Bat design and ball exit velocity in baseball - Australian Sports ...

Introduction
Bat design and ball exit velocity in baseball: Implications for player safety
R.L. Nicholls* , B.C. Elliott & K. Miller University of Western Australia
Wooden bats have been used in baseball since the founding of the game in nineteenth-century America. These bats are still exclusively used
by professional players, and in the Olympic Games and World Championships. More durable metal bats were introducted into baseball in
1972, and are used in most non-professional and youth baseball leagues.
Metal bats have been the subject of recent research as concerns arise over increasing ball exit velocity and player safety. The National Centre
for Catastrophic Sports Injury Research ranked baseball ninth in nonfatal injuries to US collegiate athletes in 1997. However, baseball had the
highest fatal injury rate of the 13 men’s sports surveyed (0.63 fatal injuries per 100,000 participants), exceeding that of gridiron and ice hockey
(NCAA News, June 8, 1998). The US Consumer Product Safety Commission reported 14 fatalities in children from blunt impact by a baseball
to the head or chest between April 1994 and April 1995 (Van Amerongen, Rosen, Winnik & Horwitz, 1997).
As the closest infielder to the hitter, the pitcher is at greatest risk of being injured by the batted-ball. In 1998, 375 Division I collegiate pitchers
were struck by linedrives (hard-hit, low-trajectory balls). Such impact injuries comprise about 3% of all injuries to pitchers, and this rate has
remained relatively constant since 1972 (Dick, 1999). However, increasing sophistication in metal bat design indicates the risk to pitchers may
be greater when facing hitters using metal bats. ’Ball exit velocity (BBV) for balls hit with metal bats has been demonstrated higher than from
wood bats (Bryant, Burkett, Chen, Krahenbuhl & Lu, 1977; Elliott, 1979). BBV values obtained from wood bats ranged between 39.62 - 44.17
m/s (143 - 159 km/h). At a distance of 16.46 m, minimum movement time for a pitcher to complete a protective motion against a linedrive is
approximately 400 ms (Cassidy & Burton, 1989). This represents a ’safe’ BBV of approximately 42 m/s (151 km/h). Greenwald, Penna &
Crisco (2001) quantified average ball exit velocity from metal bats as 47.61 m/s (171 km/h), with even high-school hitters achieving BBV
exceeding 160 km/h. It is evident ball exit speeds from metal bats present a particular danger to the pitcher.
Differences in elastic or vibrational characteristics of the bat constituent material have been proposed as factors in the superior performance of
metal bats (Ashley, 1991). However during a high-speed impact, the baseball may be in contact with the bat for as little as 2 ms (Greenwald,
Penna & Crisco, 2001). Adair (1994) estimated the impulse generated by a ball striking a bat held firmly in the hands may take up to 8 ms to
propagate from the point of impact in the barrel, to the hands, and back to the impact point - by which time the ball will have already departed.
These findings suggest factors beside bat material properties may affect ball exit velocity.
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Bat design and ball exit velocity in baseball: Implications for player safety
R.L. Nicholls* , B.C. Elliott & K. Miller University of Western Australia
This study investigated the effect of bat design on linedrive speed. The selection of wood or metal materials promotes considerable design
differences between bats. The greater density of aluminium alloys means the bat must be shaped as a hollow tube to maintain the same weight
as a solid ash bat, whose mass is distributed throughout the implement, with a greater proportion of mass located in the hitting region (barrel).
Resistance to angular acceleration is determined by the distribution of bat mass with respect to its axis of rotation. As a first class lever rotating
about the hands against the resistance of the barrel weight, for a given torque the “end-heavy” wood bat will achieve less angular acceleration
than a metal bat. Such a bat also requires greater impulse to produce a change in bat speed, resulting in decreased linear velocity of the barrel,
thereby imparting less velocity to the ball (Noble, 1998; Hay; 1973).
Statement of the Problem
To develop effective standards for equipment and maximise player safety in baseball, the effect of bat design on ball exit velocity must be
quantified. Metal bats are currently certified”“safe” before commercial sale using robotic swing testing. The purpose of this study was to
quantify the effect of bat weight distribution on ball exit velocity from metal and wood bats swung by high-performance hitters.
Methods
Baseball bats: One metal bat and one wood bat were selected for analysis. Bat specifications are listed in Table 1 (expressed in both SI and
empirical units as is traditional in baseball). Bats were selected as representative of the length and mass of bats used in high school and
collegiate baseball. The metal bat was constructed from an alloy of heat-treated zinc, magnesium and aluminium. The wood bat was a solid
northern white ash bat. Although sold as conforming to NCAA mass and length restrictions (which state the empirical length-to-weight differential
must not exceed 3), the metal bat was approximately 30 g lighter than the certified weight. While the bats were virtually identical in length and
similar in mass, the primary difference between the two bats was the location of the centre of mass (CM). The theoretical point around which
the mass of the metal bat was distributed was located using a knife-edge balance technique (Hay, 1973), and was found to be approximately
5 cm closer to the handle than that of the wood bat.
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Bat design and ball exit velocity in baseball: Implications for player safety
R.L. Nicholls* , B.C. Elliott & K. Miller University of Western Australia
Table 1: Specifications of wood and metal baseball bats used in this study.
Bat Length (m) Length (in.) Mass
(kg)
Mass
(oz.)
Empirical lengthweight
differential
Print
Bat centre of
mass
(balance
point) (m)
Wood 0.835 32.87 0.840 29.63 3.24 0.570 0.0637
Metal 0.834 32.83 0.805 28.40 4.43 0.528 0.0698
Index
Diameter at
widest point of
bat barrel (m)
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).
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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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Bat design and ball exit velocity in baseball: Implications for player safety
R.L. Nicholls* , B.C. Elliott & K. Miller University of Western Australia
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 an angle between
the global z-axis, axis of rotation and the bat tip, projected on the XZ plane (Figure 1). The
tip of the wood bat was typically located 22 degrees behind the horizontal position achieved
by the metal bat 0.005 s prior to impact. The greater angle of incidence and reflection during
the bat-ball impact may have resulted in decreased ball exit velocity from wood bats due to
greater frictional force.
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Bat design and ball exit velocity in baseball: Implications for player safety
R.L. Nicholls* , B.C. Elliott & K. Miller University of Western Australia
Table 2: Instantaneous bat tip linear velocity (m/s) for wood and metal baseball bats (0.005 s prior to ball contact)
Velocity component Metal (ms -1) Wood (ms -1) p
Bat tip (X) 37.46 ± 3.19 34.09 ± 3.86 0.0005*
Bat tip (Y) 1.09 ± 5.34 -2.69 ± 4.52 0.02
Bat tip (Z) 8.70 ± 5.74 6.81 ± 8.05 0.221
Bat tip (resultant) 39.39 ± 3.24 36.39 ± 3.25 0.0005*
* p < 0.01
Cassidy and Burton (1989) indicated 400 ms is required for a pitcher to complete a reactive movement to avoid being struck by the batted ball.
This corresponds to a ball exit velocity of approximately 42 m/s. Mean ball exit velocity from wood bats in this study (Table 4) was 40.8 m/s.
This finding supports the use of wood bat exit velocities as a “gold standard” to determine permissible bat performance in baseball. On July 19,
1999, the US National Collegiate Athletic Association (NCAA) Baseball Rules Committee ruled ball exit velocity of 93 ± 1 mph (41.57 m/s) was
the certifiable limit for all metal bats used in college baseball. This ruling was based on BBV tests conducted on solid wood bats, considered an
acceptable standard as they have been in use since the inception of the game. Our results suggest ball exit velocity from wood bats swung by
live hitters is within, but at the upper limit of, human reaction time for defensive players. The finding that average exit velocity from metal bats
was 43.98 m/s (98.95 mph), and as high as 120.97 mph, indicates a high potential for impact injury to fielding players.
Table 3: Bat orientation (deg) at the instant 0.005 s prior to ball contact for wood and metal baseball bats.
Angle Metal (deg) Wood (deg) p
TILT 120.53 ± 3.59 118.57 ± 7.26 0.176
TRANS
* p < 0.01
359.37 ± 13.93 338.89 ± 7.66 0.0005*
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Bat design and ball exit velocity in baseball: Implications for player safety
R.L. Nicholls* , B.C. Elliott & K. Miller University of Western Australia
The velocity of approximately 11% of all linedrives from metal bats exceeded 49 m/s, ranging up to a maximum ball exit velocity of 54.077 m/
s. This value is approximately 5 m/s greater than the maximum velocity achieved from the wood bat (49.115 m/s). However, a maximum BBV
of 49.17 m/s from wood bats was recorded, which again exceeds the recommended safe limit by 8 m/s (17 mph). This finding is substantiated
by the April 8, 2001 injury to Cleveland Indians’ pitcher Steve Woodard, struck and injured by a linedrive from a wood bat during the first week
of Major League competition. Houston relief pitcher Billy Wagner was hit in the head by a line drive on July 16, 1998 - a blow from which the
ball caromed into the third-base dugout (Baum, 1998). Such incidents indicate the issue of equipment design and safety in baseball may
extend beyond bats to factors including the elastic properties of the ball, the dynamics of the impact between bat and ball, increasing size and
strength of hitters, the distance of the pitcher from the batter and protective equipment for defensive players.
Conclusions
Table 4: Mean ball exit velocity (m/s) for wood and metal baseball bats (0.01 s after ball contact).
Bat Mean ball exit velocity (ms-1) p
Metal 43.98 ± 4.95 0.0257
Wood 40.80 ± 4.30
A clear relationship exists between bat weight distribution and both the orientation and linear velocity of the bat at impact. This has important
implications for ball exit velocity and player safety. Reversion to exclusive use of wood bats is not an economically viable solution for nonprofessional
baseball leagues. Current NCAA regulations for metal bats in collegiate play legislate for design features including maximum bat
diameter and length-to-weight ratio. All bats must also conform to a maximum BBV of 93 +/- 1 mph when swung at 80 mph by the Baum Hitting
Machine. Although specific to these bats and players, the results of this study clearly indicate a certified bat swung by a live hitter may produce
BBV exceeding that demonstrated when using the Baum Hitting Machine. On June 12, 1999, the NCAA Baseball Rules Committee announced
recommendations that mass distribution regulations be adopted for metal bats, although this legislation remains to be enacted. The results of
this research support such recommendations.
References
“Researchers release latest catastrophic-injury report” NCAA News June 8, 1998
Adair, R.K. (1994) The Physics of Baseball 2nd edition. Harper Collins, New York
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Bat design and ball exit velocity in baseball: Implications for player safety
R.L. Nicholls* , B.C. Elliott & K. Miller University of Western Australia
Ashley, S. (1991) Wood-composite baseball bats take the field. Mechanical Engineering August 1991: 43-45.
Baum, B. (1998) Astros reliever Billy Wagner hit in head by line drive. Associated Press: http://www.sportserver.com/newsroomfeat/archive/
071598/hou75819.html
Brody, H. (1986) The sweet spot of a baseball bat. American Journal of Physics 54(7): 640-643.
Bryant, F.O., Burkett, L.N., Chen, S.S., Krahenbuhl, G.S., Lu, P. (1977) Dynamic and performance characteristics of baseball bats. Research
Quarterly 48(3): 505-509.
Cassidy, P.E., Burton, A.W. (1989) Response time in baseball: implications for the safety of infielders and pitchers. Unpublished: University of
Minnesota, Minneapolis, MN.
Dick, R.W. (1999) A discussion of the baseball bat issue related to injury from a batted ball. NCAA News 12 April
Elliott, B.C. (1979) Performance characteristics of aluminium and wooden teeball bats. Sports Coach 3(4): 36-37.
Greenwald, R.M., Penna, L.H., Crisco, J.J. (2001) Differences in batted-ball speed with wood and aluminium baseball bats: a batting cage
study. Accepted: Journal of Applied Biomechanics, February 2001.
Hay, J.G. (1973) The Biomechanics of Sports Techniques. Prentice-Hall, New Jersey.
Messier, S.P., Owen, M.G. (1984) Bat dynamics of female fast pitch softball batters. Research Quarterly for Exercise and Sport 55(2): 141-
145.
Noble, L. (1998) Inertial and vibrational characteristics of softball and baseball bats: research and design implications. International Society
of Biomechanics in Sports: 1998 conference proceedings. http://www.isbs98.uni-konstanz.de/fullpaper/lnoble.pdf
Noble, L., Eck, J. (1985) Bat loading strategies. In: Terauds, J., Barham, J.N. (eds) Biomechanics in Sports III 58-71 Academic Publishers,
Del Mar, CA.
Van Amerongen, R., Rosen, M., Winnik, G., Horwitz, J. (1997) Ventricular fibrillation following blunt chest trauma from a baseball. Pediatric
Emergency Care 13(2): 107-110.
Williams, T.S., Underwood, J. (1971) The Science of Hitting Simon & Schuster, New York
Winter, D.A. (1990) Biomechanics and Motor Control of Human Movement. 2nd edition. Wiley, New York.
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Bat design and ball exit velocity in baseball: Implications for player safety
R.L. Nicholls* , B.C. Elliott & K. Miller
University of Western Australia
INTRODUCTION: Wooden bats have been used in baseball since the founding of the game in nineteenth-century America.
These bats are still exclusively used by professional players, and in the Olympic Games and World Championships. More durable
metal bats were introducted into baseball in 1972, and are used in most non-professional and youth baseball leagues.
Metal bats have been the subject of recent research as concerns arise over increasing ball exit velocity and player safety. The
National Centre for Catastrophic Sports Injury Research ranked baseball ninth in nonfatal injuries to US collegiate athletes in 1997.
However, baseball had the highest fatal injury rate of the 13 men’s sports surveyed (0.63 fatal injuries per 100,000 participants),
exceeding that of gridiron and ice hockey (NCAA News, June 8, 1998). The US Consumer Product Safety Commission reported 14
fatalities in children from blunt impact by a baseball to the head or chest between April 1994 and April 1995 (Van Amerongen,
Rosen, Winnik & Horwitz, 1997).
As the closest infielder to the hitter, the pitcher is at greatest risk of being injured by the batted-ball. In 1998, 375 Division I
collegiate pitchers were struck by linedrives (hard-hit, low-trajectory balls). Such impact injuries comprise about 3% of all injuries to
pitchers, and this rate has remained relatively constant since 1972 (Dick, 1999). However, increasing sophistication in metal bat
design indicates the risk to pitchers may be greater when facing hitters using metal bats. Ball exit velocity (BBV) for balls hit with
metal bats has been demonstrated higher than from wood bats (Bryant, Burkett, Chen, Krahenbuhl & Lu, 1977; Elliott, 1979). BBV
values obtained from wood bats ranged between 39.62 - 44.17 m/s (143 - 159 km/h). At a distance of 16.46 m, minimum movement
time for a pitcher to complete a protective motion against a linedrive is approximately 400 ms (Cassidy & Burton, 1989). This
represents a ’safe’ BBV of approximately 42 m/s (151 km/h). Greenwald, Penna & Crisco (2001) quantified average ball exit velocity
from metal bats as 47.61 m/s (171 km/h), with even high-school hitters achieving BBV exceeding 160 km/h. It is evident ball exit
speeds from metal bats present a particular danger to the pitcher.
Differences in elastic or vibrational characteristics of the bat constituent material have been proposed as factors in the superior
performance of metal bats (Ashley, 1991). However during a high-speed impact, the baseball may be in contact with the bat for as
little as 2 ms (Greenwald, Penna & Crisco, 2001). Adair (1994) estimated the impulse generated by a ball striking a bat held firmly in
the hands may take up to 8 ms to propagate from the point of impact in the barrel, to the hands, and back to the impact point - by
which time the ball will have already departed. These findings suggest factors beside bat material properties may affect ball exit
velocity.
This study investigated the effect of bat design on linedrive speed. The selection of wood or metal materials promotes
considerable design differences between bats. The greater density of aluminium alloys means the bat must be shaped as a hollow
tube to maintain the same weight as a solid ash bat, whose mass is distributed throughout the implement, with a greater proportion
of mass located in the hitting region (barrel). Resistance to angular acceleration is determined by the distribution of bat mass with
respect to its axis of rotation. As a first class lever rotating about the hands against the resistance of the barrel weight, for a given
torque the “end-heavy” wood bat will achieve less angular acceleration than a metal bat. Such a bat also requires greater impulse to
produce a change in bat speed, resulting in decreased linear velocity of the barrel, thereby imparting less velocity to the ball (Noble,
1998; Hay; 1973).
STATEMENT OF THE PROBLEM: To develop effective standards for equipment and maximise player safety in baseball, the
effect of bat design on ball exit velocity must be quantified. Metal bats are currently certified “safe” before commercial sale using
robotic swing testing. The purpose of this study was to quantify the effect of bat weight distribution on ball exit velocity from metal
and wood bats swung by high-performance hitters.
METHODS: Baseball bats: One metal bat and one wood bat were selected for analysis. Bat specifications are listed in Table 1
(expressed in both SI and empirical units as is traditional in baseball). Bats were selected as representative of the length and mass
of bats used in high school and collegiate baseball. The metal bat was constructed from an alloy of heat-treated zinc, magnesium
and aluminium. The wood bat was a solid northern white ash bat. Although sold as conforming to NCAA mass and length
restrictions (which state the empirical length-to-weight differential must not exceed 3), the metal bat was approximately 30 g lighter
than the certified weight. While the bats were virtually identical in length and similar in mass, the primary difference between the two
bats was the location of the centre of mass (CM). The theoretical point around which the mass of the metal bat was distributed was
located using a knife-edge balance technique (Hay, 1973), and was found to be approximately 5 cm closer to the handle than that of
the wood bat.
Table 1: Specifications of wood and metal baseball bats used in this study.
Bat Length (m) Length (in.) Mass
(kg)
Mass
(oz.)
Empirical lengthweight
differential
Bat centre of
mass
(balance
point) (m)
Wood 0.835 32.87 0.840 29.63 3.24 0.570 0.0637
Metal 0.834 32.83 0.805 28.40 4.43 0.528 0.0698
Diameter at
widest point of
bat barrel (m)

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

an angle between the global z-axis, axis of rotation and the bat tip, projected on the XZ plane (Figure 1). The tip of the wood bat was
typically located 22 degrees behind the horizontal position achieved by the metal bat 0.005 s prior to impact. The greater angle of
incidence and reflection during the bat-ball impact may have resulted in decreased ball exit velocity from wood bats due to greater
frictional force.
Table 2: Instantaneous bat tip linear velocity (m/s) for wood and metal baseball bats (0.005 s prior to ball contact)
Velocity component Metal (ms -1) Wood (ms -1) p
Bat tip (X) 37.46 ± 3.19 34.09 ± 3.86 0.0005*
Bat tip (Y) 1.09 ± 5.34 -2.69 ± 4.52 0.02
Bat tip (Z) 8.70 ± 5.74 6.81 ± 8.05 0.221
Bat tip (resultant) 39.39 ± 3.24 36.39 ± 3.25 0.0005*
* p < 0.01
Cassidy and Burton (1989) indicated 400 ms is required for a pitcher to complete a reactive movement to avoid being struck by
the batted ball. This corresponds to a ball exit velocity of approximately 42 m/s. Mean ball exit velocity from wood bats in this study
(Table 4) was 40.8 m/s. This finding supports the use of wood bat exit velocities as a “gold standard” to determine permissible bat
performance in baseball. On July 19, 1999, the US National Collegiate Athletic Association (NCAA) Baseball Rules Committee ruled
ball exit velocity of 93 ± 1 mph (41.57 m/s) was the certifiable limit for all metal bats used in college baseball. This ruling was based
on BBV tests conducted on solid wood bats, considered an acceptable standard as they have been in use since the inception of the
game. Our results suggest ball exit velocity from wood bats swung by live hitters is within, but at the upper limit of, human reaction
time for defensive players. The finding that average exit velocity from metal bats was 43.98 m/s (98.95 mph), and as high as 120.97
mph, indicates a high potential for impact injury to fielding players.
Table 3: Bat orientation (deg) at the instant 0.005 s prior to ball contact for wood and metal baseball bats.
Angle Metal (deg) Wood (deg) p
TILT 120.53 ± 3.59 118.57 ± 7.26 0.176
TRANS 359.37 ± 13.93 338.89 ± 7.66 0.0005*
* p < 0.01
The velocity of approximately 11% of all linedrives from metal bats exceeded 49 m/s, ranging up to a maximum ball exit velocity
of 54.077 m/s. This value is approximately 5 m/s greater than the maximum velocity achieved from the wood bat (49.115 m/s).
However, a maximum BBV of 49.17 m/s from wood bats was recorded, which again exceeds the recommended safe limit by 8 m/s
(17 mph). This finding is substantiated by the April 8, 2001 injury to Cleveland Indians’ pitcher Steve Woodard, struck and injured by
a linedrive from a wood bat during the first week of Major League competition. Houston relief pitcher Billy Wagner was hit in the
head by a line drive on July 16, 1998 - a blow from which the ball caromed into the third-base dugout (Baum, 1998). Such incidents
indicate the issue of equipment design and safety in baseball may extend beyond bats to factors including the elastic properties of
the ball, the dynamics of the impact between bat and ball, increasing size and strength of hitters, the distance of the pitcher from the
batter and protective equipment for defensive players.
Table 4: Mean ball exit velocity (m/s) for wood and metal baseball bats (0.01 s after ball contact).
Bat Mean ball exit velocity (ms -1) p
Metal 43.98 ± 4.95 0.0257
Wood 40.80 ± 4.30
CONCLUSIONS: A clear relationship exists between bat weight distribution and both the orientation and linear velocity of the bat
at impact. This has important implications for ball exit velocity and player safety. Reversion to exclusive use of wood bats is not an
economically viable solution for non-professional baseball leagues. Current NCAA regulations for metal bats in collegiate play
legislate for design features including maximum bat diameter and length-to-weight ratio. All bats must also conform to a maximum
BBV of 93 +/- 1 mph when swung at 80 mph by the Baum Hitting Machine. Although specific to these bats and players, the results
of this study clearly indicate a certified bat swung by a live hitter may produce BBV exceeding that demonstrated when using the
Baum Hitting Machine. On June 12, 1999, the NCAA Baseball Rules Committee announced recommendations that mass
distribution regulations be adopted for metal bats, although this legislation remains to be enacted. The results of this research
support such recommendations.
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