Bat design and ball exit velocity in baseball: Implications for player safety
R.L. Nicholls* , B.C. Elliott & K. Miller
University of Western Australia
Bat and ball motion were manually digitised using Peak Motus 2000 software (Peak Performance Technologies, Englewood, CO). Each swing was digitised from the first movement of the hitter’s hands in the negative Y direction, until one frame (0.005 s) prior to ball impact, to avoid discontinuity effects attributable to the momentum of the ball impacting the bat (Winter, 1990). Quintic spline was used as a smoothing and interpolation function. The trial producing the highest bat-tip linear velocity in the pre-impact frame was selected for further analysis for each subject.
Paired-sample t-tests were used to test for differences between the wood and metal bat in bat swing speed (linear velocity at the instant prior to impact), and ball exit velocity. The Wilcoxon Signed Ranks test was used to test for differences between bat resultant velocities, due to the chi-square nature of the distribution.
Results and Discussion
Material factors in metal bats such as greater stiffness, reduction of vibration in the stronger handle, and greater elastic distortion during impact, have been previously attributed as sources of higher ball exit velocity from these bats (Ashley, 1991). Adair (1994) indicated bat-ball contact time is too short for bat vibrational and elastic properties to significantly affect ball exit speed. Bat design strategies have focussed on maximising the energy imparted to the ball at minimal energy cost to the hitter. The results from this study suggest the design of the bat, particularly bat weight distribution, plays an important role in the production of ball exit speed. Under the assumption hitters used equal effort when swinging wood and metal bats, significant differences were evident in bat velocity and orientation at impact, with these effects reflected in the mean linedrive speeds for each type of bat.
Mean ball exit velocity (BBV) from hitters using the metal bat exceeded that from the wood bat by 3.18 m/s (Table 4). Greenwald, Penna & Crisco (2001) also reported significant differences in BBV for hitters using wood and metal bats, but did not standardise bat length or mass, so it was not clear if the source of the difference was related to bat weight distribution or simply to bat mass or length differences. Bats selected for this study were standardised for length and mass, but varied substansially in the distribution of that mass (Table 1). The greater density of alloys used in the construction of metal bats means a greater proportion of the total bat mass is located in the handle. Noble and Eck (1985) indicated knob-end loading displaced the centre of percussion (sweet spot) of the bat toward the barrel end and enlarged it. Hitting the ball from this region of the bat increases the ball exit velocity as no energy is lost to bat vibration (Brody, 1986). In addition, because it is further from
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