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.
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).
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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