In baseball, bat velocity (BV) is a key component of hitting performance. A faster BV with the same bat results in greater batted ball velocity, shorter swing time, and a longer decision time (20). Even a slight increase in batted ball speed or distance could make it possible for a ground ball that would be stopped by infielders to pass through them, or for a fly out near the fence to become a home run. One of the goals of a batter's training and pre-batting warm-up is to optimize their at-bat BV. After pre-batting warm-up with a weighted bat, despite a faster perceived bat swing by batters (11), scientific studies have demonstrated that BV actually decreases (4,11,17) or is unaffected (19). Similar results pertain if the pre-batting warm-up is performed with a bat of a lesser moment of inertia (4).
Even though pre-batting warm-up with a weighted bat has been reported not to improve BV, there is a warm-up–related mechanism that has been shown to have the potential to improve explosive performances like batting. This mechanism, termed postactivation potentiation (PAP), takes advantage of a transient enhancement in muscle contractile performance after maximum voluntary contraction (2). Thus, three sets of a 3-second long maximal isometric squat protocol resulted in a greater increase in a countermovement jump than did a maximal dynamic squat protocol (12). Although the performance enhancement effect of PAP on twitch contraction (1,5,18) and voluntary single-joint movements (1,9) have been demonstrated, the performance enhancement effect on voluntary multi-joint movements remains vague because of several interfering factors, such as fatigue from the conditioning contraction and familiarity with the conditioning contraction themselves. However, the effect of longitudinal usage and subsequent familiarity with the warm-up procedure using the isometric contraction conditioning (ISO) has not been tested.
Despite a demonstrated negative or lack of effect of the pre-batting warm-up procedure using a weighted bat, swinging a weighted bat in the on-deck circle is still common among baseball players. To enhance a batter's performance, research is required to clarify the optimal pre-batting warm-up procedure. Thus, the purpose of this study was twofold: (a) to investigate the effectiveness of a maximum voluntary isometric contraction as a pre-batting warm-up method by comparing BV after maximum voluntary isometric contractions and conventional pre-batting warm-up methods (swinging a weighted or normal bat); (b) to examine the training effect of maximum voluntary isometric contraction on BV. In this study, it was hypothesized that a pre-batting warm-up that used a maximum voluntary isometric contraction would have an acute positive effect on BV and that longitudinal intervention with maximum voluntary isometric contraction for a bat swing would augment the acute effect of the isometric contraction pre-batting warm-up procedure.
Experimental Approach to the Problem
This study consisted of two experimental approaches to investigate both the acute effects of three warm-up procedures and the training effects of conditioning activity using maximum voluntary isometric contraction. Changes in BV were used to evaluate the effectiveness of each pre-batting warm-up procedure and training program. The effect of pre-batting warm-up on BV was investigated by evaluating changes in BV after three types of pre-batting warm-up trials (five standard bat swings [SBS], five weighted bat dry swings [WBS], or four sets of 5-second maximal voluntary isometric contractions mimicking the bat swing movement pattern [ISO]). After the completion of experiment 1, experiment 2 used 12 of the 24 subjects for the experimental group who underwent an 8-week training program (3 days per week) of isometric contraction conditioning (ISO training program), after which, the effect of ISO training was evaluated.
Twenty-four male division I intercollegiate baseball field players with a height of 1.83 ± 0.06 m and body mass of 84.0 ± 12.5 kg (mean ± SD) participated. For experiment 2, the height and body mass of subjects in the experimental group was 1.85 ± 0.07 m and 85.3 ± 10.3 kg, respectively. The height and body mass of subjects in the control group was 1.82 ± 0.06 m and 83.8 ± 9.8 kg, respectively. All subjects had a minimum of 2 years of experience in strength training programs. Before the participation in this experiment, each subject read and signed an informed consent approved by the university institutional ethical review board.
An 83.8 cm (33 inches) long aluminum baseball bat weighing of 850.5 g (30 oz), which had been used as standard bat for study in baseball hitting, was used by all subjects during the measurement of BV and implementation of SBS warm-up. An additional weight of 680.4 g (24 oz) (Pow'r Wrap; Grand Enterprises West, MN, USA) was added to the standard bat for the implementation of WBS warm-up. The center of mass of the weighted bat was located approximately 28 cm (11 inches) below the top of the bat. For the ISO warm-up, subjects pulled on a handle with a steel wire rope connected to a wall or fence. Subjects performed the ISO warm-up in the early swing phase position (16). Each hand pulled the handle separately but with different posture as shown in Figure 1. Each effort took five seconds. The first and third sets of ISO were performed by the batter's lead hand (Figure 1A). The second and fourth sets of ISO were done with the batter's trailing hand (Figure 1B). To avoid an excessive load for internal rotation of the trailing hand's shoulder, the position of the trailing hand was set at internally rotated position (Figure 1B).
BV just before ball–bat contact was measured with a vertical computerized photosensing timer (BatMaxx 5500; Technasport, MN, USA; Figure 2). This device measures the amount of time a moving object takes to intercept two laser beams running to two sensors. To measure the horizontal velocity of the barrel of the bat where the ball–bat contact occurs, two sensors that received vertical laser beams from the device on top were set 10 cm (3.9 inches) and 13 cm (5.1 inches) behind the baseball tee in the pitcher-to-catcher orientation. Reliability of the corrected data can be affected by aspects of swing trajectory, such as slice angle, as well as the location of ball–bat impact in the grip-to-top direction of the bat. Therefore, the subjects were asked to perform a level swing and to drive the ball toward center field. Subjects were also instructed to hit the ball with the sweet spot of the bat. One investigator who was a former intercollegiate baseball player made a qualitative judgment as to whether each hit met the requirements for an accurate measurement. To examine the test–retest reliability of the measurement device, one investigator's 6 test dry swings were measured each day for 10 days. Mean test–retest reliability of the BV was r > 0.89. In addition, the mean difference between the investigator's BV obtained from this study's measurement device and the BV of the same swings obtained from an accelerometer attached on the barrel of the bat was less than 1%.
Testing Protocols for Experiment 1
All subjects performed a general loosening-up activity that consisted of jogging, stretching, calisthenics, and practice swings until they felt ready for batting. The height of the ball on the tee was adjusted to the level of the batter's umbilicus during the foot contact phase of the hitting motion (16). The batters were then instructed practice hitting the ball with an incremental increase in intensity until they felt they were ready to hit with a maximum effort. They were told to hit the ball straight toward the net (simulating center field) to optimize the direction of the bat entering the beam of the vertical laser. The last portion of the general loosening-up activity consisted of hitting three balls off the tee at their maximal effort. A 1-minute rest then ensued. Next, to establish the pre-batting warm-up BV, the batters hit a ball off the tee three times in each warm-up trial. The average BV of the three hits was defined as the baseline BV of the warm-up trial. The interval between each hit was 10 seconds. Right after the three hits, the subjects were assigned to one of the three different pre-batting warm-up procedures (SBS, WBS, and ISO) in a counterbalanced design among all subjects. For the SBS and WBS, the interval between each swing was 5 seconds. Subjects were verbally instructed to perform each swing with a maximal effort. For the ISO, the interval between each 5-second maximum voluntary contraction was also 5 seconds. Instruction regarding a proper ISO technique was given before the ISO trials for each subject. After completing one of the three pre-batting warm-ups, the subjects took a 1-minute rest. They then hit three balls off the tee to determine their post–warm-up BV. All subjects completed all three warm-up procedures within 1 week. Each procedure was separated by a minimum of 24 hours.
Training Protocols for Experiment 2
After completion of experiment 1, 12 of the 24 subjects formed the experimental group and underwent an 8-week training program involving isometric contraction conditioning (ISO training program). In addition to their regular baseball practice, the experimental group participated in ISO training session for 3 days a week. Each ISO training session was held during or after the subjects' regular practice and began with four sets of 5-second maximum voluntary isometric contractions followed by three hits off a baseball tee. The process was then repeated. Subjects in the experimental group were allowed to choose to participate in the training session on three of the four potential weekdays (Monday, Tuesday, Wednesday, and Tuesday). The remaining 24 subjects served as a control group and spent the 8 weeks performing only their regular baseball practice, but with an additional six hits off a baseball tee 3 days a week.
Testing Protocols for Experiment 2
At the end of the 8-week training period, to examine the effect of ISO training on their baseline BV and their acute response to the ISO warm-up, all 24 subjects were tested for their pre-ISO warm-up BV. This value served as the baseline BV and the post-ISO warm-up BV. The same procedure as in experiment 1 (except for the implementation of the SBS and WBS warm-up procedures) was used for the measurement of subjects' posttraining BV.
The PASW Statistics program (version 18.0; SPSS Japan Inc, Tokyo, Japan) was used for statistical analyses. The mean BV of the three hits during each warm-up procedure for each batter represented the BV for that warm-up procedure and that batter. In experiment 1, a two-way repeated measure analysis of variance (ANOVA; factor—group [SBS vs. WBS vs. ISO] and time [pre–warm-up vs. post–warm-up]) with post hoc Bonferroni corrected t-test was used for testing differences in BV before and after each pre-batting warm-up procedure. In experiment 2, differences in the baseline BV of the experimental and control groups, before and after the 8-week training period, were tested with a two-way repeated measures ANOVA (factor—group [EXP vs. CON] and training [pretraining vs. posttraining]) with post hoc Bonferroni corrected t-test. Differences in the change of post-ISO warm-up BV from baseline BV before and after the 8-week training were tested with a two-way repeated ANOVA (factor—group [EXP vs. CON] and training [pretraining vs. posttraining]) with post hoc Bonferroni corrected t-test. The overall alpha level was set at p < 0.05 for all analyses.
A two-way ANOVA indicated a significant interaction of the three factors (F2,46 = 16.85, p < 0.001). A one-way ANOVA revealed that the main effect of factor was not significant in pre–warm-up conditions. Another one-way ANOVA showed a significant main effect of factor in post–warm-up conditions (F2,46 = 9.55, p < 0.001). A post hoc paired t-test showed that there was (a) no significant change in post-SBS BV (−0.33 m·s−1, or −0.73 mph), (b) a significant decrease in post-WBS BV (−0.89 m·s−1, or −1.98 mph; p < 0.05), and (c) a significant increase in post-ISO BV (+0.39, or +0.86 mph; p < 0.05) (Table 1 and Figure 3).
The two-way ANOVA for the change in baseline BV (m·s−1) revealed the interaction of two factors (F1,22 = 19.10, p < 0.01; Figure 4). The post hoc paired t-test showed that the posttraining baseline BV was significantly greater than the pretraining baseline BV in the EXP group (31.15 ± 1.57 m·s−1 vs. 30.21 ± 1.84 m·s−1, or 67.58 ± 4.09 mph to 69.68 ± 3.51 mph, respectively, p < 0.001). The two-way ANOVA for the change rate in the post-ISO warm-up BV (%) revealed both a main effect of training (F1,22 = 19.10, p < 0.01) and an interaction between the two factors (F1, 22 = 19.10, p < 0.001; Figure 5). A post hoc paired t-test showed that the increase in posttraining post-ISO BV expressed as percentage baseline BV was significantly greater than the similarly calculated pretraining pre-ISO BV in the experimental group (103.75 ± 1.91% vs. 100.17 ± 3.18%, respectively, p < 0.01). A post hoc independent sample t-test also showed that the posttraining post-ISO BV expressed as percentage change from baseline in the experimental group was significantly greater than the similarly calculated posttraining post-ISO BV in the control group (103.75 ± 1.91% vs. 100.90 ± 2.70%, respectively, p < 0.001).
The purpose of experiment 1 was to compare the acute effects of different pre-batting warm-up techniques on BV. The results indicated a significant increase in BV after ISO and a significant decrease in BV after WBS. The ISO warm-up consisted of four sets of 5-second maximal voluntary isometric contraction conditioning. The ISO warm-up was designed to induce PAP, which causes an acute enhancement in the rate of force development (13,18). The positive effect of the ISO warm-up on BV was similar to that seen in other studies that used maximal voluntary isometric contraction for improvement in explosive performances (6,12). As a common pre-batting warm-up method, swinging an overweighted bat has been traditionally performed by many baseball players (17). Other batters often swing a standard bat in the on-deck circle. Statistical analysis showed a significant decrease in post-WBS warm-up BV compared with the pre-WBS warm-up BV and no significant difference between BV before and after SBS. The lack of increase in BV after WBS that was observed in this study supports previous reports (4,11,17,19). In contrast, there was a significant increase in post-ISO warm-up BV from the pre-ISO warm-up BV. Therefore, ISO warm-up can be used as an effective warm-up method to acutely increase a batter's BV.
The purpose of experiment 2 was to examine the effect of an 8-week training program of maximum voluntary isometric contraction on BV. The baseline BV during the posttraining measurement was significantly greater than that of pretraining measurement in the ISO experimental group who underwent the ISO training protocol. It has been reported that training programs composed of normal bat swings and other exercise modes, such as swinging different type of bats (3,14), power resistance training (15), and rotational medicine ball exercises (21,22), increase the baseline BV. A similar result was found for the subjects in this study who underwent the ISO warm-up followed by normal bat swings. During the ISO warm-up, subjects performed maximum isometric contraction conditioning activity at the early phase of their bat swing as shown in Figure 2. During the preswing phase and early phase of the batting swing, trailing leg muscles (semimembranosus, biceps femoris, gluteus maximus, and vastus medialis), trunk muscles (erector spinae and abdominal obliques of both sides), and lead arm muscles (posterior deltoid and triceps) reach their peak of electromyographic activity (16). Therefore, the preswing phase and early swing phase may be the crucial period of time for force production in the batting motion. The 8-week ISO training protocol might well improve the maximal strength and rate of force development during the isometric contraction at the posture of the ISO warm-up. Kubo et al. (8) reported that 12 weeks of maximal voluntary isometric contraction conditioning training promoted an increase in muscle size, maximum voluntary contraction torque, tendon stiffness, and squat jump performance for recreationally active young men. There is also a correlation between isometric maximum strength and performance in resistance training and jumping (7). ISO training should be able to improve phase-specific force production of the batting swing motion. The improvement of baseball performance through a sport-specific training program as observed in this study corresponds to the findings of previous research. Szymanski et al. (21,22) reported that 12 weeks of medicine ball training, which is specific to the rotational motion of baseball hitting significantly improved not only rotational strength but also BV. Therefore, training with a swing-like exercise seems to be an effective way to improve hitting performance.
Because of the change in baseline BV before and after training, the rate of change in post-ISO warm-up BV was used for a comparison of the extent of change in BV between pretraining measurement and posttraining measurement. The rate of increase in post-ISO warm-up BV of the experimental group was significantly greater in the posttraining measurement. Maximal voluntary isometric contraction conditioning activity, which was used in the ISO warm-up procedure, was expected to elicit PAP. There are two dilemmas in the utilization of PAP mechanism for performance enhancement. First, a conditioning contraction with greater intensity and duration evokes greater potentiation and fatigue simultaneously (1,13). The second dilemma is that a longer recovery time promotes greater recovery and depresses the potentiation effect (13). To ensure an increased BV with the ISO warm-up procedure, the 8-week ISO training program was used. It has been reported that intense training promotes an increased work capacity and recovery rate (10). The increased work capacity during the four sets of 5-second maximal voluntary isometric contraction conditioning and recovery rate during the 1-minute resting period may explain the significantly greater enhancement of post-ISO warm-up BV after the training session in this study.
Based on the findings in this study, collegiate baseball batters can achieve a higher BV by using an ISO pre-batting warm-up procedure compared with swinging a weighted or standard bat. In agreement with previous research, swinging a standard bat with additional weight produced no positive effect on post–warm-up BV with a standard bat. Furthermore, the after effects of swinging a bat with a larger moment of inertia not only decreased the BV but also changed the batter's swing pattern (17) and the batter's perception of bat swing (11).
Implementation of isometric contraction conditioning training, such as the ISO training program used in this study, can be an effective way for college baseball players to improve their baseline BV and also increase their post-ISO warm-up BV. The isometric contraction conditioning in the ISO warm-up targeted an increase in the maximal strength and rate of force development at the position of the early phase of the bat swing. The positive effect of ISO training can be explained by the principle of specificity and overload of training, which are confirmed by other effective training programs for batting (e.g., swinging bats with a different moment of inertia (3), resistance training (15), and rotational medicine ball exercises (21,22).
The authors would like to thank Dr. Larry I. Crawshaw and Dr. Naokazu Miyamoto for his contribution in manuscript development. This study was supported by the Yamaha Motors Foundation for Sports. This study was also supported by the Grant-in-Aid for the Global COE, Waseda University “Sport Sciences for the Promotion of Active Life”, from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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