor units may be attributable to inconsistencies in SSC performance measures (18). Although it is acknowledged that the neural regulation of leg stiffness and RSI are more effective in adults in comparison to that in children (27) and that both measures of SSC appear to increase with age (20), the effect of age on the neuromuscular adaptive response to plyometrics training within a more homogenous age range remains unclear. Therefore, this study aims to examine the effects of a 4-week plyometric training program on measures of leg stiffness and RSI in 9-, 12-, and 15-yearold children.

METHODS Experimental Approach to the Problem

A between-group, repeated-measures design was used to examine the effect of a plyometric training program on measures of SSC in young male subjects. The participants were placed within either an experimental (EXP) or control (CON) group. The EXP groups participated in plyometric training sessions twice weekly for 4 weeks, whereas the CON groups completed their regular physical education lessons. Both EXP and CON groups for each age range were tested for

| www.nsca.com

contact time, flight time, jump height, RSI, and absolute and relative leg stiffness pretraining and posttraining intervention. These independent variables were selected to examine the effects of a plyometric intervention on measures of cyclical fast-SSC function. Data were collected from a mobile contact mat, and measures of leg stiffness and RSI were calculated based upon contact time, flight time, and jump height data. Subjects

One hundred and fifty male participants were recruited for the study, inclusive of 50 from each age group (9-, 12-, and 15-year-olds). To be included in the final analyses, the participants were required to complete at least 75% of the total training sessions. As a result of this stipulation, 21 subjects were removed from the study. Consequently, 129 male subjects were included for the final analyses from 3 different age groups: 9-year-olds (N = 41), 12-year-olds (N = 44), and 15-year-olds (N = 44). Mean values (6SD) for group characteristics are provided in Table 1. The participants were subsequently divided into either an EXP or CON group within their respective age groups, based on their weekly school timetable for Physical Education lessons. Despite not pair matching individuals based on an independent variable, there were no significant differences between CON and EXP group descriptive data. None of the participants reported any injury at the time of testing. All the participants were involved in regular physical education lessons; however, none were involved in any formalized strength and conditioning training programs. Parental consent and participant assent were collected before testing, and the University Research Ethics Committee granted ethical approval. Procedures

Testing. Testing was completed at the same time on each testing day, at the same indoor venue and by the same tester. The participants were asked to wear the same clothing and footwear and to avoid drinking, eating, and participating in exercise activities up to 1 hour before testing. Testing included

TABLE 1. Mean (6SD) values for descriptive details for both EXP and CON groups per age group.* Group

N

Age (y)

G9CON G9EXP G12CON G12EXP G15CON G15EXP

21 20 22 22 24 20

9.60 6 0.19 9.44 6 0.54 12.23 6 0.28 12.29 6 0.31 15.29 6 0.33 15.33 6 0.27

Standing height (cm) 135.48 133.20 151.67 151.89 174.11 174.35

6 5.73 6 8.66 6 6.93 6 7.94 6 9.20 6 6.63

Sitting height (cm) 68.50 6 67.98 6 75.94 6 75.60 6 87.68 6 87.00 6

3.20 3.50 4.05 4.10 5.13 2.75

Body mass (kg) 32.75 32.64 47.38 44.78 63.70 64.96

6 6.95 6 7.02 6 13.91 6 9.42 6 11.43 6 8.89

Maturity offset (y) 23.76 23.86 21.80 21.86 1.13 1.07

6 0.39 6 0.55 6 0.60 6 0.55 6 0.83 6 0.36

*G9CON = age 9 control group; G9EXP = age 9 experimental group; G12CON = age 12 control group; G12EXP = age 12 experimental group; G15CON = age 15 control group; G15EXP = age 15 experimental group.

VOLUME 26 | NUMBER 10 | OCTOBER 2012 |

2813


SSC Training Adaptations in Male Youths

TABLE 2. Plyometric training program outline.* Wk 1 Exercise Squat jump Countermovement jump Pogo hopping Standing long jump Lateral hops Hop scotch Bilateral !power" hops Ankle jumps !Power" skipping Unilateral pogo hops Max rebound hops Drop jumps Hurdle !power" hops Total foot contacts

S1 2 2 2 2 2

36 36 38 38 38

72

Wk 2 S2

23 23 43 43

S3 8 8 4 8

3 2 4 3 4

80

38 33 38 34 33

86

Wk 3

Wk 4

S4

S5

S6

S7

S8

2 3 10

2 3 10

438

4 3 10

4 3 10

335 338 2 3 10 335

3 3 2 3

94

94

2 3 10 335 434 335 106

2 3 10 335 434 335 106

35 38 3 10 35

3 2 4 2

38 38 35 35

102

*S = session number.

maximal and submaximal hopping, both of which have been shown to be reliable and valid measures in pediatric populations (19). The participants were given 4 trials of maximal hopping, with the mean of the best 2 trials subsequently used for further analysis. The participants were then given a single trial of a submaximal hopping test where 10 consecutive hops closest to the designated metronome rate were used for analysis (19). All jumps were performed on a mobile contact mat (Smartjump, Fusion Sport, Coopers Plains, Australia) and data instantaneously collected via a handheld personal digital assistant (PDA) (iPAQ, Hewlett Packard, Palo Alto, CA, USA). Reactive strength index (RSI) was determined during the maximal hopping test, which involved the participants performing 5 repeated bilateral maximal vertical hops on the contact mat (19). The participants were instructed to maximize jump height and minimize ground contact time (3). The first jump in each trial was discounted, whereas the remaining 4 hops were averaged for the analysis of RSI (19). The RSI variable was calculated from the equation of Flanagan and Comyns ([7]; equation 1). Absolute leg stiffness was measured during the submaximal bilateral hopping test, which was performed at a hopping frequency of 2.5 Hz. This frequency was selected to ensure that movement patterns were reflective of typical spring-mass model behavior (19). Relative leg stiffness was also normalized to leg length and body mass (24). The participants were asked to hop 2 legged on top of the contact mat for 20 consecutive hops. Hopping frequency was maintained using a quartz metronome (SQ-44, Seiko, Berkshire, United Kingdom). Leg stiffness (kilonewtons per meter) was calculated using measures of body mass, contact times, and flight times ([3]; equation 2), which is known to be a valid and reliable method

2814

the

for youth populations (19). Within the equation, KN refers to leg stiffness, M is the total body mass, Tc is equal to ground contact time, and Tf represents the flight time.

RSI ¼ jump height ðmillimetersÞ= ground contact time ðmillisecondsÞ; KN ¼ ½M 3pðTf þ Tc Þ&= #! " $ Tc2 Tf þ Tc =p ' ðTc =4Þ :

ð1Þ ð2Þ

Training. Experimental groups from all the 3 age groups followed a twice-weekly plyometric training program for 4 weeks on an indoor surface (content detailed in Table 2). The length of the training intervention was selected in accordance with previous research that has reported positive adaptation of SSC measures after 4 weeks of plyometric training (30,34) and to allow for a test week before and after the training intervention within the educational term. The EXP group completed 25–40 minutes of plyometric training twice a week, whereas the CON group participated in their normal physical education lessons. Total training session time was dependent on the planned intensity of the session, with more rest provided after those exercises eliciting greater eccentric loading. The participants had at least 48 hours of recovery time between any 2 training sessions (25). All the training sessions were preceded by a 10-minute warm-up that included low-intensity aerobic activity and a range of mobility exercises that provided appropriate activation of the lower limb musculature (5). To minimize the risk of injury, all the sessions progressed from low to moderate-high intensity drills, thus gradually imposing a greater eccentric stress on the musculotendon

TM



the

TM


| www.nsca.com

toward RSI thresholds, to ensure the participants gained TABLE 3. Mean (6SD) values for reactive strength index during maximal hopping. feedback based not only on how quickly they interacted 21 Reactive strength index (mm(ms ) with the ground (contact time) but also how much impulse Mean difference Pre Post (95% confidence interval) Group they could produce (reflected by jump height). This transition G9CON 0.82 6 0.26 0.82 6 0.27 0.01 (20.07–0.09) of plyometric training feedback G9EXP 0.90 6 0.25 0.90 6 0.24 0.01 (20.08–0.09) to optimize fast-SSC function G12CON 0.82 6 0.19 0.76 6 0.20 20.07 (20.15–0.01) G12EXP 0.91 6 0.24 1.01 6 0.26* 0.09 (0.01–0.017)*† has previously been advocated G15CON 1.26 6 0.27 1.19 6 0.28 20.06 (20.14–0.01) in the literature (7,18). Reactive G15EXP 1.46 6 0.28 1.52 6 0.26 0.06 (20.2–0.14) strength index thresholds were set in accordance with group *Significant increase from pretraining value. †Performance change is significantly greater than G12CON. means and SDs attained from baseline data. Such an approach allowed a range of thresholds to be used for each set per exercise, thus maximizing the likelihood of each participant training at, or near, their unit. The intensity of the program was increased in accoptimal training threshold. ordance with previous plyometric training guidelines (18). Training volume was defined by the number of foot contacts Statistical Analyses made during each session, starting with 72 contacts in the Descriptive statistics (means 6 SD) in addition to the mean first session, increasing to 106 contacts in the final 2 sessions. differences (95% confidence interval [CI]) in performance are Plyometric drills lasted approximately 5–10 seconds, and at provided based on pretraining and posttraining intervention least 90 seconds rest was allowed after each set (25). data. Separate mixed-model analyses of variance (ANOVAs) Plyometric drills included standing vertical and horizontal were used to test for significant differences in performance jumps, lateral jumps, ankle hops, skipping, single leg hopping, variables pretraining to posttraining, between EXP and CON maximal hopping, and low-level drop jumps (20 cm). Owing groups and between age groups and interaction effects to the relative lack of plyometric experience, verbal feedback between these terms. For all models, age group served as the in the initial stages was focused on correcting take-off and independent variable (3 levels: 9-, 12-, and 15-year-olds). landing mechanics. Technical guidance was provided for Differences in all performance variables were analyzed using stance and spinal posture, linearity of jumping kinematics, and a 3 3 2 3 2 model ANOVA (age 3 group 3 time), where age landing !quietly with soft knees" for all basic vertical and refers to the 9-, 12-, and 15-year-old age groups, group horizontal jumps. Additionally, and even in the early stages represents CON or EXP subgroups, and time refers to of the program, children were exposed to repeated subpretraining to posttraining data. Sphericity of data was tested maximal hopping in a bid to maximize the likelihood of by Mauchly’s statistic, and where violated, Greenhousesimultaneous development of fast elastic recoil and stretchGeiser adjustment was used. Bonferroni and Games-Howell reflex use. This training approach was augmented by realpost hoc tests were used to determine the origin of any time feedback from a mobile contact mat (Smartjump, Fusion between-group differences, when equal variance was or was Sport) in which different visual and auditory output was not assumed, respectively. Changes in performance were projected dependent on ground contact times, above or calculated as percentage changes, and a 1-way ANOVA was below predetermined performance thresholds, as recomused to determine any significant between-group differences mended in previous research (18). These thresholds ranged in performance change. Bonferroni post hoc analysis was from the lower and higher SDs surrounding group mean subsequently used to determine the location of any values. For example, the performance threshold during the significant differences. Statistical significance for all the tests first set of a given exercise would be set at the lowest standard was set at alpha level p # 0.05. Descriptive statistics were deviation (based on the initial pretraining intervention test calculated through Microsoft Excel", whereas all ANOVAs data for contact time), the second set would use the group were computed via SPSS V.17 for Windows. mean value, and the third set would use the highest SD. This approach was implemented to ensure that all the participants were exposed to a gradual increase in intensity, and to provide a spectrum of intrasession training intensities to allow for individual differences. In the final week of the training program, the emphasis of feedback was shifted

RESULTS Reactive Strength Index

Means (6SDs) and mean differences (95% CI) for RSI for each group are presented in Table 3. For maximum hopping VOLUME 26 | NUMBER 10 | OCTOBER 2012 |

2815


the


Contact time (ms)

Absolute leg stiffness (kN(m21)

Relative leg stiffness

TM

Group

Pre

Post

Mean difference (95% CI)

Pre

Post

Mean difference (95% CI)

Pre

Post

%Mean difference (95% CI)

G9CON

166.46 6 21.52 155.33 6 18.52 174.17 6 18.11 184.22 6 29.25 189.49 6 23.25 192.15 6 22.28

166.13 6 20.31 160.89 6 22.16 178.81 6 25.84 172.95 6 22.30† 196.79 6 25.84 180.13 6 17.84†

20.33 (27.87 to 8.53) 5.55 (22.85 to 13.96) 4.63 (23.38 to 12.64) 211.26 (219.27 to 3.25)† 7.30 (20.37 to 14.97) 212.02 (220.43 to 3.62)†

17.65 6 3.22 19.39 6 4.04 23.56 6 6.02 21.08 6 5.57 28.89 6 7.45 28.40 6 5.83

17.66 6 3.71 18.58 6 3.47 22.82 6 5.81 22.71 6 4.69‡ 27.09 6 4.93† 30.96 6 5.37‡

0.01 (21.61 to 1.6) 20.81 (22.46 to 0.83) 20.74 (22.31 to 0.83) 1.63 (0.62– 3.20)‡ 21.80 (23.30 to 0.29)† 2.56 (0.91– 4.21)‡

37.73 6 8.28 39.94 6 6.08 39.03 6 5.16 37.09 6 9.36 40.02 6 6.91 39.29 6 8.50

37.38 6 6.89 38.45 6 6.32 38.08 6 7.61 40.06 6 7.66‡ 37.99 6 6.68 42.75 6 7.37‡

20.34 (22.23 to 2.93) 21.49 (21.16 to 4.14) 20.95 (23.48 to 1.58) 2.97 (0.44 to 5.50)‡ 22.04 (24.46 to 0.39) 3.46 (0.81– 6.11)‡

G9EXP G12CON G12EXP G15CON G15EXP

*G9CON = age 9 control group; G9EXP = age 9 experimental group; G12CON = age 12 control group; G12EXP = age 12 experimental group; G15CON = age 15 control group; G15EXP = age 15 experimental group. †Significant decrease from pretraining measure (p , 0.05). ‡Significant increase from pretraining measure (p , 0.05).


SSC Training Adaptations in Male Youths

2816 TABLE 4. Mean (6SD) values for contact time, absolute and relative leg stiffness during submaximal hopping.*

the

TM

Journal of Strength and Conditioning Research only the 12-year-olds performing plyometric exercise demonstrated improvements in RSI (p = 0.022) when compared with the age-matched control. Irrespective of time, there was a significant main effect for age (F(1,127) = 63.438, p , 0.001), with 15-year-olds recording significantly greater RSI values than the younger 2 age groups (both p , 0.001). Leg Stiffness

Means (6SDs) and mean differences (95% CI) for contact times, absolute leg stiffness, and relative leg stiffness for each group are presented in Table 4. There were significant increases in both absolute (F(1,127) = 5.181, p , 0.05) and relative (F(1,127) = 4.839, p , 0.05) leg stiffness for both 12- and 15-year-old experimental groups after the training intervention program. There was also a significant decrease in absolute leg stiffness (p = 0.020) in the 15-year-old CON group. Regardless of time, the results revealed a significant main effect for age (F(1,127) = 50.588, p , 0.001) with 15-year-olds recording significantly greater absolute leg stiffness values than the younger 2 age groups (both p , 0.001). In comparison with the age-matched controls, both 12- and 15-year-old experimental groups revealed significant decreases in contact times after the training intervention period (F(1,127) = 5.432, p , 0.01). Results revealed a significant main effect for age (F(1,127) = 18.901, p , 0.001) with both 9- (p , 0.001) and 12year-old (p = 0.020) boys recording significantly lower contact times than the 15-year-old age group.

DISCUSSION Although previous research has demonstrated the effectiveness of youth-based plyometrics on a range of performance measures (5,15,32), the effects of plyometric training on leg stiffness and RSI, remained unclear. The results of this study revealed that a 4-week plyometric training program significantly improved leg stiffness performance in both 12and 15-year-old boys. These positive training effects remain for relative leg stiffness, which suggests that these training effects are not body size dependent. Additionally, the 12year-old boys who participated in the plyometric program significantly improved their RSI performance. None of the CON groups made any significant improvements in either absolute or relative leg stiffness, or RSI. The results revealed a consistent trend that 15-year-olds produced better results for RSI values during maximal hopping. Additionally, the cohort used shorter ground contact times and greater absolute and relative leg stiffness during submaximal hopping. This would suggest that age and maturational status have an effect on the natural development of SSC performance, irrespective of training intervention, a finding that is commensurate with previous literature (20). Analysis of the group 3 time interaction revealed that the experimental intervention had some positive benefit on RSI during maximal hopping. The same beneficial training effects were apparent for contact time and absolute and relative leg

| www.nsca.com

stiffness during submaximal hopping. Subsequent analysis revealed that there was no effect of age on the training response for RSI. In contrast, submaximal hopping contact time, and absolute and relative leg stiffness all showed a significant age 3 group 3 time interaction, which suggests that age had an influence on training adaptation. Although the interaction effects were related to significant positive training responses in the experimental groups, it should be noted that there existed a negative response in some of the CON groups. However, such a trend was only significant for the 15-year-olds in absolute leg stiffness during submaximal hopping. From these findings, it can be deduced that although the training program intervention had a beneficial effect on performance measures, age also influenced the training effect of certain variables collected during both submaximal and maximal hopping. This study revealed that the plyometric training program was successful in inducing a significant training effect on RSI in 12-year-old boys. This increase in RSI is commensurate with the findings of previous research that has reported improvements, albeit nonsignificant, in reactive strength of 13-year-old boys in response to an 8-week plyometric training program (25). Previous authors have described RSI as representing the strain placed on the musculotendon unit (23), and owing to the significant improvements reported by the 12-year-old experimental group, it is suggested that the participants developed an increased tolerance to the eccentric loading placed on the musculotendon unit during maximal hopping. More specifically, greater stretch-reflex contribution (34), rate-of-force development (22), and increased desensitization of Golgi tendon organs (11,28) might represent the neural mechanistic adaptations that enabled the participants to better tolerate and overcome impact forces experienced in the maximal hopping protocol. Additionally, increased motor unit recruitment might explain the positive training adaptation displayed by the 12-year-olds for RSI, as previous research has shown that prepubescent boys were able to activate approximately 10% more motor units after 10 weeks of strength training (31). Despite being nonsignificant, both 9- and 15-year-old experimental groups showed improvements in RSI, suggesting that both young children and adolescents can enhance reactive strength capabilities following plyometric training. However, the large SDs reported highlight the individual variation associated with pediatric populations and the varied training response of children within a chronological age group owing to differences in biological maturity (1). Although a growing body of evidence exists for the benefits of plyometric training on various components of jump performance in youths, such as vertical jump height (5), rebound jump height (25), and rate-of-force development (22), there is a paucity of research examining the effects of plyometric training on stiffness properties. In adults, joint stiffness has been shown to increase in response to a 12-week plyometric training program, with concomitant increases in VOLUME 26 | NUMBER 10 | OCTOBER 2012 |

2817


SSC Training Adaptations in Male Youths squat, countermovement and drop jump heights (16); however, there currently exists minimal evidence into the adaptations of leg stiffness in youth after plyometric training. Data revealed that 15-year-olds produced significantly greater absolute leg stiffness than both 12- and 9-year-olds. This finding reflects the influence of body mass on leg stiffness, with older participants who possess greater body mass, requiring greater overall stiffness to maintain center of mass displacement during ground contact (24). Both the 12and 15-year-old experimental groups made significant improvements in absolute leg stiffness. Both groups also showed the largest performance changes in terms of mean differences from preintervention to postintervention data (increases of 1.63 and 2.56 kN(m21, respectively). However, subsequent analysis revealed that these performance changes were not significantly greater than the other CON or EXP groups. Conversely, the 15-year-old CON group reported a significant decrease in performance. When normalized to body mass and limb length, the significant differences between preintervention and postintervention data for the 12- and 15-year-old experimental cohorts remained, suggesting that body mass and limb length were not responsible for the training adaptation. Consequently, the improvements in leg stiffness might be related to the significant reductions in ground contact time, suggesting that the training program was successful in enhancing the rebounding properties of the musculotendon unit. In relation to the spring-mass model (24), decrements in ground contact time would require greater rate-of-force development and more effective use of elastic energy reutilization to maintain center of mass displacement. Additionally, previous research has identified that with reductions in ground contact times, there is a concomitant increase in stretch-reflex activity reliance (26,27) and that neural regulation of the leg extensors were strong predictors of leg stiffness in boys (27). Previous research has revealed that 4 weeks of plyometric training can induce significant improvements in the excitability of the soleus muscle shortlatency stretch reflex (34), and it could be suggested that similar adaptations were experienced by the 12- and 15year-old experimental cohorts of this study. Short-latency stretch reflexes reflect the spinal voluntary command to activate the muscle during the 30- to 60-millisecond time phase of ground contact (10), and owing to the reduced contact times displayed by the older experimental groups, it is reasonable to suggest that the plyometric training program enhanced the reflexive contribution to overall leg stiffness by increasing muscle activity before, and during, ground contact. Additionally, both groups may have enhanced their ability to regulate leg stiffness in a feedforward manner during the 100 milliseconds before ground contact (preactivation) and in the immediate 30 milliseconds post ground contact (background muscle activity), because these factors are known to contribute to stiffness regulation in children (17) and adults (29).

2818

the

PRACTICAL APPLICATIONS This study has revealed the potential benefits of short-term plyometric intervention programs in augmenting youth SSC function. Previous research has highlighted the effectiveness of a 4-week training program in adults (30,34); however, this study appears to be the first to examine the effects of a low duration training program in youth populations. This finding is of particular interest to practitioners and educators alike, owing to the fact that educational term times are typically short in duration and that children are likely to respond more favorably to regular changes in a periodized training program (4). If performance adaptations in the SSC function can be obtained in such a short timeframe, then it is reasonable to suggest that such programs could be effectively integrated within an academic term, or as a short-term training block within a periodized training program. With respect to program design, although the youngest age group did not make significant improvements in either RSI or leg stiffness, neither did they show any reductions in performance, and consequently, it is suggested that they should focus mainly on fundamental plyometric movement competency. The 12- and 15-year-olds made significant improvements in leg stiffness, with 12-year-olds also significantly improving their reactive strength abilities. Owing to more mature musculoskeletal systems and the enhanced training response demonstrated by these age groups, adolescents should focus on more advanced plyometric activity, providing technical competency can be demonstrated first.

REFERENCES 1. Beunen, GP and Malina, RM. Growth and physical performance relative to the timing of the adolescent spurt. Exerc Sport Sci Rev 16: 503–540, 1988. 2. Bret, C, Rahmani, A, Dufour, A-B, Messonnier, L, and Lacour, JR. Leg strength and stiffness as ability factors in 100 m sprint running. J Sports Med Phys Fit 42: 274–281, 2002. 3. Dalleau, G, Belli, A, Viale, F, Lacour, JR, and Bourdin, M. A simple method for field measurements of leg stiffness in hopping. Int J Sports Med 25: 170–176, 2004. 4. Faigenbaum, AD, Kraemer, WJ, Blimkie, CJ, Jeffreys, I, Micheli, LJ, Nitka, M, and Rowland, TW. Youth resistance training: Updated position statement paper from the National Strength and Conditioning Association. J Strength Cond Res 23: S60–S79, 2009. 5. Faigenbaum, AD, McFarland, JE, Keiper, FB, Tevlin, W, Ratamess, NA, Kang, J, and Hoffman, JR. Effects of a short-term plyometric and resistance training program on fitness in boys age 12 to 15 years. J Sports Sci Med 6: 519–525, 2007. 6. Farley, CT and Gonzalez, O. Leg stiffness and stride frequency in human running. J Biomech 29: 181–186, 1996. 7. Flanagan, EP and Comyns, TM. The use of contact time and the reactive strength index to optimize fast SSC training. J Strength Cond Res 30: 32–38, 2008. 8. Gerodimos, V, Zafeiridis, A, Perkos, S, Dipla, K, Manou, V, and Kellis, S. The contribution of SSC and arm-swing to vertical jumping performance in children, adolescents, and adult basketball players. Pediatr Exerc Sci 20: 379–389, 2008. 9. Harrison, AJ and Gaffney, S. Motor development and gender effects on SSC performance. J Sci Med Sport 4: 406–415, 2001.

TM



the

TM

Journal of Strength and Conditioning Research 10. Hobara, H, Kimura, K, Omuro, K, Gomi, K, Muraoka, T, Iso, S, and Kanouse, K. Determinants of difference in leg stiffness between endurance- and power-trained athletes. J Biomech 41: 506–514, 2008. 11. Hutton, RS and Atwater, SW. Acute and chronic adaptations of muscle proprioceptors in response to increased use. Sports Med 14: 406–421, 1992. 12. Kerdok, AE, Biewener, AA, McMahon, TA, Weyand, PG, and Herr, HM. Energetics and mechanics of human running on surfaces of different stiffnesses. J App Physiol 92: 469–478, 2002. 13. Komi, PV. Stretch-shortening cycle: A powerful model to study normal and fatigued muscle. J Biomech 33: 1197–1206, 2000. 14. Korff, T, Horne, SL, Cullen, SJ, and Blazevich, AJ. Development of lower limb stiffness and its contribution to maximum jumping power during adolescences. J Exp Biol 212: 3737–3742, 2009. 15. Kotzamanidis, C. Effect of plyometric training on running performance and vertical jumping in prepubertal boys. J Strength Cond Res 20: 441–445, 2006. 16. Kubo, K, Morimoto, M, Komuro, T, Tsunoda, N, Kanehisa, H, and Fukunaga, T. Influences of tendon stiffness, joint stiffness, and electromyographic activity on jump performance using single joint. Eur J Appl Physiol 99: 235–243, 2007.

| www.nsca.com

23. McClymont, D. Use of the reactive strength index (RSI) as an indicator of plyometric training conditions. In: Science and Football V; The Proceedings of the 5th World Congress on Science and Football, Part VI—Football Training. T. Reily, J. Cabri, and D. Arau´jo, eds. Oxon, United Kingdom: Routledge, 2005. pp. 408–416. 24. McMahon, TA and Cheng, GC. The mechanics of running: how does stiffness couple with speed? J Biomech 23: 65–78, 1990. 25. Meylan, C and Malatesta, D. Effects of in-season plyometric training within soccer practice on explosive actions of young players. J Strength Cond Res 23: 2605–2613, 2009. 26. Moritani, T, Oddsson, L, and Thorstensson, A. Phase-dependent preferential activation of the soleus and gastrocnemius muscles during hopping in humans. J Electromyogr Kinesiol 1: 34–40, 1991. 27. Oliver, JL and Smith, PM. Neural control of leg stiffness during hopping in boys and men. J Electromyogr Kinesiol 20: 973–979, 2010. 28. Ovalle, WK. The human muscle-tendon junction. A morphological study during normal growth and maturity. Anat Embryol (Berl) 176: 281–294, 1987. 29. Padua, DA, Arnold, BL, Perrin, DH, Gansneder, BM, Carcia, CR, and Granata, KP. Fatigue, vertical leg stiffness, and stiffness control strategies in males and females. J Athl Train 41: 294–304, 2006.

17. Lazaridis, S, Bassa, E, Patikas, D, Giakas, G, Gollhofer, A, and Kotzamanidis, C. Neuromuscular differences between prepubescents boys and adult men during drop jump. Eur J Appl Physiol 110: 67–74, 2010.

30. Potach, DH, Katsavelis, D, Karst, GM, Latin, RW, and Stergiou, N. The effects of a plyometric training program on the latency time of the quadriceps femoris and gastrocnemius short-latency responses. J Sports Med Phys Fitness 49: 35–43, 2009.

18. Lloyd, RS, Oliver, J, and Meyers, RW. The natural development and trainability of plyometric ability during childhood. J Strength Cond Res 33: 23–32, 2011.

31. Ramsay, J, Blimkie, C, Smith, K, Garner, S, Macdougall, J, and Sale, D. Strength training effects in prepubescent boys. Med Sci Sports Exerc 22: 605–614, 1990.

19. Lloyd, RS, Oliver, JL, Hughes, MG, and Williams, CA. Reliability and validity of field-based measures of leg stiffness and reactive strength in youths. J Sports Sci 27: 1565–1573, 2009.

32. Thomas, K, French, D, and Hayes, PR. The effect of two plyometric training techniques on muscular power and agility in youth soccer players. J Strength Cond Res 23: 332–335, 2009.

20. Lloyd, RS, Oliver, JL, Hughes, MG, and Williams, CA. The influence of chronological age on periods of accelerated adaptation of stretchshortening cycle performance in pre- and post-pubescent boys. J Strength Cond Res 25: 1889–1897, 2011.

33. Verkhoshansky, Y. Supertraining (6th ed.). Rome, Italy: Verkhoshansky, 2009.

21. Lloyd, RS, Oliver, JL, Hughes, MG, and Williams, CA. Specificity of test selection for the appropriate assessment of different measures of stretch-shortening cycle function in children. J Sports Med Phys Fitness. In press. 22. Matavulj, D, Kukolj, M, Ugarkovic, D, Tihanyi, J, and Jaric, S. Effects of plyometric training on jumping performance in junior basketball players. J Sports Med Phys Fitness 41: 159–164, 2001.

34. Voigt, M, Chelli, F, and Frigo, C. Changes in the excitability of soleus muscle short latency stretch reflexes during human hopping after 4 weeks of hopping training. Eur J Appl Physiol Occup Physiol 78: 522–532, 1998. 35. Wang, LI, Lin, DC, and Huang, C. Age effect on jumping techniques and lower limb stiffness during vertical jump. Res Sports Med 12: 209–19, 2004. 36. Witzke, KA and Snow, CM. Effects of plyometric jump training on bone mass in adolescent girls. Med Sci Sports Exerc 32: 1051–1057, 2000.

VOLUME 26 | NUMBER 10 | OCTOBER 2012 |

2819
