Subjects: Informed consent was obtained from sixteen hitters from an Australian Baseball Federation summer baseball league. The subjects’ mean age was 22.81 ± 4.58 years, height 1.77 ± 0.74 m and mass 83.06 ± 8.59 kg. A minimum batting average of .300 from the 1999-2000 season was required (mean .366 ± 0.04). Ten subjects were right-handed, six were left-handed.
Data collection: All participants attended a familiarisation session at the indoor hitting facility prior to filming. Batting practice was undertaken with the test bats in a net-mesh batting tunnel (3 m x 4 m x 26 m). During data collection, experienced pitchers were used to pitch baseballs to each subject from a distance of 10.67 m. The mean pitch velocity was 20.47 m/s, corresponding to a speed of approximately 36 m/s over the regulation baseball pitching distance of 18.44 m, and representative of collegiate pitching speeds. Each subject hit with both wood and metal bats in a random order. Hitters were instructed to swing at pitches only in the mid-section of the strike zone, and practiced until they achieved a consistent pattern of linedrives directed toward centrefield. Each subject was subsequently filmed until producing five linedrives. Five to eight minutes recovery were permitted between use of each bat.
High-speed video data was collected using two electronically-synchronised 200 Hz cameras with a minimum shutter speed of 1/1000 s. Direct linear transformation was used to obtain three-dimensional (3D) coordinates for the motion of the bat and ball in time increments of 0.005 s. The global coordinate system to represent bat orientation in 3D space is illustrated in Figure 1, where the positive X-axis is directed toward the pitcher, positive Y is vertical, and positive Z is represented by the cross-product of the X and Y axes (out of the page).
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 the axis of rotation, the sweet spot will potentially develop greater linear velocity during the swing, thereby imparting greater velocity to the ball. The results of this study suggest such design practices increase the risk to infield players of being struck by a linedrive. Hitters in this study achieved significantly greater bat resultant linear velocity when swinging a metal bat (Table 2). The primary manifestation of the difference in bat velocity was in the x-component - that directed toward the pitcher.
The final vertical (y) velocity of the metal bat was opposite in sign to that of the wood bat, indicating the y-component velocity was directed upward (Table 2). An increase in vertical velocity prior to contact has been described as “positioning the bat to meet the ball” (Messier & Owen, 1984), and characteristic of the “optimal power swing” (Williams & Underwood, 1971). In this study, the heavier barrel of the wood bat may have affected the hitter’s ability to position the bat for maximum power, although the effect was not significant. This result is reinforced by the lack of significant difference in TILT (Table 3), which describes the orientation of the bats with respect to the vertical (y) axis.
Great hitters such as Ted Williams have previously indicated the value of contacting the ball with the bat directly over the home plate for maximum ball exit speed (Williams & Underwood, 1971). A less oblique horizontal impact between bat and ball increases linedrive speed through the minimal loss of ball energy as friction, heat and spin (Hay, 1973). This was reflected in the mean position achieved by hitters using metal bats (Table 3). In this study, the orientation of the bat on the transverse plane (TRANS) was described by