HSOA Journal of Sports Medicine and Injury Care Journal Effect of a 12-Week Off-Season Strength Training Program on Athletic Performance in Adolescent Baseball Players

competence, build musculoskeletal capacity and resilience, and to promote muscle memory for future seasons. Off-season strength and conditioning programs have popular in baseball, as a means to improve athletic performance as well as reduce injury risk. Significant variability exists in baseball-specific training, ranging from resistance training (weightlifting, resistance band and medicine ball workouts, plyometrics) to interval throw ing programs to weighted ball throwing Previous studies have demonstrated that lower body power, upper body power, and torso rotational power are correlated to bat swing velocity in adolescent, high school (HS), and college baseball players. Similarly, throwing velocity increases have been demonstrated in HS baseball players that have completed baseball-specific 4-week training programs involving Abstract Purpose: Off-season strength training programs have become common place in baseball, as a means to improve athletic perfor mance and reduce injury risk. No study has assessed analytics and advanced metrics during the season following an off-season, base -ball-specific strength training program. We hypothesized that players completing a 12-week baseball-specific strength and conditioning program would experience significant gains in running speed and bat exit velocity, as well as in-season hitting performance metrics. Methods: 25 healthy high school baseball players were recruited into an intervention group. 60-yard dash and batting exit velocity were measured pre-program. The intervention group then performed an off-season 12-week strength training program, with measurements repeated post-program. Hitting performance metrics were assessed in the subsequent baseball season, utilizing a comparison group of teammates who did not perform the program. Changes from base line were evaluated using paired t-tests within, and the training effect was assessed using repeated-measures analysis of variance to assess differences in changes between the intervention and the control groups. A two-tailed significance level was set at P < 0.05. Results: Running speed showed a statistically significant improve - ment (0.278 seconds average reduction; 95% CI 0.161, 0.394; P < 0.001) while bat exit velocity did not (0.476 mph average gain; 95% CI -1.267, 2.219; P = 0.575). There was no statistically significant change in hitting performance among the intervention group in the season following the program when compared to the season immediately preceding the program. No statistically significant changes in hitting performance statistics were observed when comparing inter vention players versus the comparison group. No participants were injured at any time during the study. Conclusion: Performing a 12-week strength training program in - creased running speed but not bat exit velocity or post-program hitting performance statistics. Baseball-specific strength training programs are safe and effective interventions that can improve ath - leticism in adolescents. adher ing to a normal (Gaussian) distribution using the Shapiro-Wilk test and histogram visualization within each group and presented using means and standard deviations. Paired t-tests were performed to eval-uate changes from baseline in bat exit velocity, 60-yard dash time and in-season performance metrics. The training effect was assessed using repeated-measures analysis of variance to assess differences in seasonal changes (deltas) between the intervention and the control groups. Changes in continuous outcomes were presented with 95% confidence intervals and P values. A two-tailed significance level was set a priori at P < 0.05.


Introduction
Strength, power, speed, and agility are key elements of a baseball player's athletic performance. Like most youth sports, baseball has become a three-season sport, with players often playing 50-60 game summer schedules in addition to their high school seasons as more youth athletes trend towards early specialization [1]. With this early specialization, however, comes an increased risk of overuse injury. Pasulka et al., examined 1,190 youth athletes age 7-18 and found that 26% were single-sport specialized; defined as participating in a single sport for greater than 8 months of the year [1]. Additionally, single-sport athletes were more prone to overuse injury than other athletes, 44.3% versus 32.2% [1]. Equally concerning as the trend in youth sport specialization is the temporal trend of youth muscular fitness (i.e., muscular strength, power and endurance) which indicates that today's youth are weaker and slower than previous generations [2]. Young athletes who do not address neuromuscular deficits and build strength reserves may be more likely to suffer a sports-related injury and less likely to attain elite-level performance [2][3][4].
One key to combating injury in youth sports could be proper off-season conditioning, both to allow for recovery, promote movement variability and competence, build musculoskeletal capacity and resilience, and to promote muscle memory for future seasons. Off-season strength and conditioning programs have become popular in baseball, as a means to improve athletic performance as well as reduce injury risk. Significant variability exists in baseball-specific training, ranging from resistance training (weightlifting, resistance band and medicine ball workouts, plyometrics) to interval throwing programs to weighted ball throwing [5][6][7]. Previous studies have demonstrated that lower body power, upper body power, and torso rotational power are correlated to bat swing velocity in adolescent, high school (HS), and college baseball players. Similarly, throwing velocity increases have been demonstrated in HS baseball players that have completed baseball-specific 4-week training programs involving resistance training [8]. More recently, studies involving weighted ball throwing programs have demonstrated increased pitch velocity following a 6-week program in adolescent pitchers, but at the cost of an increased injury rate (24% injury rate in experimental group vs. 0% in control group) [7]. No current studies have assessed analytic improvements in performance following an off-season, baseball-specific strengthening and conditioning program.
The purpose of our study was to prospectively investigate the effects of 12-week strength and conditioning program on certain sport-specific performance metrics on HS baseball athletes. We hypothesized that athletes participating in strength and conditioning workouts will experience significant improvements in showcase metrics (e.g., 60-yard dash times, batting exit velocity) as well as hitting performance measures in their post-program seasons.

Subjects
HS baseball players between the ages of 14 and 18 years were recruited for the study during the off-season. Players was excluded if they had a current injury which precluded participation in the program. A total of 25 male baseball players from HS teams within a 50mile radius of Providence, RI met these criteria and agreed to participate. Each participant completed intake forms consisting of medical history, prior experience with strength and conditioning and baseball background. Participants under 18 years of age provided written assent and their legal guardians provided informed consent. The subjects' mean (standard deviation) age, weight, and height were 15.4 (1.0) years, 73.2 (10.3) kg, and 174.4 (4.2) cm, respectively. Years of baseball experience were 9.7 (1.5).
A comparison group of 15HS baseball players between the ages of 14 and 18 years from one of the local high schools and one of the regional travel teams was created to compare in-season performance metrics. Members of the comparison group were teammates of the participants but did not participate in the program and did not complete pre-/post-program performance measures (e.g., 60-yard dash, batting exit velocity). Participants under 18 years of age provided written assent and their legal guardians provided informed consent. Anthropometric data (weight in kg, height in cm, BMI) were collected prior to program participation utilizing a physician scale (Henry Schein Model 1127157, Melville, NY).
A 12-week strength and conditioning program was developed collaboratively by fellowship-trained sports medicine specialists and certified physical therapists utilizing established strength and conditioning references (Appendix 1) [9][10][11]. The 12-week duration was determined based on previously published literature regarding off-season strength and conditioning programs in this age group [5,12]. Athletes were to complete workouts twice a week under the direct supervision of certified physical therapists and sports medicine physicians, and once a week on their own. All athletes were given a "workout log" to track their workouts and promote compliance; this notebook included video links to the various exercises so participants could review proper form and technique (Appendix 2).
A showcase-type setting on a HS varsity baseball field (grass surface) was utilized to collect pre-program performance data on September 16, 2018. Post-program data was collected on December 17, 2018 at a university field house which included an indoor batting cage and synthetic surface indoor track. Participants performed a dynamic warm-up prior to data collection. Participants designated their primary and secondary positions, and were grouped according to the following positions: infield, outfield, pitcher, catcher. Average length of time from completion of the program to the beginning of the HS baseball season for most participants was 12 weeks.

Bat Exit Velocity
Participants designated their individual preference of bat material [e.g., wood, metal (alloy, composite)] which was recorded. Participants used the same bat for pre-and post-program data collection. All alloy and composite bats were Bat-Ball Coefficient of Restitution (BBCOR) certified. After performing warm-up swings on a tee, participants faced live pitching in a batting cage from an adult baseball coach who threw fastballs behind an L-screen. Bat exit velocities from 10 swings were recorded using a handheld radar gun (Pocket Radar Model PR1000, Santa Rosa, CA) situated 5 feet behind the hitter outside the cage. The best effort (mph) was utilized.

60-yard dash
Participants performed pre-program 60-yard dashes on the grass in foul territory along the first base side of the field. Distances were measured using an open reel measuring tape. Cones were used to demarcate the start and finish lines. Participants ran the distance one at a time, and times were recorded using a stopwatch, with recordings to the 100th of a second. Hand signals were used to start the heat, and a researcher recorded the participants' times at the finish line. Post-program 60-yard dashes were performed on a synthetic indoor track in a university field house. An identical protocol was used to measure/ demarcate distances and times.

In-Season performance metrics
All participants consented to allow researchers to access their 2018 and 2019 GameChanger statistics from their HS(varsity, JV) and summer (travel, American Legion) seasons, which were their baseball seasons immediately before and after the strength and conditioning program. GameChanger is a free mobile app and website (https://gc.com) that provides scorekeeping, advanced statistics, and live game updates for baseball teams and their fans. For the HSand summer seasons, the participant's primary team's statistical data was used (e.g., varsity OR JV for HS, travel OR American Legion for summer). The comparison group's in-season performance metrics were manually compiled from publicly available box scores available in GameChanger. Intervention group and comparison group players with at least 20 plate appearances in both 2018 and 2019 with retrievable data in GameChanger were included in the analysis.

Statistical analysis
All data were collected and analyzed using SPSS (version 25, IBM Corporation, Armonk, NY) and Stata (version 15.0, StataCorp LLC, College Station, TX).All continuous data were confirmed as adhering to a normal (Gaussian) distribution using the Shapiro-Wilk test and histogram visualization within each group and presented using means and standard deviations. Paired t-tests were performed to evaluate changes from baseline in bat exit velocity, 60-yard dash time and in-season performance metrics. The training effect was assessed using repeated-measures analysis of variance to assess differences in seasonal changes (deltas) between the intervention and the control groups. Changes in continuous outcomes were presented with 95% confidence intervals and P values. A two-tailed significance level was set a priori at P < 0.05. Evaluation of change in hitting performance from 2018 and 2019 seasons among players in the intervention group is shown in table 2. Of the 25 participants, 18 players had at least 20 plate appearances in each season with retrievable data in Game Changer, which were criteria for inclusion in the analysis. Batting average (BA), on base percentage (OBP), slugging percentage (SLG), and on base plus slugging percentage (OPS) performance statistics were calculated using standard formulas (www.baseball-reference.com). No statistically significant change in hitting metrics was observed for the intervention group in the season following the strength training program compared to the season prior to the program.
Hitting performance statistics from 2018 and 2019 seasons were also analyzed among players in the comparison group (Table 3). Of the 15 players in the comparison group, 11 players had at least 20 plate appearances in each season with retrievable data in Game Changer.
No statistically significant changes in hitting metrics were observed for the comparison group in the season following the strength training program compared to the season prior to the program.
Hitting performance metrics were evaluated between the intervention group and the comparison group (Table 4). No statistically significant changes in BA, OBP, SLG, or OPS were observed between these groups in the season prior to and following the strength training program. Figure 1 summarizes the hitting performance metrics of the intervention and comparison groups in seasons 2018 and 2019.

Discussion
A 12-week off-season strength training program for HS baseball players safely and effectively enhanced physical performance with regards to 60-yard dash times. Also, while a statistically significant increase in bat exit velocity was not found, our study did show an increase in bat exit velocity of nearly half a mile per hour. This slight increase could be the difference between a hard-hit single up the middle and a diving defensive play by a middle infielder resulting in a routine out at first. No injuries were documented during the training process, and no injuries which could be attributed to the training program occurred in the subsequent baseball season. This is important in validating the safety and effectiveness of such a program in order to best develop youth athletes, both with regards to physical performance as well as injury prevention. As single-sport specialization becomes more common among youth athletes, strength training programs that emphasize sport-specific training while allowing adequate recovery from the prior season will be essential. Moreover, our results support the previous claims made by Faigenbaum   Data are presented using mean ± standard deviation for each performance measure.
The evaluation of the change in performance measures from pre-to post-program was done using paired t-tests. Players with at least 20 plate appearances in both 2018 and 2019 were included in the analysis.
Data are presented using mean ± standard deviation for each year. The evaluation of the change from 2018 to 2019 was done using paired t-tests. Players with at least 20 plate appearances in both 2018 and 2019 were included in the analysis.
Data are presented using mean ± standard deviation for each year. The evaluation of the change from 2018 to 2019 was done using paired t-tests. regular participation in a well-designed youth resistance training program can alter physical activity trajectories, increase muscular fitness, enhance performance and reduce associated injury risks in children and adolescents.
To our knowledge, our study is the first to investigate the effect of an off-season strength training program on performance metrics for athletes using publicly available data, comparing the season immediately prior to the season following the training program, in any organized sport. There are however some studies that examine the short term ability of strength and conditioning programs with regards to maximum velocity [8,13]. Escamilla et al., showed an increase in maximum pitching velocity in 11-15 year old athletes following a 4-week organized conditioning program [8]. The authors noted that this was a short term increase and an association with in-season performance was not investigated. Escamilla et al., performed a similar study in a group of 14-17 year old athletes, which is more consistent with the current study [13]. Three separate 6-week throwing programs, Throwers Ten (TT), Keiser Pneumatic (KP), and Plyometric (PLY), all resulted in a significant increase in throwing velocity compared to the control group [13]. Once again the authors did not examine the effect of the program on injury rates or in-season performance.
With regard to performance, our current study did not demonstrate statistically significant improvement in any of the major offensive statistical categories (BA, OBP, SLG, OPS) for the intervention group versus the comparison group. This could have been due to limitations in sample size in both the study and comparison groups. Additionally, as demonstrated by the aforementioned Escamilla study, 6 physical gains in strength training programs may potentially be short-term and transient in nature. It is possible that athletes whom perform pre-/ off-season strength training programs achieve their peak level of fitness early in the season, as athletic (e.g., games, practices, travel, suboptimal meal schedules), academic (e.g., homework, exams, standardized testing, tutoring), and social (e.g., video games, dating) scheduling during the season interferes with ideal strength and conditioning programming and nutrition/hydration. Consequently, sustaining off-season strength training gains in-season can be very challenging to attain, and inevitably requires an in-season strength training program with modified volume/intensity that can be practical to perform [14,15].
Our study had several limitations. First, participants were not randomized into intervention and control groups. Second, sample size (participant, comparison groups) was limited, and, for this reason, participant and comparison groups were not equal. Additionally, it was our intent to stratify the participant group by the percentage of supervised training sessions they attended (e.g., ≥75-90%, ≥50-74%, ≥25-49%, < 25%) and compare performance metrics among the four groups. However, a small sample size prevented stratified analysis. Fourth, due to winter weather conditions in the Northeast, we were unable to perform pre-and post-program 60-yard dash testing and bat exit velocity testing in the same environments. It is possible that the improved mean 60-yard dash times achieved on an indoor synthetic surface could be partially due to differing extrinsic factors. Brechue et al., [16] evaluated 68 National Collegiate Athletic Association (NCAA) Division II college football players to determine the effects of football equipment and running surface on the 40-yard dash times. Running surfaces used included a rubberized indoor track in the field house, and outdoor natural grass game field at the football stadium. Sprint performance was significantly impaired when running on natural grass compared with running on an indoor rubberized track (linemen: 0.15 ± 0.09-second increase on grass vs. indoor surface; backs: 0.12 ± 0.06-second increase), and the magnitude of the sprint performance impairment was similar between linemen and backs.
While it is likely that the improved sprint performance result in our study was a direct result of surface type, the mean decrease in sprint performance time was 0.278 seconds, the magnitude of change which unlikely can be explained by running surface alone. Even a 0.100 second improvement in 60-yard dash time in baseball is clinically significant, and can mean the difference between an infield single versus an out, or a stolen base versus caught stealing. Also, with regards to performance and injury we did not perform a longitudinal analysis of performance or injury rate in the study group. As we only have one year's worth of data, it may be truly difficult to determine the cumulative effect that the strength and conditioning program had on performance and injury prevention; this could be improved with further follow up. Additionally, because the training program was completed by December, it is possible that any gains imparted by the program may have been lost by the time the spring season began in March. Finally as with any study of adolescent athletes, it was not possible to determine if physical gains were solely a result of their growth and maturation as the transition to skeletally mature athletes.

Conclusion
Performing a 12-week strength training program significantly increased running speed but not bat exit velocity or post-program hitting Players with at least 20 plate appearances in both 2018 and 2019 were included in the analysis.
Data are presented using mean ± standard deviation for each year. The evaluation of the change from 2018 to 2019 was done using paired t-tests.  performance statistics. Baseball-specific strength training programs are safe and effective interventions that can improve athleticism in adolescents, and likely result in clinically significant gains in both sprint performance and bat exit velocity. Future research utilizing prospective, randomized controlled trials assessing the effect of combined off-season and in-season strength training programs on baseball-specific metrics (e.g., showcase metrics and in-season advanced metrics for hitting) and injury can provide essential data targeting injury prevention in a single-sport specialization era.

Conflict of Interest
The authors have no relevant financial disclosures or conflicts to report. No external funding was received for this project.