In a competitive sport environment, improvement in muscle performance characteristics is desirable. For example, in the sport of baseball, a hitter would prefer to wait as long as possible before deciding to swing at a pitch. This extra time allows the hitter to more accurately recognize the speed, movement, and location of a pitch, all of which increase the chances of making solid contact with the ball. This requires optimal pitch recognition (reaction time) plus bat speed (movement time) (20). Therefore, requiring less time to complete the swing, as a function of decreased movement time (via increased bat speed), translates to an increase time permitted to react before executing the swing.
Traditionally, weighted bat warm-ups have been used with the intention of immediately increasing bat speed. It was hypothesized that additional motor units would be activated by the overweighted implement, which would persist when the extra load was removed, resulting in greater speed (16,18). Since that time, however, research has been conducted examining this method of warming up, and the general consensus is that warming up with too heavy a bat can in fact decrease bat speed (15,17,18,25,32). Southard and Groomer (32) suggested that swinging too heavy of a bat before swinging the normal bat alters the batter's swing pattern, leading to decreased bat speed. Other hypotheses have suggested that by swinging a weighted bat, the central nervous system may preferentially activate slow-twitch fibers because of the slower rate of muscle shortening with the slower swing (16,18,27). The popularity of this warm-up method is very common because of its sport specificity; however, given its lack of empirical support, and in fact contraindicative results, other warm-up exercises need to be investigated.
The relationship between the speed of the bat and the distance the ball will travel postimpact has been well established (1). Bat speed during a baseball swing is generated through a sequential recruitment pattern of muscles in the entire body. A hitter generates most of the bat speed by using a kinetic link, defined as “large base segments ‘passing’ momentum to smaller adjacent segments” (40). The larger, stronger muscles of the lower body initiate the movement and transfer the momentum to the upper body. Additionally, early in the swing, there is rotation of the upper body, in respect to the lower body, creating a “stretch” in the torso during the stride phase. The added “stretch” activates muscle spindles, which through a short-loop reflex, can lead to increased force production, by way of the stretch-shortening cycle (40).
Previous studies have demonstrated the importance of lower body function in generating bat speed. Shaffer et al. (31) demonstrated the importance of legs in the initiation of maximal bat speed. Katsumata (23) reported the use of ground reaction forces, further establishing the importance of back-to-front weight transfer, for producing maximal bat speed. Pertaining to strength and power, researchers have displayed significant correlations between lower body strength and power with bat speed (33-36). Therefore, training methods that involve improving leg strength and power are starting to be incorporated into conditioning programs to increase bat swing speed.
One technique that has been shown to have potential benefits on strength and power is whole-body vibration (WBV). Whole-body vibration refers to stimulating muscles via an external vibrating platform. Exercises performed on the vibrating platform may include squats, hops, jumps, and isometric holds in various positions. The vibration stimulus is typically applied through the feet, which then resonates throughout the body (22). A number of studies have used an acute bout of vibration exposure, lasting between 60 seconds and 10 minutes, in which an increase in dynamic and static leg strength and power was reported (8,9,12,21). Although numerous studies have demonstrated WBV's effectiveness, not all WBV studies have proven beneficial for increasing muscle strength and power (10,11,26,28,38).
Of the mechanisms proposed to account for the enhanced muscle performance after WBV, the “tonic vibration reflex” has received a great deal of support (5,6). When a muscle is vibrated, the primary nerve endings of muscle spindles are stimulated, which results in excitation of the alpha-motor neurons (Ia afferents) (5,6,21,23). Through the stimulation of the muscle spindles, it has been suggested that they retain an enhanced level of sensitivity and consequently contribute to force production. This mechanism is thought to be responsible for the increase in muscle performance (7,14,22,24). With the potential to increase force production during a baseball swing, the effects of WBV during the performance of upper and lower body exercises warrant investigation. As such, this study aimed to investigate the potential additive effects of these exercise modes on baseball swing performance.
This study was conducted to determine the effect of exercise order in performing selected upper and lower body exercises with and without WBV on subsequent baseball bat speed. In addition, we examined the relationship between upper and lower body strength on bat speed.
Experimental Approach to the Problem
The design of this study was developed for the purpose of determining the efficacy of a warm-up incorporating the order of performing upper and lower body exercises with and without WBV on subsequent baseball bat speed. This repeated-measures experimental design required that all subjects complete 4 experimental sessions presented to each subject in random order. The independent variables were WBV exposure and exercise order, whereas the dependent variable was baseball bat speed. The exercises performed were chosen to include movements that could be performed in a game situation on the on-deck circle or in the dugout immediately before a player's time at bat. By manipulating the order of performing upper and lower body exercises with and without WBV, recommendations may be possible for specific warm-up procedures to enhance subsequent bat speed. A preliminary session was conducted for the purpose of subject familiarization of experimental protocol and for the determination of bench press and back squat 1 repetitive maximum (1RM).
Sixteen male subjects (21.6 ± 2 years, 84.7 ± 9 kg, 181.4 ± 7 cm) currently enrolled in a university participated in this study. All subjects had at least a minimum of high school varsity baseball playing experience. Four of the 16 subjects had playing experience beyond high school (junior college or National Collegiate Athletic Association Division II). None of the subjects were playing, or were preparing to play, competitive baseball during the time of testing. All subjects were considered active at the time of the study in that they had been involved in a strength training program for at least 6 months before the study. All subjects were free of any musculoskeletal injuries and were given specific instructions of the procedures and asked to provide informed consent. All testing was conducted during the spring. The study was reviewed and approved by the university's institutional review board.
A Pneumex Pro-Vibe (Pneumex, Sandpoint, ID, USA) was used to administer the WBV (Figure 1). This vibration platform has a surface area of 81 × 101 cm2 and can support at least 450 kg.
A Basler piA640-210gm high-speed video camera (Basler Vison Technologies, Ahrensburg, Germany) recorded each subject's baseball swings. Bat speed video analysis was conducted by using Streampix 4.13.1 video program (Norpix, Montreal, Quebec, Canada).
The first testing session required all subjects to undergo assessments of muscular strength. After a series of dynamic warm-up exercises, upper body strength was measured via 1RM barbell bench press and 1RM barbell back squat to assess lower body strength. The order of the 2 muscular strength tests was randomized for each subject. Testing procedures for muscular strength followed those outlined and recommended by the National Strength and Conditioning Association (3). The highest resistance successfully performed one time was recorded as the 1RM.
After the day of muscular strength testing, subjects reported to the laboratory on additional days to complete the 4 experimental treatments separated by at least 1 day of rest. Subjects were required to refrain from performing any strenuous strength training activities at least 24 hours before testing. A standardized dynamic movement warm-up began each testing session consisting of arms circles, trunk twists, and hip mobility exercises. After the warm-up, subjects stood on the ground and performed 3 sets of 5 bat swings at balls placed on a stationary hitting tee with a 20-second rest period between swings. To standardize the height of the ball for each subject, across all testing sessions, the height of the ball was set level with the greater trochanter of the hitter's forward leg. A standard wooden bat was used for all swings, measuring 83.82 cm in length and 870 g in weight. Before each swing, the hitters were verbally reminded to swing with maximal effort. To represent bat speed for each of the 3 sets separately, the 5 swings within each of the 3 sets were averaged.
The 4 separate sessions were used to manipulate WBV and Leg/Arm exercise order, and were randomized across subjects. The control (CTRL) condition consisted of no WBV or exercises with 5 minutes of rest between the 3 testing sets. For all vibration sessions, the frequency and amplitude were set at 30 Hz and 4 mm, respectively. These WBV conditions were selected because they represent WBV conditions typical of many previously published studies evaluating WBV and acute neuromuscular performance (2,7,10-13,21,28,29,38). In the “Leg-Arm VIB” treatment, lower body exercises with WBV (Leg VIB, Figure 1) were performed after the first set of swings, whereas upper body exercises with WBV (Arm VIB, Figure 2) were performed after the second set of swings. In the “Arm-Leg VIB” treatment, upper body exercises with WBV (Arm VIB) were completed after set 1, with lower body exercises with WBV (Leg VIB) being performed after set 2. Finally, in the “Arm-Leg NOVIB” treatment, the order replicated the “Arm-Leg VIB” treatment minus the vibration exposure (Arm NOVIB, Leg NOVIB). The testing protocols are illustrated in Figure 3 and in Tables 1 and 2.
Bat Speed Measurement
To measure bat speed, all subjects stood on the ground and hit an indoor ball placed on a hitting tee. A meter stick aligned in the field of view of the video camera served as a distance reference, which was used to calculate bat displacement per videotape frame. The camera was positioned perpendicular to the hitting zone, with the field of vision focused on the area immediately before bat-ball impact on the hitting tee. The camera recorded at 200 frames per second. Reflective tape was applied to the top of the bat to serve as a maker during filming.
Bat speed was recorded immediately prior to impact, based on recommendations by Tabuchi, Matsuo, and Hashizume (37). Measuring the distance the leading edge of the top of the bat traveled in each frame of the video determined bat speed, recorded in meters per second. To represent bat speed for each of the 3 sets within each treatment, the 5 speeds were averaged.
To determine significant correlations between bat speed and muscular strength, a linear regression analysis was calculated. A repeated-measures analysis of variance was implemented to investigate any statistical differences in bat velocities between sets of swings and across all exercise treatments. The level of significance for rejecting the null hypothesis for all tests was set at p ≤ 0.05.
Reliability of the baseball bat speed was determined during the CTRL treatment, in which no Leg or Arm or WBV was applied. Intraclass correlation coefficient (ICC) was 0.93 between swing sets 1 and 2, and 0.95 between swing sets 2 and 3. The coefficient of variation between all swing sets was calculated at 2%. Based on an effect size of 0.24 (20), a total sample size of 16, and ICC of 0.93-0.95, a statistical power of 0.9 was estimated.
Relationship of Muscular Strength and Bat Speed
Leg strength, as measured by the 1RM back squat, was significantly related to CTRL bat speed (R2 = 0.406; p = 0.008, Figure 4). In contrast, the relationship between CTRL bat speed and upper body strength was not significant (R2 = 0.093; p = 0.25).
Effect of Exercise Treatments
The exercise order of Leg-Arm VIB elicited a significant increase in bat speed of 2.6% (p = 0.02; Table 3). When performing Leg-Arm NOVIB bat speed significantly decreased by 2.1% (p = 0.0039; Table 3). In the CTRL, Leg VIB, Leg-Arm VIB, and Arm VIB sessions, no significant differences were reported for mean bat speed between the 3 sets (Table 3).
Results from this study revealed a significant relationship between bat swing speed and lower body muscular strength as determined from the back squat 1RM. The bat swing originates from the rotational forces produced in the lower body, as such emphasizing the importance of strength and power in that region is critical, if generating maximal bat speed is the goal. To produce a swing with high bat speed, the hitter shifts their weight from the back foot to the front foot. During this stage of the swing, ground reaction forces are as high as 140% of the hitter's weight (23). If a hitter does not have adequate strength to control and transfer the load, bat speed will be lost during the swing. The results from this study coincide with previous bat swing research as the subjects with stronger and more powerful lower bodies generated the highest bat speed (33-36).
Upper and lower body exercises when performed independently with WBV (Arm VIB and Leg VIB) did not significantly enhance bat speed. Although the difference was not significant, after completion of arm and leg exercises alone did we observe an increase in bat speed by 1%. Studies have examined vibration effects on arm performance through the use of vibrating dumbbells and barbells (10,26,28). From these studies, it was reported that acute vibration exposure before exercise did not significantly enhance the performance of upper body muscles and is in agreement with results from this current study. Given that the upper body is responsible for translating the forces and torques generated by the lower body, it is possible that any fatigue may prove detrimental to the performance of the bat swing.
When WBV is applied to lower body performance, the results have been mixed. A few studies have displayed no significant changes in performance after acute WBV exposure (11,14,29,38), whereas others have reported positive benefits (2,4,7,13,21). As a function of the mixed results of localized vibration on upper and lower body performance, there has been some controversy on the potential ergogenic effects of vibration exposure. The controversy exists because of differences in skills being tested, intensity of vibration stimulus, and type of vibration stimulus (vertical, sinusoidal, see-saw, etc.).
A unique feature of this study is the combination of upper and lower body exercises to produce a total-body exercise vibration exposure. When upper and lower body WBV exercises were combined, in that order, bat speed acutely increased 2.6%. Previous studies addressing the effects of WBV on performance have demonstrated improvements lasting for up to 2 minutes. As such, it was hypothesized that the exercise order ending with the lower body would acutely increase bat speed, which was supported by the results from this study. Lower body exercises with WBV alone did not significantly change bat speed performance, possibly because of swinging a baseball bat with maximal speed is a total-body skill. Therefore, the addition of upper body exercises to lower body could explain the performance changes.
When the legs were performed first in order, there was approximately a 5- to 7-minute period between treatment and posttesting, which is 2-3 times longer than suggested by previous studies for WBV benefits to remain. Torvinen et al. (39) and Adams et al. (2) demonstrated lower body performance increases persisted up to 2 minutes after WBV exposure, consequently, waiting too long after WBV exposure could dissipate the vibration effect, nullifying its potential benefits. When the lower body was performed second, the recovery period was approximately 30 seconds. In this situation, the vibration stimulus was likely still in effect to acutely facilitate acute neuromuscular adaptations (2,4,39).
It is believed that an adequate WBV exposure time was used in this study. When all exercises in a WBV trial were completed, total exposure equaled 9 minutes. At the selected frequency and amplitude (30 Hz, 4 mm), the results of this study agree with previous research demonstrating the acute effectiveness of WBV on muscular performance with this specific combination of vibration and time (7-9). To what extent the WBV stimulus was transmitted throughout the upper versus lower body remains unknown. Given the ability of the human body to attenuate the vibration amplitude as it rises through the body, future research will need to test this further.
To investigate the effect of performing the exercises alone, subjects performed the exercises minus the vibration (“Arm-Leg NOVIB”). A significant decrease in bat velocity in subsequent sets suggest that these exercises may have been somewhat fatiguing or at least influenced the skill in some way to decrease bat speed. It could be suggested that the fatigue effects in the WBV trial were buffered by the increased sensitivity and responsiveness of the intrafusal fibers. We did not anticipate this decrease; therefore, a mechanism to explain this finding was not investigated. Performing exercises with WBV exposure may indeed “prime” targeted musculature for enhanced performance to a greater degree than exercises performed without WBV, similar to that seen with postactivation potentiation protocols (30). Increased muscle performance from this potentiation effect may be because of the greater number of motor units being recruited, enhanced coordination and synchronization of motor unit recruitment, and more sustained recruitment of higher threshold muscle fibers. Any of these can lead to enhanced force production and higher rate of force development (19,30).
We recognize that there are essentially an infinite number of combinations of WBV frequencies and amplitudes that may have been employed in this study. Our selection of 30 Hz and 4 mm stems from research published before the conductance of this study, which suggests these WBV parameters could acutely enhance neuromuscular performance (7-14,21,28,29,38). Further studies are required to determine the relative efficacy of WBV frequency and amplitude combinations on subsequent baseball bat speed. Additionally, the results of this study should not be referred beyond the baseball bat swing skill.
In conclusion, the results from this study demonstrate the positive effect of WBV when combined with total-body preparatory movements immediately before performing a complex, whole-body power movement. The conditioning exercises, when combined with WBV, may in fact “prime” relevant musculature leading to enhanced power. Additionally, the current study further clarifies the importance of lower body strength in determining bat swing speed in a baseball player, whereas upper body strength has less of a role in the process.
The results from this study may suggest to coaches and players that exercising in a specific sequence of upper body then lower body movements with WBV exposure can acutely increase bat speed by about 2-3%. A small, yet statistically significant increase in bat speed can provide significant, practical advantages during competition. In addition to increasing the time available for pitch recognition with the decrease of movement time (high bat speed), the distance a ball will travel will also increase. A 3% increase in bat speed will increase the linear momentum of the bat, translating into greater batted-ball distance. These exercises with WBV may be incorporated either on the on-deck circle (WBV platform embedded within the circle) or within the dugout area, if space provides. A direct relationship was verified between lower body strength and bat speed, advising coaches that increasing leg strength will have a positive impact on bat speed. Therefore, training programs can be appropriately designed with specific focus on the legs. Bat speed is of extreme importance to hitters at any level and the results from this study can help to suggest methods of enhancing swing performance, leading to on-field advantages for hitters.
The bats and batting gloves used in this study were donated by Hillerich & Bradsby: Louisvile Slugger. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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