Comparison of the Effects of Compound Training, Plyometric Exercises, and Kettlebell Exercises on Strength, Power, Dynamic Balance, and Pitched Ball Velocity in 30 Male High School Baseball Pitchers Aged 16–19 Years

Introduction

Athletic training aims to maximize the physical and mental performance required for competition. Individualized training methods have been proposed to suit the specific needs of each sport [1,2]. Baseball involves pitching, hitting, catching, and base running. Regardless of the position played, physical performance is a critical factor in performing these tasks effectively [3]. In addition to the physical components of baseball players’ training, when designing a comprehensive seasonal periodization program, the seasons can be classified as off-season, pre-season, in-season, post-season, or restoration [4]. Off-season training can include moderate- to low-intensity (50–75%, 1 RM) resistance and circuit trainings for muscle adaptation to develop the best and most evenly distributed physical performance for the sport [5] and high-intensity (30–95%, 1 RM) performance training for maximum power in the pre-season phase [6]. High intensity (50–93%, 1 RM) and low training volume should be applied during the season to maintain fitness and perform at peak performance [7]. Although expert opinion on the timing of strength training differs, the consensus is that off-season strength gains should be maintained during the competitive season [8]. The pitching motion of a pitcher is a complex movement involving all body segments [9]. The pitching motion consists of wind up, stride, arm cocking, arm acceleration, arm deceleration, and follow-through [10,11]. Ball speed (BS) is important for pitcher success [10].

Anaerobic power is correlated with pitchers’ BS, and anerobic training can improve pitchers’ BS [12]. Not only in baseball, but in all sports, strength, power, and speed are considered essential components of athletic performance [13], which is why researchers and practitioners suggest a variety of training approaches to improve specific neuromuscular abilities [14]. In this regard, plyometric training and strength training are among the most frequently used strategies to improve sports performance [15,16]. Systematic strength training is primarily used to achieve significant increases in maximal strength and hypertrophy [17], while plyometric training (PT) is better suited to improving stretch-shortening cycle (SSC) function and strength capacity [18]. The anerobic performance of baseball pitchers is directly related to their overall performance [19]; plyometric and kettlebell exercises are typical training methods used to improve anerobic function [20]. Among them, plyometrics has been used for decades in the training of track and field athletes in Russia and Eastern Europe. Verkhoshanski, a famous Russian track and field coach, initiated the concept of what he called impact training or jump training. The word plyometric is actually derived from the Greek words plythein or plyo, which means increase, and metric, which means measurement. Both lower and upper extremities use plyometric concepts as part of functional movement patterns and techniques when performing sports, and the explosive and ballistic nature of baseball makes plyometrics a strong consideration [21]. PT’s SSC constitute a training method for jump-like movements based on neurophysiological mechanisms [22], which combines speed and strength to produce maximum force within a brief timeframe, to improve the sensitivity and responsiveness of the nervous system [23]. Controversies exist on the appropriate training duration and jump height when constructing a PT program for each muscle group, which involves rapid centrifugal muscle action followed by centripetal muscle action, increases force development, while high-intensity resistance training increases strength and acceleration [24,25]. Elastic exercise training may be a better way to increase BS in baseball players than resistance training [21]. A correlation was observed between plyometrics and pitching restraint [26]. Resistance training is a collective term referring to a method of conditioning the body that progressively uses a wide range of resistance loads, varying exercise speeds and using different training methods [27]. Resistance training has previously been shown to be effective in improving strength and power [28], but performing resistance training in a mixed-methods approach may improve speed performance. Kettlebell (KB) training has become one of the most popular fitness trends in the United States in recent years. Described as “cannonballs with handles,” KBs can be used in a variety of ways with the goal of improving overall physical conditioning. Anecdotal reports about KBs indicate that they are easy to use, require little space, are highly portable, and are very efficient for people who have little time to exercise. They can also be used as an alternative to traditional weightlifting or powerlifting because the exercises can be performed unilaterally or bilaterally in any plane. KT requires proper technique and intensity to perform. KBs come in a variety of weights, and KT can progress from simple full-body exercises to more complex exercises that include rotations. Kettlebells are smaller and require less physical space than barbells or weight plates, and are more accessible and convenient [29]. KT also provides both aerobic and anaerobic exercise and integrates muscle and joint training to improve strength, power, and throwing ability [30–32]. The concept of 2 biomechanically similar exercises can be described as a compound pair [33], which may be the optimal training strategy for developing motor skills because this form of compound training enhances neuromuscular activity, which in turn enhances strength and power production, and compound training is sport-specific [34]. Combining the 2 exercises is effective [5]. Resistance training and PT are common training methods for improving strength and power. Combining 2 exercise programs is more effective than using a single program [35]. A study on long jumps and ball throwing in adolescents found that a combination of resistance and PT was effective [36]. Previous studies have reported that exercise programs during the off-season increase exercise performance, but research on the effect of pitcher BS on players in-season is insufficient. Therefore, this study aimed to compare the effects of combination training, plyometric training, and kettlebell training on strength, power, dynamic balance, and pitching speed in 30 male season-long high school baseball players aged 16–19 years.

Material and Methods

ETHICAL STATEMENT:

This study was approved by the Sahmyook University Research Ethics Committee (IRB approval number: SYU 2022-06-012-003), and targeted high school baseball pitchers who expressed their intention to participate. Information was provided to coaches and trainers at the schools, and the trainers helped implement the study. All methods were performed in accordance with relevant guidelines and regulations. The study followed CONSORT guidelines for randomized controlled research, and the study was registered with the Clinical Research Information Service (CRIS) (KCT0008922).

PARTICIPANTS:

Thirty in-season high school baseball pitchers were selected as participants, and the experimental groups were divided into CTG (n=10), PTG (n=10), and KTG (n=10). The data were analyzed using the G*Power, 3.1.9.7 program [37]. Based on software power analysis (effect size=0.6, significance level=0.05, power=0.8), 30 participants were included in the final sample. Thirty pitchers from 3 high schools in Seoul with at least 5–6 years of pitching experience were included. Pre-intervention (or baseline) measurements were taken after a familiarization period. The inclusion criteria for this study were male subjects aged 16–19 years, who played competitively at least once a month, had no pain from injury 3 months prior to the first test, had not participated in a similar study, and voluntarily agreed to participate. We excluded those with a history of orthopedic surgery, those who dropped out during the intervention, and those who dropped out due to personal reasons.

A familiarization session was conducted to make the subjects aware of the testing and training procedures before the commencement of the study. A computer-generated random number table was generated to randomly assign subjects to 3 groups: compound training group (CTG), plyometric training group (PTG), and kettlebell training group (KTG). Randomization and assignment of participants was performed by an independent examiner not affiliated with the study. Each group consisted of 10 participants. Independent assessments were performed on the dependent variable at baseline and after completion of the training protocol. Outcome assessors remained blinded to participant assignment.

PROCEDURES:

The exercise protocol was implemented over a 4-week period from July to August. The evaluation consisted of strength, power, dynamic balance, and pitching ball velocity, and was conducted before the intervention. The 30 participants in the study were divided into 3 groups for each exercise. All 3 groups performed the exercises during the season, and all 3 groups followed a warm-up routine of 1 minute of jogging and stretching, followed by 10 minutes of stretching after training, for 4 weeks. All 30 participants in all 3 groups underwent a post-intervention assessment to measure strength, quickness, and dynamic balance.

STRENGTH TEST: Isokinetic testing helps determine explosive force [38], and isokinetic equipment has a validity of 0.94 and a reliability of 0.93 [39]. Maximal torque of the knee extensors and flexors at 60° was measured using isokinetic and isometric dynamometers (Humac NORM, CSMIC, Stoughton, MA). To ensure that the knee flexion-extension repetitions were not subjected to forces from other parts of the body other than the lower extremities, the subject was seated in a measuring chair and the trunk and femur were immobilized by a belt connected to the chair. The shin pad was adjusted in length so that it was 1 cm above the lateral malleolus of the ankle, which is the point of force, and secured with a strap, the axis of motion was aligned with the axis of the dynamometer, and the subject was audibly encouraged to exert maximum strength during the test. After matching the knee epicondyle movement axis and the machine rotation axis at 90 degrees of the knee in a sitting position on an isokinetic machine, the range of motion was fixed at 0–135 degrees, and was performed 4 times at 60/s [40]. Maximum strength is the highest value of the maximum strength of both extension and flexion, expressed in Nm.

POWER TEST: Power was assessed using the vertical jump test. The reliability of this test was 0.97 [41]. The starting position is to stand 20 cm away from the wall in the sagittal direction with both feet together, then jump up as high as possible and make a dot with a fingertip on the wall. We measured and recorded the distance between the point taken while standing and the point taken by jumping. Two attempts were made and the best result was recorded [42].

DYNAMIC BALANCE TEST: A Y-balance test (YBT) kit was used to measure the dynamic balance. The inter-examiner reliability of the test was 0.67–0.96 [43]. The dynamic balance test uses a specialized kit for assessing dynamic balance ability. Subjects stand on 1 leg with the big toe touching the center of the starting point and extend the opposite leg as far as possible in the frontal, posterolateral, and posteromedial directions. Subjects perform a total of 3 repetitions to measure the maximum distance reached. The sum of the distances reached in the 3 directions (anterior, posterior medial, and posterior lateral) was divided by the leg length multiplied by 3 and then multiplied by 100. Leg length was measured by the examiner as the distance from the ASIS to the center of the medial radius bone with the subject lying on the examination table [44].

PITCHING BALL VELOCITY TEST: Participants had sufficient warm-up time (20 minutes), including an individual warm-up jog, dynamic stretching, box step-up drills, and catching a pitcher’s fungo ball (20 reps), before throwing a total of 10 fastballs from their usual pitcher’s mound to home plate, a distance of 18.44 meters, and having their pitching velocity measured from behind the pitcher [45]. A speed gun (STALKER Pro-2, USA) was used to measure pitch velocity. While there is a lack of confidence in the equipment, the performance specifications for the equipment indicate a speed range of 1–890 MPH, a target acquisition time of 0.01 seconds, and an accuracy of ±0.3%.

INTERVENTION:

The PT program was used [46,47]. The program included 10 min of jogging and stretching as a warm-up and 10 min of stretching following PT. The participants performed PT twice weekly for 4 weeks. Shows the training methodologies (Figure 1). The training difficulty was set to a level that would not be overwhelming during the competition (Table 1).

A KT program was used [48,49]. The 1RM was calculated by measuring the 10 RM of the exercise group according to the guidelines of the National Strength and Conditioning Association (NSCA), and exercises were performed with a kettlebell weight corresponding to 45–75% of the 1RM. The participants jogged and stretched for 10 min to warm up and stretched for 10 min after KT. The participants performed KT twice weekly for 4 weeks. Figure 2 shows the training methodology. The training intensity was set to a level that did not lead to overexertion during the competition (Table 1).

STATISTICAL ANALYSIS:

Statistical analyzes were performed using IBM SPSS Statistics version 22.0. Descriptive statistics were used to assess general characteristics. Paired t-tests were used to compare before and after group CTG, PTG, and KTG dependent variables, and one-way analysis of variance was used to analyze CTG, PTG, and KTG comparisons. Tukey (HSD) test was used for post hoc testing. All statistical significance levels were set at p<0.05.

Results

CHANGE IN STRENGTH IN RESPONSE TO INTERVENTION:

Maximal torque of the knee extensors and flexors was measured using isokinetic and isometric dynamometers (Humac NORM, CSMIC, Stoughton, MA). Maximum strength is the highest value of the maximum strength of both extension and flexion, expressed in Nm. The results measured during knee extension and flexion were as follows: strength in the left knee joint extension and flexion for left and right foot angular velocities of 60°/s significantly increased within the groups from pre- to post-exercise (p=.000). Tukey’s post hoc test for changes in the strength of extension and flexion between groups showed that the strength of extension for a right foot angular velocity of 60°/s increased more effectively in the CTG than in the PTG and KTG, except that the strength of extension increased more in the PTG than in the KTG. The strength of flexion increased more effectively in the CTG than in the PTG and KTG (Table 2).

CHANGE IN POWER IN RESPONSE TO INTERVENTION:

All assessments increased significantly in all groups (p=.000). Tukey’s post hoc test for change in power between groups showed that CTG increased power more than PTG and KTG (Table 3).

CHANGE IN DYNAMIC BALANCE IN RESPONSE TO INTERVENTION:

The results of measuring Y-balance are as follows. Within the group, left and right Y-balance increased across all score outcomes post-intervention across all assessments (p=.000). Tukey’s post hoc tests for changes in left and right dynamic balance between groups showed that CTG increased left dynamic balance more than PTG and KTG (Table 4).

CHANGE IN PITCHING BALL VELOCITY IN RESPONSE TO INTERVENTION:

To measure the pitch speed behind the pitcher, a speed gun (STALKER Pro-2, USA) with a speed range of 1–890 MPH was used, and the measured ball speed was found to increase in all groups (p=.000). Tukey’s post hoc test for changes in ball speed between groups showed that the CTG was more effective than the PTG and KTG (Table 5).

Discussion

After a 4-week complex exercise program, strength, VJ, Y-balance, and throwing speed increased significantly in all 3 training groups. These results demonstrate that a short-term resistance training program results in increased physical prowess and pitching velocity in high school baseball players. Because in-season high school baseball players have long travel times and often play games in quick succession, baseball-specific short-term complex exercise programs may be an attractive alternative to longer-term programs (eg, 8-8-8 hours). A study by Carter et al [50] examined the effects of an 8-week upper-extremity PT training program on strength and pitching velocity in college baseball players and found that the PT-trained group had significantly faster pitching velocities after 8 weeks. Specifically, there was an increase of 2.4% in pitching speed (38.1 m·s, 85.2 mi·h) compared to the average pitching speed before training (37.2 m·s, 83.2 mi·h). This agrees with the study by Grezios et al [51], who reported that PT training benefited baseball throwing based on the type of muscle contraction that predominates when throwing overhead. Investigating the effect of resistance training on pitching velocity in high school baseball players, Escamilla et al [52] reported that adolescent (11- to 15-year-old) baseball players significantly increased their pitching velocity (4.0% increase from 25.1 m·4 to 26.1 m·4) after performing resistance training and interval throwing for 4 weeks. However, because of the interval throws, it is difficult to know how much it has increased. However, the literature examining the effect on pitching velocity in high school baseball players supports the evidence that resistance training increases pitching velocity [50].

Before and after the intervention, the strength of the left and right knee flexors and extensors significantly increased in all groups, with the combined exercise group showing statistically significantly better results (p<0.05) than the PTG and KTG. Plyometrics, which consists of rapid centrifugal muscle action followed by centripetal muscle action, improves the rate of force development. High-intensity resistance training, which increases strength and acceleration, is effective when combined with other methods [5]. Combining PT and KT for in-season pitchers improves lower-extremity strength, VJ, and Y-balance and affects pitching velocity through physical function development [53]. Additionally, the change in the strength of the right knee flexors was significantly greater in the PTG than in the KTG (p<0.05). A possible reason for this significant difference may be that most participants were right-handed, which may lead to greater strength in the dominant arm [54]. Increased knee hip flexion during the stride process in right-handed participants in the pitching motion, knee joint extension, and hip extension plays a translational role in moving the center of gravity forward [55]. Greater movement during this process may allow for a greater ability for this muscle to generate force, contributing to the strength of the right knee extension muscle. Consistent with our results, a previous study on pitchers’ performance reported higher muscle activity of the vastus lateralis muscle in the second of 6 stride segments in baseball pitchers and that the degree of trunk extension when throwing a ball at a stride length of 18.44 meters could make it feel as if the ball is being thrown closer to the batter, resulting in a 140 km/h ball appearing to the batter as a 145 km/h fastball [56]. PT is an effective training method for improving knee joint flexor and extensor strength outcomes. Controversies exist on the appropriate training duration and jump height when designing a PT program for each muscle group [57]. The optimal training duration for improving power is 4–6 weeks for high-intensity training, which can be performed without causing the central nervous system to experience undue strain or fatigue. This study’s results are consistent with those of research on the effects of biweekly KT on knee joint strength [58]. These strength gains are believed to improve the pitcher’s stride length. These strength results showed statistically significant differences between the left and right knee joint flexor and extensor groups at 60°/s and between the PTG and KTG. These results suggest that compound movements positively affect the maximum torque values of the knee joint muscles in high school baseball pitchers.

Significant increases were seen in all groups when looking at the pre- and post-power changes. When comparing between groups, the combined exercise group showed statistically significant results than those of the PTG and KTG (p<0.05). These results may be because PT activate the sensitivity of the nervous system using the elastic components of muscles and tendons and the stretch reflex through the SSC and improve the responsiveness of the muscular and nervous system to increase power [59].Vertical jumping of in-season handball players who performed PT was significantly better than that of the control group [2]. KT, a resistance exercise, can improve whole-body fitness, providing both aerobic and anaerobic exercise, and integrating muscle and joint training to improve strength, power, and throwing power [30–32]. KT can engage the entire body simultaneously by integrating all connective tissues of muscles and joints, which form the kinetic chains of the human body [31,49] and effectively increases strength and power [60]. The results of this study are consistent with those of previous research on the effects of a combined strength and jumping exercise program on lower-body power [61].

The changes in left and right Y-balance before and after the intervention in this study were significantly increased in all groups. When comparing groups, the combined exercise group showed significantly better results (p<0.05) than the groups that practiced PT and KT. This may be because plyometrics consists of jumping and landing movements, which increases proprioception of the ankle joint through weight-bearing exercises, and improvements in Y-balance reduce the risk of lower-extremity injury by activating appropriate muscles before functional adaptation, landing, and proprioceptive input [62]. Consistent with our findings, a previous study reported improvements in proprioception following plyometrics [63], whereas another study reported significant differences in balance ability following lower-extremity plyometrics of proprioception and postural stability in collegiate soccer players with postural instability [64]. The reason for the improvement is that plyometric training enhances reflex enhancement through enhanced neural recruitment or nerve firing frequency of motor units and/or alters the elastic properties of muscles and connective tissues, which in turn increases neuromuscular adaptability [65]. These plyometric exercises play an important role in improving lower-body stability [66]. Unlike weight training, KT require synchronized and interactive upper- and lower-extremities movements. The combination of core activation, balance, neuromuscular control, and coordination may have contributed to the YBT scores, which is a positive outcome, as it reduces the risk of injury due to improved balance. Our results are consistent with previous studies that have reported positive effects on balance following the application of KT [67]. Based on these results, the combined exercise program positively affected the Y-balance of high school baseball pitchers than the stand-alone program.

BS from pre- to post-intervention in this study was significantly increased in all groups. The combined exercise group showed statistically significant results (p<0.05) of the PTG and KTG. The increase in BS with increased PT is consistent with previous studies showing a correlation between plyometrics and BS [26]. A significantly greater effect was observed on ball-throwing speed in seasonal handball players who performed PTG compared to the control group [2]. KT has similar effects as traditional training, owing to its safety, versatility, and ease of location [30], and many high school baseball players could likely utilize KT as a binding-rise training or strength maintenance program to fit into their in-season training. A previous study on the effects of combined training reported that a combination of resistance and PT was more effective for long jump and ball throwing in adolescent males [36]. This is likely because plyometrics, which consists of rapid centrifugal muscle action followed by centripetal muscle action, enhances the rate of force development, while high-intensity resistance training increases strength and acceleration, resulting in a positive multimodal effect when the 2 exercises are combined [5]. Thus, plyometrics and resistance training are commonly described in the literature as the best methods for improving athletic performance. Their combined effects are reported to be greater than those of any program performed alone [5,35]. Recent studies have suggested that male adolescents may benefit from a combination of PT and resistance training methods to induce specific acute adaptations in lower-extremity vertical jump and acceleration abilities [68].

This study confirmed the effects of combined PT and KT on strength, VJ, Y-balance, and BS in seasonal high school baseball pitchers. Plyometrics enhances momentary explosive movements and produces better gains than resistance training. Recent studies have reported that the appropriate training period for PTs is recommended to be 8–12 weeks [69]. However, it does not provide specific insight or recommendations on these issues. These results require further study of optimal training times and jump heights for different muscle groups. The results of this study show that baseball has a positive effect on athletic ability in performing instantaneous movements. Additionally, compound exercises can dramatically increase dynamic movement, improve anaerobic power and other athletic abilities, and quickly provide an efficient workout that will help prepare for competition.

Limitations of this study include the short duration of the study (4 weeks) and the intervention period, which makes it difficult to determine the effectiveness of the exercise program over an entire season (6 months or more). In addition, there was a fixed pitching roster at one school, and this particularity resulted in a small sample size; therefore, future studies should take these limitations into consideration and increase the sample size through including more schools, and longer-term studies over an entire baseball season to evaluate the long-term effects of an integrated exercise program on physical performance and performance.

Conclusions

The purpose of this study was to determine the effects of the combination PT and KT on strength, power, dynamic balance, and pitching ball velocity of high school baseball pitchers during the season. The intervention lasted 4 weeks and patients were divided into 3 groups: CTG, PTG, and KTG. The results of this study compared the effects of CT, PT, and KT on strength, VJ, Y-balance, and pitching speed in 30 high school baseball players aged 16–19 years old during the season, and showed a high increase in CTG. Although long-term research is needed throughout the entire baseball season, our results demonstrate an efficient method and short-time training program to maintain or increase the effects of training during the off-season to demonstrate the best performance, and it can be used for high school baseball players during the season. Our study shows that physical ability and pitching speed can be increased by performing compound exercises.