David H. Janda, MD, Cynthia A. Bir,, MS, RN, David C. Viano, PhD, and Stephen J. Cassaria, MSE
Objective:
To compare various soft-core baseballs for their ability to reduce the risk of fatal chest-impact injury.
Design:
This study used a three-rib biomechanical surrogate to quantitatively analyze chest impacts from nine soft-core baseballs and one standard baseball, which served as the control. Impacts were achieved with an air cannon system, with the velocity of impact being 40, 50, and 60 mph.
Materials and Methods:
The deflection of the three-rib structure at the sternum was measured and used to calculate the viscous criterion, which correlates with risk of chest-impact injury.
Measurements and Main Results:
Analysis showed that baseballs with lighter mass had a significantly lower viscous criterion (p < 0.05). Those with a similar mass had no change in the viscous criterion, and the heaviest soft-core baseball had a significantly higher viscous criterion at an impact velocity of 60 mph.
Conclusion:
The results of this study indicate that soft-core baseballs may not differ from a standard baseball with regard to the risk of fatal chest-impact injury while playing baseball. Other techniques, such as preventive coaching, need to be implemented when trying to improve baseball safety.
Key Words:
Baseball, Injury prevention, Commotio cordis, Cardiac arrhythmia, Viscous criterion.
It has been estimated that more than 19 million Children are involved in youth baseball in the United
States.1 It has also been estimated
that softball and baseball lead to more injuries necessitating emergency room
visits in the United States than any other sport. Between 1983 and 1994, more
than 4.8 million injuries secondary to softball and baseball were documented
through selected emergency rooms throughout the United States by the Consumer
Product Safety Commission.2 In 1981,
the Consumer Product Safety Commission concluded that more fatalities occur in
the 5- to 14-year age group as a result of baseball than any other sport. There
were 88 deaths related to baseball in this age group between 1973 and 1995: 68
were caused by impacts with the ball, 13 were caused by impacts with the bat,
and 7 were from another cause or were of unknown cause. Baseball impacts to the
chest caused 38 of the 68 deaths attributable to the ball, whereas impacts to
the head and other regions accounted for the other 30 deaths.2 These data
translate into one death in youth baseball for every 4.75 million
participants.
In 1995, Maron et al.3 reported on 25 individuals, aged 3 to 19
years, who died secondary to blunt chest impacts while participating in a
variety of sports. The majority of these cases (18 of 25) occurred while playing
baseball or softball. Of the 25 deaths, 19 had cardiopulmonary resuscitation
initiated within about 3 minutes of their collapse, and 28% of the victims were
wearing some sort of chest protector. Autopsies were performed on 22 of the 25
subjects. All were found to be free from cardiac structural abnormalities, with
only small contusions evident on the left side of the chest in 12 of the
victims.
Episodes of sudden death without structural cardiac damage in individuals
participating in sports have been referred to as commotio cordis.4 Commotio cordis has been hypothesized to
occur as a result of apnea, vasovagal reflex, or ventricular arrhythmia: The
occurrence of blunt chest impacts leading to ventricular arrhythmia has been
supported elsewhere. Liedtke et al.5
demonstrated that a blunt trauma caused a redistribution of the blood flow in
the small vessels of the heart. It was thought that this disruption in the
perfusion of the myocardium was the cause of hemodynamic and electrophysiologic
changes within the heart. It is these electrophysiologic changes, or arrhythmia,
that appear to be the foundation of commotio cordis, an injury often not
associated with serious, gross myocardial damage. In animal experiments, Viano
et al.6 reported the presence of
ventricular fibrillation (VF) in 7 of 16 chest impacts with baseballs. These
impacts, occurring at a velocity of 95 mph (42.8 ni/s), consistently produced
the fatal arrhythmia and traumatic apnea. In most of the reported clinical cases
of commotio cordis, however, the velocity of impact was thought to be between 30
and 50 mph.3 The velocity of impact, therefore, is not the sole factor for
blunt-impact fatalities.
In an effort to determine other factors associated with
commotio cordis and traumatic cardiac arrest, the timing of the chest impact has
been studied by researchers.7'8
Cooper et al.7 investigated the
biomechanics of nonpenetrating chest impacts in an animal model. Several tests
were performed, including electrocardiography of the heart. It was demonstrated
that VF was the most serious arrhythmia produced by these impacts. Upon analysis
of the results, the T wave of the electrocardiogram was identified as a
vulnerable period of the cardiac cycle. Impacts occurring at this time appear to
initiate VF. In six impacts resulting in acute VF, four occurred during the T
wave. This finding was corroborated by Kroell et al.8 who found that five of eight impacts
resulting in immediate VF occurred during the T wave. These
findings seem consistent with the fact that this period in the cardiac cycle
represents a time when cardiac cells may he repolarized, depolarized, or
partially repolarized. Electrically, therefore, the heart is more
vulnerable.8
Along with functional injuries
associated with blunt thoracic impact, there exist structural injuries, such as
cardiac contusion and rupture. In an effort to evaluate all parameters of
cardiac injury, Icroell et al.8
conducted a series of experiments. The studies investigated the
interrelationship between velocity of impact, compression of the chest, and
injury in an animal surrogate. The responses evaluated included Abbreviated
Injury Scale score, presence of arrhythmia including ventricular fibrillation,
number of rib fractures, heart rupture, and the evaluation of biomechanical
responses. The biomechanical responses included the impact velocity, maximum
chest compression, and maximum viscous response.
The viscous criterion (VC) is a time-dependent product of the
velocity of the chest deformation (V) and the amount of compression (C).9 The chest compression is defined as the
displacement of the chest in relationship to the spine normalized by the initial
thickness of the thorax. This criterion is thus dependent not only on the amount
of compression but also on the rate at which the compression occurs. The viscous
response is related to the energy absorbed by the body, including the heart,
during the rapid phase of chest compression.
The ability to determine the viscous
response or VC of an impact on a large scale becomes problematic. Case studies
of human fatalities leave the researcher with limited postmortem data. Although
an animal surrogate could serve as an in vivo model of the event, there is a
lack of an ability to complete a large number of tests. Logically, a
biomechanical surrogate validated against an animal model would allow for
reproducible data to be acquired and analyzed.
It is the purpose of this study to use an experimental model,
the three-rib system, to evaluate the effectiveness of various soft-core and
lighter-mass baseballs in their ability to reduce the magnitude of chest impact
and risk of fatality. This study was designed to evaluate several soft-core
baseballs at the velocities likely to be seen during little league play: 40, 50,
and 60 mph. The standard baseball served as the control for this
study.
METHODS
Baseballs
Nine soft-core baseballs were tested as part of the
experimental group in the present study. The 'Official League" baseball
manufactured by Rawlings served as the control (Fig. 1). The mass of each ball
was determined before testing. Table 1 lists the types of balls tested and the
respective mass of each.
 Figure 1. click image to
expand
Nine soft-core baseballs were tested, with the standard baseball serving as
the control
Click on Image to enlarge
 Table 1.
click image to expand
Three-Rib Structure
Chest-impact measurements were obtained using a three-rib
chest structure that was constructed with the rib subunits of the BIOSID test
dummy. The BIOSID is a biomechanical surrogate that has been extensively tested
and was developed for purposes of determining biomechanical responses in
blunt-impact scenarios. The entire system was not used for this study, however,
because it is a lateral-impact system suitable for high-velocity, blunt-impact
studies. Only the rib components of the BIOSID were used in the development of
the three-rib system.
The three-rib system was developed because of the need for a portable,
low-cost surrogate that has biofidelity to human chest-impact responses. It was
considered ideal for the chest impacts that were performed for this study
because the rib structures were continuous and therefore provided an adequate
loading surface for the concentrated impacts of a baseball. This system is
similar to the Hybrid III in its ability to measure chest-deflection data from
which VC can he calculated.
The basic structure involved three thorax ribs mounted to a spine box
opposite the impact side (Fig. 2). Dampening material on the inside of the rib
provided for viscous bending resistance and allowed for the dissipation of
energy. Nylon supports were mounted to the sides of the spine box to prohibit
the upward and downward motion of the ribs. A 15.5 cm 7 23 cm urethane bib tied
the three ribs together on the impact side. The urethane was further covered
with padding made of Ensolite (Robatex, Bedford, Va) approximately 2.5 cm thick
to simulate overlying skin and subcutaneous tissue.
 Figure 2
- click image to expand
To measure the amount of chest deflection, a position transducer was
mounted to the interior of the ribs in the anterior-posterior position. This
transducer was connected directly behind the urethane bib in the sternal region.
The base of the transducer was mounted within the spine box. This arrangement
allowed for the total amount of chest displacement to be measured.
The linearity of the position transducer was verified
statically over a range of 50 mm and dynamically for velocities up to 10 m/s.
Differentiation of the rib displacement curve verified that this velocity was
not exceeded during the testing. Calibration tests with the three-rib system
demonstrated an average 6.8% coefficient of variation in VC and 3.4% in chest
deflection for 11 tests with 2.3% coefficient of variation in test
speeds.
The chest structure was placed on a free-moving sled to
allow for horizontal movement with each impact, simulating whole-body motion in
an impact. A pressurized air cannon, which discharged the baseballs, was placed
approximately 39 inches in front of the chest structure. The projectile
velocities were 40,50, and 60mph (17.9, 22.4, and 26.8 mis). The sled was
repositioned to the same starting point after each impact, and the amount of
displacement of the entire system was recorded. Velocities of the baseballs were
recorded with an Oehler Research, Inc. (Austin, Tex), model 35P
chronograph.
Analysis and Interpretation of Biomechanical
Responses
The baseballs impacted anteroposterior to the midpoint of
the sternal and thoracic area. As described above, the amount of deflection in
the chest was measured with a linear chest-deflection transducer located at the
center of the impact region. A Texas Micro Systems (Santa Barbara, Calif) 486
computer with a RC Electronics (Houston, Tex) A-to-D conversion board and
software served as the data-acquisition system used to record these
responses.
The variables collected included velocity of ball at impact,
total displacement of the chest, and linear displacement of the entire
three-rib/sled unit. The velocity of the ball at impact dictates, along with the
ball mass, the amount of energy going into the system. The chest displacement is
indicative of the amount of impact energy absorbed by the chest. In addition, it
allows for calculation of the viscous criterion to which cardiac injury can be
correlated. The distance that the entire three-rib/sled unit traveled on the
rail system is indicative of the amount of energy being absorbed by the entire
unit. It represents the amount of energy transferred into the system minus the
amount of energy absorbed by the thorax structure and rebound of the
ball.
In 1986, Kroell et al.8 validated the viscous criterion as the
best predictor of the probability of heart rupture and thoracic injuries with
Abbreviated Injury Scale scores > 3. Further analysis by Viano and Lau
10 resulted in the determination that the
VC was the best indicator of soft-tissue injuries for many body regions when the
velocity of deformation was in the 3 to 30 mis range. The difficulty of
determining how VC relates to commotio cordis exists because of the lack of
previous analysis and the enigma surrounding the event of commotio cordis
itself.
To determine the trends that are prevalent within the data,
the average VC for each of the baseballs at each of the velocities was
normalized. The standard ball at 60-mph (26.8 m/s) served as the normalization
factor for this process. The standard ball was chosen because it served as the
control for the study, and the velocity of 60-mph (26.8 rn/s) was chosen because
it provides the most severe impact.
Each ball was 'thrown" at the three-rib structure a total of
10 times at each velocity. The balls were randomly sampled without replacement
until all 10 balls were tested, at which time the process was repeated with the
same set of 10 balls. A coefficient of variation was calculated for each group
within the analysis of the data to ensure repeatability.
Statistical Methods
Differences in the data were statistically evaluated using an analysis of
variance. Further analysis involved a Bonferroni (Dunn) t test when the
analysis of variance was significant (p < 0.05). The probability of injury
was determined by logistical analysis using SAS software (SAS, Inc., Cary, NC).
The values of X2, R, and
p are reported, as well as the statistical parameters for the sigmoidal
model for injury risk.
RESULTS
Viscous Response
The results of the testing were stratified according to
the three different velocities and the variables of VC, sternal displacement,
and sled displacement. The trends in the data can be demonstrated by normalizing
the data. The VC values were used when evaluating the trends because they have
been shown to be the greatest indicator of injury.9 As stated above, the normalizing factor
was the standard baseball response at 60-mph (26.8 rn/s). As is evident in our
graphic analysis (Fig.3), there is generally an increase in the VC values as the
velocity increases.
 Figure 3 -
click image to expand
One ball, the Incrediball, had a significantly lower VC
value for all three velocities (p < 0.05). This was also the lightest
baseball tested. Five of the other balls (RIFI, SafeT, Incrediball Pro,
ADStarr5, and Flexiball) all had a significant reduction in VC values for at
least one of the tested velocities. There was one ball that produced a
significantly higher VC than the standard to the p < 0.05 level, the
ADStarr10 at 60 mph (26.8 rn/s). Table 2 shows the results of all the baseballs
stratified at each velocity.
 Table 2 -
click image to expand
Sled Displacement
The results of the entire sled displacement are presented in Figure 4.
These results are indicative of how the whole body would respond to an impact
with a given ball. It would seem logical that whole-body motion would be more
advantageous than deflection within the thorax from an injury standpoint. What
this displacement means from an injury standpoint is inconclusive, however,
because the coefficient of friction of the sled is unknown. It should be noted
that the baseballs of lower mass will provide a lower-energy input at a greater
velocity; therefore, less energy is available to move the sled or cause thoracic
deformation. Further testing will be needed to determine how the sled
displacement relates to other types of injury mechanisms.
 Figure 4 -
click image to expand
DISCUSSION
This study was prompted by the need to address fatalities related to
blunt chest impact that is occurring on the baseball fields of the youth
population. The use of a biomechanical surrogate in a laboratory setting
provided values for an injury criterion, namely, the viscous criterion. The VC
was developed to analyze chest-impact data by considering both the variable of
compression and the velocity of compression. This criterion has been used
extensively in the automotive industry, where blunt thoracic trauma also poses a
safety concern.
The results of this study indicate that the use of soft-core baseballs
does not necessarily protect the player from the possibility of chest-impact
injury to a significant level compared with the standard baseball. In fact, one
of the soft-core baseballs actually generated a significantly higher VC than the
standard baseball: the ADStarr10 at 60-mph (26.8 m/s). Only one baseball
demonstrated a significant reduction in the VC at all three velocities: the
Incrediball. This baseball also had the lowest mass of those tested. Several
baseballs had a significantly lower VC at 60 mph (26.8 m/s): RIFI, SafeT,
Incrediball, Incrediball Pro, ADStarr5, and Flexiball.
In addition to the results reported above, at least one death has
occurred on the field while a soft-core baseball was being used.3 The occurrence of chest-impact injury is
therefore possible even with the soft-core baseballs. In a recent study, the
risk of injury with soft-core baseballs was 1 injury per 6,855
player-hours, compared with 1 injury per 7,889 players-hours with standard
baseballs.11 Even though there was a
decrease in the VC values of impacts with some of the soft-core baseballs, the
timing of the impact remains a crucial, uncontrollable variable in actual
baseball impacts.7 It is impossible,
therefore, to provide players with a guarantee that a ball will prevent all
injuries and fatalities.
The problems that exist when using a soft-core baseball need to be
addressed. The first factor to consider is that the majority of players using
the soft-core baseball are among the youth population. This is a population in
which chest-impact injury is more prevalent because of the increase in
compliance of the thorax structure in comparison with the adult. Second, the
marketing and use of a potentially safer ball can give a false sense of
security. Children may be more likely to take a greater risk in fielding the
ball or not moving away from a wild pitch if they believe that there is a
decreased risk of injury.
Unfortunately, the production of a baseball that will significantly
reduce the rate of injuries may not be possible when considering the
chest-impact scenario. Adequate coaching techniques may be more likely to
address these concerns. Teaching a child how to "get out of the way" or to turn
his or her chest away from a wild pitch may be more effective in reducing the
rate of injuries than any other technique. Before players take the field, they
should realize that protective equipment will not prevent all injuries or
fatalities and that other preventive measures should be taken.
Based on the results of this study, further analysis and testing
regarding the biomechanics of commotio cordis is encouraged. When all other
factors are equal, however, the presented test method and the calculation of VC
provides a means of determining the risk of injury related to blunt chest
impacts. Based on this test method, only the baseballs with the lower VC values
would have a lower risk of injury. Those baseballs with VC values that are not
significantly lower than those of the controls should not be considered to be
safer than the standard baseballs. Furthermore, the efficacy of educational
coaching regarding injuries related to baseball and softball should be
assessed.
Acknowledgments
The authors express their appreciation to Mr. Brian Czach for his assistance
with the development of the three-rib structure. Thanks must also go to Angela
Cheney for her efforts within the testing laboratory. And finally, we thank Dr.
Anthony Schork for his assistance with the statistical
analysis.
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