EFFECTS OF IMPACT LOCATION ON SOFTBALL BAT VIBRATIONS AND DISCOMFORT

Abstract

Studies examining the effects of interior loading on the location of the center of percussion (COP) of aluminium softball and baseball bats 'have resulted in recommended procedures for the systematic displacement 'of the COP. While the effect of distal displacement of the COP has been shown to improve pat performance, interior loading procedures have also been shown to dramatically change fundamental frequency and distance between the distal node of the fundamental mode and COP. Previous research with softball bats has shown that the COP a-d node of the fundamental mode are the preferred areas for ball-bat impact when they are relatively close together (less than 2 cm) with no significant difference between the two impact sites regarding preference. When these two parameters are farther apart, one site may be preferred over the other, as has been suggested in a recent study with tennis racquets. This study compared vibrational characteristics d perceptions of discomfort resulting from impacts on the Center of Percussion (COP) and fundamental vibrational node of bats with varying COP-node distances. Seventeen adult skilled slow-pitch softball players (9 females, 8 males) aged 25-40 years held each of four aluminium softball bats while they were impacted at the following locations by softballs at an approximate speed of 26 m/s: COP, distal node of fundamental mode, and midway between these two points. Results of a questionnaire regarding annoyance and discomfort were correlated with mechanical characteristics and vibrational characteristics associated with each impact location. The data indicated that: (1) discomfort was directly related to the degree of excitation of both the fundamental and first overtone vibrational modes, which was directly related to nodeimpact distance; (2) bats with higher fundamental frequencies &ere more comfortable than bats with lower fundamental frequencies: and (31 with bats having large fundamental node-COP distances (greater' than 9 cm) COP impacts are significantly less comfortable than impacts on the node.