Participants
Twenty-seven healthy boys who participated in locally organized sports (principally baseball and American football) volunteered to take part in this study. The methods and procedures used in this study were approved by the Institutional Review Board for use of human subjects at the College, and informed written consent from the parents and assent from the children were obtained. Participants were randomly assigned to either a resistance training group (n = 14) or a combined resistance training and plyometric training group (n = 13). Baseline physical characteristics are presented by group in Table 1. Participants were excluded if they had a chronic pediatric disease or had an orthopedic condition that would limit their ability to perform exercise.

Study procedures
All study procedures took place at a school athletic facility. Even though all participants had prior experience performing the fitness tests used in this study, prior to data collection all participants participated in one introductory session during which time proper form and technique on each fitness test were reviewed and practiced. During this session research assistants demonstrated proper testing procedures and participants practiced each test. Any questions participants had were answered during this time. Participants were asked not to perform any vigorous physical activity the day before or the day of any study procedure. The same researchers tested and trained the same participants and the fitness tests were performed in the same order with identical equipment, positioning, and technique. Pre-testing was performed the week before the training period and post-testing was performed the week after the training period.

Fitness testing procedures
Power, acceleration, speed and agility were evaluated using the vertical jump, long jump, seated medicine ball toss, 9.1 m (10 yd) sprint and pro agility shuttle run. These tests are often used to assess performance in athletes (Arthur and Bailey, 1998). Lower back and hamstring flexibility were evaluated by the v-sit flexibility test in a temperature controlled environment. Standardized protocols for fitness testing were followed according to methods previously described (Harman and Pandorf, 2000; Safrit, 1995). The best score of two trials for each test was recorded to the nearest 0.5 cm. or 0.01 sec.

Briefly, the vertical jump was measured using the Vertec Jump Training System (Sports Imports, Hilliard, OH, USA). The Vertec has 49 color-coded, moveable vanes that are spaced 1.27 cm apart. Subjects were instructed to jump as high as possible and touch the highest vane. The vertical jump was calculated by subtracting a subject's standing reach height from his maximal jump height. The standing long jump was measured on a mat which was fixed to the floor. Subjects were permitted to perform a countermovement (i.e., an active prestretch of the hip and knee extensors) prior to jumping vertically or horizontally.

The seated medicine ball toss was performed with a 3.6 kg medicine ball (about the size of a shotput). The participants sat on the floor with their back against a wall and were instructed to toss the ball as far as they could with both hands at an approximate angle of 45° (similar to a chest pass). Prior to each toss the ball was coated with magnesium carbonate (e.g., weightlifting chalk) so that when the ball landed on the floor it left a distinctive mark that allowed for a precise measurement. The distance from the wall to the near edge of the mark on the floor made by the ball was measured. The electronic Speed Trap II Timing System (Brower Timing Systems, Draper, Utah, USA) was used to time the 9.1 m sprint and pro agility shuttle run. The 9.1m sprint test was used to assess acceleration. For the pro agility shuttle run, the subjects started on a centerline facing the researcher. The subjects sprinted 4.55 m to the left, then 9.1 m to the right, and finally 4.55 m back to finish as they crossed the centerline. Scores resulting from improper technique or incorrect body positioning during any fitness test were discarded. Test-retest reliability intraclass Rs for all the dependent variables was R > 0.85. Test-retest reliability was established by testing 15 boys on two separate days. We did not assess maximal strength in this study because the variables of primary interest were upper and lower body power performance.

Training procedures
Both exercise groups trained twice per week on nonconsecutive days (Tuesday and Thursday) for six weeks under carefully monitored and controlled conditions. Prior to each training session, all subjects participated in a 10 minute warm-up period which included jogging at a self-selected comfortable pace followed by calisthenics. After the warm-up session, subjects in the resistance training group performed static stretching exercises (~25 min.). Although the potential benefits of an acute bout of static stretching have recently been questioned (Faigenbaum et al., 2005; Zakas et al., 2006), no studies suggest that regular stretching diminishes performance (Shrier, 2004). Subjects in the combined resistance training and plyometric training group performed plyometric exercises (~25 min). Following completion of the static stretching or plyometric training protocols, all subjects participated in the same resistance training program (~50 min). Each training session ended with ~5 min. of cool-down activities. The daily training duration for both study groups was purposely designed to be 90 minutes.

Throughout the study period, subjects exercised in small groups and an instructor to subject ratio of at least 1:4 was maintained. Experienced physical education teachers and certified strength and conditioning coaches discussed and demonstrated proper exercise technique throughout the study period. Teachers and coaches consistently encouraged the subjects to maintain proper technique performance. If a subject fatigued and could not perform an exercise correctly, the exercise was stopped.

Static stretching: Subjects in the resistance training only group performed seven static stretching exercises in a slow, deliberate manner with proper body alignment during the six week training period. Subjects held each stretch for 30 seconds at a point of mild discomfort, relaxed for 5 seconds, then repeated the same stretch for another 30 seconds before progressing to the opposite leg (when necessary). The specific stretches (in order of performance) were hip/low back stretch, chest/hamstring stretch, quadriceps stretch, calf stretch, triceps/hip stretch, adductor stretch, and v-sit hamstring stretch. The stretching protocol used in this study was consistent with general flexibility recommendations for school-aged children (American Alliance for Health, Physical Education, Recreation and Dance, 1999).

Plyometric training: The progressive plyometric training program used in this study was based on findings from previous investigations as well as observations from conditioning coaches and sports medicine professionals (Chu et al., 2006; Hewett et al., 1999; Myer et al., 2005). The components of this program included preparatory movement training and plyometric training. Prior to the performance of the plyometric exercises, subjects performed one or two sets of six to ten repetitions on two or three preparatory exercises (e.g., push-up, body weight squat) which prepared them for the demands of more advanced training. The inclusion of these exercises was especially important for subjects in this study who had no experience participating in a progressive plyometric program. The purpose was for these movements to become 'automatic' so the skill learned could be 'tapped' later on when subjects performed more advanced plyometric exercises.

The plyometric training program progressed from level one (weeks one and two; 1-2 sets of 10 repetitions) to level two (weeks three and four; 1-2 sets of 8 repetitions) and finally level three (weeks five and six; 1-2 sets of 6 repetitions). During weeks one, three and five, subjects performed only one set of each exercise because the plyometric training program stressed proper technique performance. During weeks two, four and six, subjects performed two sets of each exercise. Subjects performed 11 plyometric exercises during weeks one and two and 12 plyometric exercises during weeks three through six. A summary of the plyometric exercise program is outlined in Table 2.

Subjects were encouraged to perform all plyometric exercises in an explosive manner. Level one included low intensity exercises (e.g., double leg hop) in order to safely introduce subjects to plyometric training. In addition, level one exercises provided the subjects with an opportunity to gain confidence in their abilities to perform basic plyometric movements before progressing to more advanced drills at levels two (e.g., double leg hurdle hop) and level three (e.g., single leg hurdle hop). Each exercise session included upper body plyometrics, lower body plyometrics and plyometric speed and agility drills which were specifically designed to enhance a subject's ability to accelerate, decelerate, change direction, and then accelerate again. Subjects performed each plyometric speed and agility drill once during weeks one, three and five and twice during weeks two, four and six. Subjects were provided with adequate time for recovery between exercises and sets. One abdominal exercise (e.g., medicine ball pullover sit-up) was incorporated into the plyometric training program to allow for additional recovery between upper and lower body plyometric exercises. A lightweight medicine ball (1-2 kg) was used for upper body medicine ball training.

Resistance training: Following static stretching or plyometric training, all participants participated in the same progressive resistance training program. The first 10 minutes of each resistance training session included a weightlifting progression (e.g., modified cleans and snatches) with a light load (wooden dowel or unloaded aluminum bar [~7 kg]). Subjects performed one to three sets of four repetitions on each lift. Following the weightlifting progression, subjects performed additional resistance training exercises. On Tuesdays all subjects performed three sets of 10 to 12 repetitions on the following exercises: squat, bench press, overhead press, lat pull- down, standing calf raise, and biceps curl. On Thursdays subjects performed three sets of 10 to 12 repetitions on the following exercises: front squat, incline press, lat pulldown, upright row, standing calf raise, and tricep extension. The last repetition of the third resistance training set on each exercise represented momentary muscular fatigue whereby participants were unable to perform additional repetitions. Following every resistance training session, subjects in both groups performed two sets of 12 to 25 repetitions of abdominal (e.g., abdominal curl), lower back (e.g., kneeling trunk extension) and rotator cuff (e.g., external rotation) strengthening exercises. Subjects were taught how to record their data on workout logs and did so throughout the training period. The instructors reviewed the workout logs daily and made appropriate adjustments in training weight and repetitions throughout the study period.

Data analysis
Descriptive data were calculated for all variables. Group differences at baseline were evaluated using independent sample t-tests. Separate two-way (group x time) repeated measures ANOVA were performed to assess group differences for the variables of interest including vertical jump, long jump, seated medicine ball toss, 9.1 m sprint, pro agility shuttle run and flexibility. When significant main effects and interactions were observed, post-hoc paired t-tests corrected for alpha inflation (Bonferroni correction) were utilized for identifying the specific differences. All analyses were carried out using SPSS version 11.0 (SPSS, Inc. Chicago, IL) and statistical significance was set at p < 0.05.

All participants attended all training sessions (100% compliance) and there were no injuries resulting from either training program. The PRT and RT groups did not differ significantly at baseline in any physical characteristics (Table 1). Likewise, there were no significant differences between groups at baseline with respect to the fitness performance measures. Significant main effects for time were observed on the vertical jump, long jump, pro agility shuttle run, medicine ball toss and flexibility, F(_1,25) = 20.2, 17.9, 6.9, 80.8, and 19.9, respectively, p < .05. Post-hoc analysis revealed that PRT made significant improvements on the vertical jump, long jump, pro agility shuttle run, medicine ball toss and flexibility whereas RT made significant improvements on the medicine ball toss and flexibility only. Significant group by time interactions were noted for the long jump, pro agility shuttle run, and medicine ball toss, F(_1,25) = 7.9, 4.8, and 11.5, respectively, p < .05, with the PRT group making significantly greater improvements in performance than RT. There were no significant interaction effects between groups for the vertical jump, 9.1 m sprint and flexibility, F(_1,25) = 3. 6, 0.1, and 1.6, respectively, p > .05. Baseline and post-training fitness performance data are presented in Table 3.