DECELERATION PHASE MOMENTS: A POTENTIAL CAUSE FOR ELBOW INJURY IN COLLEGIATE LEVEL BASEBALL PITCHERS

Background: Over the past three decades there has been an increase in the incidence of elbow and shoulder pain experienced by baseball pitchers, which can limit or lead to an end of pitching activities. Although there are a number of theories that suggest poor pitching mechanics or throwing breaking pitches prior to skeletal maturity may be the cause, biomechanical investigations have yet to elucidate a single cause for this rise in injuries. It is also well established that the highest stresses and fastest angular velocities experienced by pitchers occurs during the acceleration phase of the pitch cycle, which has led to extensive biomechanical investigations of this portion of the pitching cycle. However, the deceleration phase of the pitch, although 150% longer than the acceleration phase, still requires an abrupt reversal of motion to allow pitchers to get into a fielding position after they have delivered the pitch. Therefore, the purpose of this study was to determine if the elbow joint was subjected to an additional varus stress during the deceleration phase of the pitch cycle. Methods: NCAA Division I and Division III baseball pitchers were recruited for this study and underwent a comprehensive biomechanical pitching evaluation. All pitchers were injury free at the time of data collection and reported no history of an upper extremity injury within the previous six months of the analysis date. Additionally, all pitchers had at least two years of pitching experience. All participants pitched from a 10” mound towards a target with a designated strike zone set 60’6” away. Kinematic data was collected using a 12-camera motion capture system, and kinetic data was calculated using standard inverse dynamic techniques. The typical pitching cycle, starting with lead foot contact and ending with maximum internal rotation of the glenohumeral joint (MIR), was expanded to end when the pedestal foot reached its maximum height; allowing for the analysis of deceleration phase moments at the elbow joint. The deceleration phase elbow varus (EV) moment was compared across multiple pitch types (i.e. fastball, curveball, slider, and change-up) using the type III effects from a random intercept mixed effects model. Additionally, the deceleration phase EV moment was compared to the peak EV moment occurring during the acceleration phase of the pitch cycle. Results: The results of this study are based on 87 baseball pitchers with a mean age of 19.9 ± 1.4 years. All participants pitched a fastball, 78 pitched a curveball, 31 pitched a slider, and 60 pitched a change-up. The results indicated that there was the presence of an elbow varus moment for all pitch types that occurred during the deceleration phase of the pitching cycle after MIR that was on average about half of the peak acceleration phase moment (Table 1). Overall 26% of pitchers pitching a fastball, 33% of pitchers throwing a curveball and change-up, and 55% of pitchers throwing a slider had deceleration EV moments greater than 50% of their peak acceleration phase EV moment. There was a statistically significant difference in the number of pitchers with a deceleration phase EV moment greater than half of the acceleration phase EV moment when pitching the slider when compared to the other pitch types (p=0.029). Conclusion/Significance: The majority of pitching biomechanics research focuses on the acceleration phase of the pitching cycle because the highest speeds and moments are achieved during this portion of the pitch. However, the pitcher’s need to rapidly decelerate during the pitch does expose them to an additional elbow varus moment. This additional moment could be a potential source of injury as it is a second stress exposure for the UCL. Additionally, given that the highest deceleration EV moments were noted in the slider this may potentially explain why pitchers and coaches believe that sliders are more harmful than other pitch types. [Table: see text]