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home > publications > softball injuries >
softball injuries

An Analysis of Preventive Methods for Baseball-Induced
Chest Impact Injuries

David H. Janda, M.D., *David C. Viano, Ph.D., *Dennis V. Andrzejak, and +Robert N. Hensinger, M.D.
Institute for Preventative Sports Medicine, Ann Arbor, *General Motors Research Laboratories, Warren, and +Pediatric Orthopaedic Surgery Division, University of Michigan, Ann Arbor, Michigan, U.S.A.

See the Abstract

The financial costs of and morbidity from recreational sports injuries in the pediatric and young adult population and the importance of injury prevention are major public health concerns [4,13]. Fortunately, many factors that predispose the recreational athlete to injury, such as poor technique, inadequate coaching, lack of conditioning, or unsafe equipment, are preventable. For example, Peterson's l970 analysis of injury rates in football led to the elimination of cross-body blocking and hence a substantial reduction in football injuries [8]. However, even when competitive games are officiated, it may be difficult to control and regulate adherence to safety rules. As a consequence, passive preventive measures need to be continually developed and tested in order to minimize the risk of sports injury. Examples of semi- or pure-passive preventive sports equipment that have significantly reduced serious injuries include the use of protective eye wear during racquet sports [8], head gear during batting, and the utilization of breakaway bases in recreational baseball and softball [3-6].

However, accidental injury remains the leading cause of death in the pediatric age group and sports injuries constitute an important subgroup of this population. The national electronic injury surveillance system of the United States Consumer Product Safety Commission has estimated that softball and baseball are the two leading sports resulting in emergency room visits in the United States [1]. Between 1983 and 1989, 2,655,404 baseball and soft ball injuries were documented from selected emergency rooms throughout the United States [1]. This figure is only an approximate estimate of the overall magnitude of the current problem because it does not include nonhospitalization physician visits.

As described in a previous paper [12], the Consumer Product Safety Commission conducted a study on sports injuries in children between S and 14 years of age. The study found more baseball related fatalities in the 5- to l-year-old age group than for any other sport [2, 12]. In a follow-up study, 51 baseball-related deaths of children were documented [9]. The most frequent type, 21 cases in total, involved impact of a ball to the child's chest. Of those fatalities, 11 occurred during organized games and the remainder in unorganized recreational play. The Consumer Product Safety Commission has since reported another 11 deaths in children secondary to chest impact from a ball between 1983 to 1990 [10].

Currently, various manufacturers have developed protective chest gear that could attenuate the forces being delivered to the chest by baseball impact. In addition, softer baseballs have been developed for similar reasons. However, many products that claim to prevent injuries have not been independently tested, evaluated, or certified.

In the previous study, an animal model was utilized to investigate fatal blunt impact of baseballs to the upper torso [13]. In that investigation, pigs subjected to midsternal blunt chest trauma by a base ball accelerated to 95 mph (42.8 m/s) developed fatal arrhythmias including ventricular fibrillation, ventricular tachycardia, and asystole. A sharp dropoff in cardiac arrythmias causing death was found at lower projectile speeds and survival was related to the use of ventilatory support. It was the purpose of the present study to investigate the usefulness of a second experimental model, namely the biomechanical Hybrid III crash dummy, for the purpose of independently testing protective equipment in low-mass, high-velocity chest impacts from baseballs. A second purpose of this study was to determine the characteristics of impacts produced to the chest area of the crash dummy by various types of "protective" baseballs used in combination with various chest protectors. In this study, the standard hard ball was used as a test control to compare the impact characteristics of various softer core baseballs.

METHODS

Baseballs and chest protectors
Several different softer core baseballs were employed to impact both animal and dummy surrogates. Various commercial vests and other foams proposed as adequate protection were interposed between the surrogates and the impacting balls. The various types of softer core baseballs and chest protectors utilized in this study are described in Table 1.

Table 1. A list of the baseballs and chest protectors used in the experiments with their abbreviations encloded in backets


Baseballs
      Standard baseball types
          Worth Regular (WorReg)
          Rawlings Regular (RawReg)
      Softer core baseball types
          Reduced injury factor 1.0 (RIF1)
          Reduced injury factor 5.0 (RIF5)
          Reduced injury factor 10.0 (RIFF10)
          Kenko Pro (KenPro)
          Kenko Orange (KenOrg)
          Easton Incrediball (Easton)
   Chest protectors
      International Protective Athletic Safety
         Corporation (IPASC)
      Sangmoo
      DF Foam
      AH Foam
      ABS hard plastic

Animal surrogates
The impact experiments were performed using six juvenile swine (Sus scrofa) weighing 2-25 kg. The rationale and experimental protocol for the use of an animal model in this program was reviewed by the Research Laboratories' Animal Research Committee. The research follows procedures outlined in the "Guide for the Care and Use of Laboratory Animals," U.S. Department of Health and Human Services, Public Health Service, National Institute of Health NIH Publication 8S-23, revised 198S, and Public Health Service "Policy on Humane Care and Use of Laboratory Animals by Awardee Institutions," NIH Guide for Grants and Contracts, Vol. 14, No. 8, June 25, l985, and complies with the provisions of the Animal Welfare Act of l966 (PL 89-544), as amended in 1970 (PL 91-579) and 1976 (PL 94-270), and the Food Security Act of 1985 (PL 99-158). The experimental protocol followed the procedure previously reported by Viano et al. [13]. The swine were first restrained without excitement by injection of ketamine (20 mg/kg, i.m.) and acepromazine (200 mg/kg, i.m.), and then mask induced to a surgical plane of anesthesia with Entrane (2%) and N2O/O2 (1:1).

A tracheostomy was performed through an anterior, midline incision. Intubation was achieved with a standard, cuffed, 8-French endotracheal tube. The anesthesia gases were reconnected to the endotracheal tube and the tube was secured in place with umbilical tape. The tracheal incision was closed with surgical silk suture. The animal was al lowed to breathe spontaneously but was assisted, if required, by a volume-cycled ventilator at a respiratory rate of 16 breaths/min. Respiratory activity was monitored with a temperature-sensitive respirometer device mounted inline at the end of the endotracheal tube.

Bilateral cutdowns were performed at the medial aspect of the rear leg, exposing the femoral artery and vein. A pressure transducer was inserted via the femoral artery to the level of the thoracic viscera. This position was initially estimated by correlation to body surface anatomy and later confirmed to be in correct position at necropsy.

The surgical procedures were performed on a motorized examination table that allowed the animal to be rotated to the spine vertical position for the test [13]. Five-lead electrocardiographic monitoring vas established using wires placed under the skin and connected to a chart recorder and Holter monitor. Sternal and spinal accelerometers were placed under the skin and subcutaneous tissue at the level of impact, and secured in place with a silk suture. The animal was then fitted with a nylon sling that facilitated positioning in the spine vertical orientation. The sling also served to prevent excessive pooling of blood in the rear extremities by applying pressure to the lower abdominal region during the vertical position.

After surgical preparation, the animal was moved to the test area. The impacts were delivered by a pneumatically powered tube that propelled a standard baseball with an average weight of 150 + 5 g to a speed of 95 mph (42.8 m/s). This speed was previously determined by Viano et al. [14] to cause fatal injury in the animal model while impacts of lower velocity resulted in complete recovery.

The animal was rotated to the spine vertical orientation and the sternum positioned 12 in. from the impactor tube. The tube was directed anteroposterior to the animal, which enabled the baseball to strike the sternum one-third of the distance from the xyphoid to the manubrium. The animal was struck with the chest protector interposed between the surface of the sternum and the impacting ball. The hardest and the thickest chest protectors were utilized first with progressively thinner and softer chest protectors used in subsequent tests. After each impact, the animal was rotated back to the supine position and monitored until completion of the procedure. The test sequence (rotation/ impact/ rotation) was accomplished in approximately 10 min. Each animal was monitored after the chest impact. When the induced arrhythmia reverted to normal sinus rhythm, the animal was used for testing again. In some cases, the animal survived six impact tests. In other cases, the animals died after three or four tests, but the test matrix was completed to get a full set of biomechanical responses.

Data collected throughout each procedure included the respiratory minute rate and tidal volume, electrocardiographic monitoring using a chart and Holter recording apparatus, and spinal and sternal acceleration measurements used to estimate deflection/compression values. The decision to terminate an individual procedure was based on cardiac and respiratory responses.

Thoracoabdominal necropsy was performed on each animal and photographs of gross and microscopic organ pathology were taken.

6-Year-old child dummy
The thorax of a 6-year-old surrogate child dummy was modified as follows: The stiff chest structure was replaced with a crushable styrofoam padding that was representative of the resistance of the body. This was placed in front of the spine and was covered with a 0.5 in. piece of Ensolite to simulate overlying skin and subcutaneous tissue.

The dummy was suspended in the same orientation and apparatus as used in the animal surrogate, i.e., a vertical position approximately 12 in. from the pneumatically powered tube. This orientation allowed whole-body motion resulting from chest impact to occur and be measured.

The baseball was delivered anteroposterior to the midpoint of the thoracic area at the sternal surface. Acceleration of the thoracic spine and sternum adjacent to the site of impact was measured. The projectile velocity was 95 mph except for tests con ducted using chest protectors (see Fig. 4B) in which the projectile speed was 80 mph.

The crushable foam technique, using a load cell mounted to a rigid support, allowed direct measurement of the force and momentum transferred during impact by the various baseball types and the differences in momentum resulting from the use of chest protectors. Momentum transfer was determined by the integral of the impact force behind the sup ported styrofoam and indicates the momentum transfer to the load cell.

Fifth percentile female dummy
The Hybrid III dummy is a sophisticated anthropomorphic test device that includes chest, head, and neck structures that closely mimic the human response to frontal impact. The Hybrid III family of dummies includes the fifth percentile female dummy, which approximates the size of a 10-year old boy.

The Hybrid III female dummy can measure the viscous injury criterion (VC) [5]. This criterion relates to the risk of soft tissue and organ injuries to the body, including the risk of functional injuries to the heart such as ventricular fibrillation [11]. The VC is determined from chest compression secondary to blunt impact and the speed of that same compression.

Again, the baseball was delivered anteroposterior to the midpoint of the thoracic area at the sternal surface. In these tests, chest compression, VC, and sternal and spinal acceleration were measured with various baseball impacts and chest protectors.

RESULTS

Animal model
In the animal model, baseball impacts to the chest at 95 mph (42.8 m/s) produced death by cardiac arrhythmia and traumatic apnea when the standard hard ball was used with the generic closed cell foam chest protective devices (Table 2). When the softer core baseball was used, similar fatal arrhythmias occurred only without chest protection (Table 2). When the generic foam protectors were covered with a hard ABS plastic shell, only heart arrhythmias were found (Table 2). Sternal accelerations were lower with the chest covered by the protective vests, indicating a reduction in the 'sting" of the ball striking the chest.

Table 2. Result of baseball impact tests on Sus Scrofa



Sternal      Spinal
                       Test    Heart     acceleration  acceleration
                        no.   rhythms        (g)          (g)

Worth regular           [Baseball impact at 95 mph (42.8 m/s)]
  No protector          13    TWave-      1,029.00       31.30 
                        25    VFib*         571.90       45.20 
                        32    VFib*         595.80      117.40 
  0.50 in. DF Foam      24    VFib*         955.60       45.40 
                        30    VFib*         718.90      119.50 
  0.75 in. DF Foam      23    VTach-VFib    862.00       42.70 
                        29    VFib*         665.80      116.50 
  Sangmoo               22    VTach         721.70       41.60 
                        28    VFib*         277.50      103.10 
  ABS2 + 0.50 in. DF    21    VTach         944.20       54.60 
                        27    VTachVFib     450.60      113.60 
  ABS1 + 0.50 in. DF    20    AFib/ST+      921.20       45.20 
                        26    ST+           468.50       92.30 
RIF1 Baseball           [Baseball impact at 95 mph (42.8 m/s)]
  No protector          19    Asystole      599.30       41.90
                        38    VTach-VFib    716.10       62.60
  0.50 in. DF Foam      18    HBlock        514.20       45.00
                        37    ST+           455.30      116.90
  0.75 in. DF Foam      17    ST+           594.20       39.90
                        36    ST+           487.50       99.50
  Sangmoo               16    ST+           712.60       63.50
                        35    VTach-ST+     358.30      101.70
  ABS2 + 0.50 in. DF    15    ST+           486.80       55.70
                        34    Twave Inv     380.10      102.60
  ABS1 + 0.50 in. DF    14    ST+           492.80       49.60
                        33    ST+	    365.20      1O5.50

Cardiac rhythms, and sternal and spinal accelration results following 
baseball impact at 95 mph to the chests of sus scrofa (n = 6). 
For a detailed description of the baseballs and chest protectors used, 
see Table 1. Test numbers 14-25 were loosely 
supported torsos, whereas tests 26-38 were made with the animal tightly 
wrapped in the upright support. The impact tests conducted on each animal 
began with the thickest chest protector (ABS1 + 0.5 in DF). If a fatal 
arrhythmia did not result, the next thickest chest protection was used 
until no chest protector was used. In several cases, the animal experienced 
ventricular fibrillation but the test series was completed to collect 
biomedical data. The fatal outcome from an earlier test is noted with an 
asterisk.   TWave- = T wave depression, VFib = ventricular fibrillation, 
VTach = ventricular tachycardia, AFib = atrial fibrillation, ST+ = ST 
wave elevation, HBlock = complete heart block, TWave Inv = T-wave inversion 

Child crash dummy test
For the impact tests using the crushable styrofoam modified child crash dummy, we observed substantially reduced chest deformation with the use of chest protectors. However, we found similar, or even increased values for force and momentum transfer using three different chest protectors and a softer core baseball (Table 3). Using the standard hard ball and protective vests, the force increased 6-43% and the momentum delivered increased by 10-15% (Table 3). When a softer core ball plus protective vests were tested together, the impact force increased by 15-58% and the momentum delivered increased by 14-18% (Table 3).

The possible reasons for this are shown in the data in Fig. 1. The protective closed cell foams studied have a rate-dependent stiffness that is effective in reducing the depth of penetration of the ball; however, an increase in the impact force and a shortened impact duration can lead to an increase in the momentum transfer and power delivered (Fig. 1). Thus, reductions in acceleration and deflection may not provide complete protection from the functional effects of the ball impact.

Figure 2 gives the spinal acceleration data from impact tests using several baseball types. These data indicate that four of the highest values were obtained using softer core baseballs.

Figure 3 shows peak sternal and spinal acceleration results from impacts using a standard and a soft core baseball with the International Protective Athletic Safety Corporation vest interposed. In both cases, the vest reduced the sternal acceleration slightly but higher values were recorded with the softer core (RIF 1) baseball.

Table 3. Results of baseball impact tests on crushable styrofoam



Force       %        Momentum       %
			(kN)    Difference    kg*m/s    Difference

Regular Baseball	    
 No padding		3.37        -         7.41           -  
 0.75 in. DF Foam	4.07	  (+20)       8.49	   (+15)
 Sangmoo		3.56	  (+ 6)       8.12	   (+10)
 ABS + 0.5 in. DF	4.82	  (+43)       8.23	   (+11)
RIF I Baseball		    	                  	      
 No padding		3.15	  (- 7)       7.52	   (+ 2)
 0.75 in. DF Foam	4.49	  (+33)       8.75	   (+18)
 Sangmoo		3.87	  (+15)	      8.70	   (+17)
 ABS + 0.5 in. DF	5.33	  (+58)       8.41	   (+14)

Results of data comparing impact forces and momentum resulting from 95 mph 
impact with the standard baseball, a softer core baseball (RIF 1), and three 
separate chest protectors. For a detailed description of the baseballs and 
chest protectors used, see Table 1.


Fig. 1. Load cell measurements supporting crushable styrofoam
show the effect of various combinations of baseballs and protective
equipment. For a detailed description of the baseballs and chest protectors
used, see Table 1. A: Impact force measurements.
B: Impulse momentum computed as the integral of reaction force over
the time of impact.


Fig. 2. Peak spinal acceleration (g) data from the 6-year-old
child dummy impacted in the chest by various commercially available
baseballs.  For a detailed description of the baseballs and 
chest protectors used, see Table 1.


Fig. 3. Peak sternal (dark bars) and spinal (light bars) 
acceleration results from imapcts using a standard and a soft 
core baseball with the International Protective Athletic Safety 
Corporation (IPASC) vest interposed.

Hybrid III crash dummy tests
Detailed data on sternal acceleration, spinal acceleration, chest deflection, and the viscous response criterion are presented in Table 4. Measurements of the viscous response criterion using the fifth percentile Hybrid III female dummy with standard baseballs and softer core baseballs showed little protective effect alone or in combination with a variety of chest protectors (Table 4 and Fig. 4). In fact, one of the softer core baseballs (RIF 1) resulted in the highest viscous response observed (Fig. 4A). Protective foams as well as ABS hard shell covered forms had only a marginal effect in reducing levels of viscous response to baseball impact (Table 4).

Table 4. Results of baseball impact tests on fifth percentile Hybrid III female dummy



Chest
                          Sternal Spinal  Chest          defl.   VC
                    Test  accel.  accel.  defl.   VC     adj.    adj.
                     no.   (g)     (g)     (mm)  (m/s)   (mm)   (m/s)

0.50 in. AH Foam    
  Worth Regular      11    603      71     8.5   0.073   48.5   1.46
  Worth Regular      12    707      83     8.3   0.069   48.3   1.38
  RIF 1O             13    711      72     8.8   0.085   48.8   1.70
  RIF 10             14    744      71     8.7   0.082   48.7   1.64
  RIF 5              15    654      71     8.8   0.085   48.8   1.70
  RIF 5              16    730      72     8.9   0.088   48.9   1.76
  RIF 1              17    423      56     9.3   0.090   49.3   1.80
  RIF 1              18    491      48     9.3   0.097   49.3   1.94
Rawlings            
  0.25AH + 0.05ABS   19    455      55     8.7   0.076   48.7   1.52
  0.25AH + 0.05ABS   20    656      70     8.2   0.069   48.2   1.38
  0.50DF + 0.05ABS   21    484      36     7.6   0.061   47.6   1.22
  0.50DF + 0.05ABS   22    464      43     7.9   0.063   47.9   1.26
  0.25DF + 0.05ABS   23    530      46     7.8   0.060   47.8   1.20
  0.25DF + 0.05ABS   24    629      39     8.2   0.067   48.2   1.34
  0.50AH + 0.05ABS   25    507      29     7.6   0.063   47.6   1.26
  0.50AH + 0.05ABS   26    563      30     7.9   0.063   47.9   1.26
No protection       
  Rawlings                 760     100     9.6   0.093   49.6   1.86
  Rawlings                 780      90     8.8   0.071   48.8   1.42
  Worth Regular       2    762      82     9.3   0.096   49.3   1.92
  Worth Regular       3    820      67     8.8   0.076   48.8   1.52
  RIF 10              4    896      63     8.7   0.075   48.7   1.50
  RIF 10              5    844      69     8.5   0.072   48.5   1.44
  RIF 5               6    857      62     9.1   0.088   49.1   1.76
  RIF 5               7    867      66     9.4   0.103   49.4   2.06
  RIF 1               8    624      60     9.5   0.097   49.5   1.94
  RIF 1               9    492      57     9.4   0.097   49.4   1.94

Sternal and spinal acceleration (accel.) and absolute and adjusted (adj.)
values for chest deflection (defl.) and viscous response criterion 
(VC) in the Hybrid III female 
dummy. The adjusted deflection and viscous responses are computed by 
adding 40 mm for the skin deformation to the intemal deflection and 
multiplying the internal viscous response by 20 to approximate the 
external value of viscous loading by the baseball. For a detailed 
description of the baseballs and chest protectors used, see 
Table 1.


Fig. 4. Viscous response criterion values computed from chest
deflection data obtained using the fifth percentile Hybrid III dummy
to test different baseballs (A) and chest protectors using the 
standard baseball (B). For a detailed description of the baseballs and 
chest protectors used, see Table 1.

Electrocardiography
In the animal model, when various foam vests consisting of ABS plastic and 0.5 in. foam were tested using the hard ball, the electrocardiographic changes were suggestive of ischemic changes and progressed to an arrhythmia pattern such as ventricular fibrillation with less protection. It should be noted that in testing the same type of plastics and foams with the softer core baseballs, ventricular fibrillation or asystole did not occur but arrhythmias were observed (Table 2).

Pathology
The gross pathological changes in one test were consistent with a cardiac contusion extending from the epicardium through the myocardium into the endocardium. Minor contusions were found when using either the softer core baseballs or the hard ball. The photomicrographs were consistent with epicardial hemorrhage along with interstitial hemorrhage in the myocardium and, in some eases, hemorrhage in the subendocardium (Fig. 5). However, myocardial contusion was neither a frequent or necessary injury related to death in these or the previous animal tests [13].


Fig. 5. A: Gross pathologic findings in the pig heart consistent
with a full-thickness cardiac wall contusion. B: Micropathology 
of the same pig heart following a baseball impact. The findings 
are consistent with epicardial hemorrhage, interstitial, and 
subendocardial hemorrhage.

DISCUSSION

The events leading to this study relate to the sudden, unexplained deaths of children from blunt chest trauma in baseball. Case files of the deaths have indicated that most of the fatal injuries display no gross physiologic findings at autopsy C.7,12). Similarly, in the animal model of baseball impact to the chest previously reported, few significant anatomical or pathological changes were found [13].

It was the purpose of this study to examine the effects of soft core baseballs and commercial chest protectors in reducing the forces from baseball impacts. In order to do this, three experimental models were used: (i) an animal model using the swine sus scrofa, (ii) a child crash dummy, and (iii) a fifth percentile Hybrid III female crash dummy. Al though several independent measurements may be used to define the severity of impact, the single best measurement is the VC. The exposures in this study resulted in an average approaching VCmax = 2.0 m/s, which is within the range for producing life threatening injury by high-speed impact [5].

In the animal study (Table 2), utilizing a standard baseball and one softer core baseball with a variety of generic chest protectors, serious arrhythmias and death occurred with the softer core (RIF 1) base balls but the arrhythmias were generally of lower severity.

Impact studies of softer core baseballs compared with two models of regular baseballs indicated little protective benefit with the softer core versions. For example, it was found that the RIF I baseball impact did not sufficiently reduce the momentum transfer compared with the standard baseball (Fig. 1). In addition, the softer core baseball RIF 1 was found to produce a slightly increased average momentum in the unprotected chest condition in comparison to the standard hard ball (Fig. 1). Although the softer core baseball reduced sternal and to a lesser extent spinal acceleration in the animal model (Table 2) and fifth percentile Hybrid III female dummy (Table 4), acceleration is not as meaningful an indicator of chest injury as the viscous response or chest deflection [12]. An examination of chest deflection and viscous response data (Table 4, Fig. 4) indicates that these values are as high or higher in the softer core baseballs. As illustrated in Fig. 2, spinal acceleration, an indicator of energy transfer to the viscera, was greatest with several of the softer core baseballs.

Likewise, little protective effect was found with the use of commercially available chest protectors. In the crushable foam tests, increased momentum transfer was found when these protectors were used (Table 3). When utilizing the standard hard ball and closed cell foam padding, the force in creased between 6 and 43% and the momentum in creased between 10 and 15%. When a softer core baseball was utilized with chest padding, the force measured increased between IS and 58% and the momentum increased between 14 and 18%. These results suggest that the closed cell foam protection may actually act as a conductor of energy and therefore potentiate impact injury.

CONCLUSION

All the tests and comparisons, either in the animal model, the child dummy, or the S% Hybrid III female dummy, failed to demonstrate a significant advantage with respect to impact force reduction using softer core baseballs or a variety of chest protectors. In some cases, the protective equipment exacerbated the baseball impact effects. Currently, the efforts of manufacturers are focused on the development of softer baseballs and various types of chest protection to eliminate chest impact injuries and fatalities. However, to date, no effective preventive approach has been developed to eliminate or significantly reduce chest impact fatalities in the pediatric population exposed to the rare direct, high-speed baseball impacts. Further research and development of products must be instituted with emphasis placed on independent testing utilizing reproducible biomechanical models as described in the present study.

Acknowledgments

The authors wish to express their appreciation to Mr. Howard Bender for his assistance with the animal model. Thanks must also go to Mr. Todd Townsend for his skill and creativity in the modification of the child dummy, and especially Mr. Joseph McCleary for his diligence in assisting with the impact tests. In addition, the authors express their appreciation to Dr. T. Wentz for her help with the pathological specimens in this study and Dr. Kurt Holland for his skill with and interpretation of the electrocardiograms. The authors would like to thank Mr. John Marcello and Danmar Products for their help and guidance in procuring and developing the generic materials utilized in this study.

REFERENCES

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This article was published as:
"An Analysis of Preventative Methods of Baseball 
Induced Chest Impact Injuries"
Clinical Journal of Sports Medicine
Vol. 2, 1992, pp. 172-9
Janda DH, Viano DC, Andrzejak DV, Hensinger RN

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Copyright 2001 The Institute for Preventative Sports Medicine. All rights reserved.