Biomechanical Comparison of the Interval Throwing Progression and Baseball Pitching: Upper Extremity Stresses in Training and Rehabilitation

OBJECTIVES: The interval throwing progression is a hallmark of the rehabilitation program designed for baseball pitchers or position players returning from shoulder or elbow injury. It typically begins with flat-ground throws at a short distance and progressively increases to 180 feet or more. For pitchers, this phase is then followed by throwing off the mound, progressing from partial-effort to full-effort pitches. Theoretically, the progression of throwing phases allows an injured athlete to gradually recover his flexibility, arm strength, and mechanics while moving from less stressful activities to more stressful activities. While this throwing program has been a part of baseball rehabilitation and conditioning for decades, little is known about the biomechanical stresses generated during flat-ground throwing or variable effort pitching off the mound. METHODS: Twenty-nine healthy, college baseball pitchers were analyzed using a quantitative motion analysis system. The participants threw from flat ground at distances of 60-ft, 90-ft, 120-ft, and 180-ft, being instructed to throw “hard, on a horizontal line”. The pitchers then threw fastballs from a mound at 3 different efforts: 60% effort, 80% effort, and full-effort. Biomechanical parameters of position, velocity, and kinetic values were recorded. Mean values were calculated for humeral internal rotation torque (HIRT) and elbow valgus load (EVL) for each throw type. This data was then used to compare shoulder and elbow stresses between the various throws. The differences among mean values were analyzed with a repeated-measures analysis of variance (ANOVA). Post hoc paired t tests were performed when the ANOVA revealed a significant difference. RESULTS: Statistically significant differences exist across all mound intensities (60%, 80%, and 100% effort) for nHIRT (p=0.03) and nEVL (p=0.04), as both parameters increased with percentage throwing effort. No statistically significant differences were found across all flat ground distances in either nHIRT or nEVL (p>0.05). No statistically significant difference in peak HIRT or peak EVL were found when comparing full effort pitching to flat ground throws at any distance (p>0.10). The longer flat ground throws at 180 feet did show significantly different kinematics and biomechanical patterns when compared with pitching from the mound, while shorter flat ground throws had patterns similar to those of pitching. CONCLUSION: Variable effort pitching off the mound demonstrates significantly lower stresses on the shoulder and elbow during partial-effort throws, illustrating the importance of these throws during the recovery and rehabilitation process. Flat ground throwing at distances as short as 60 feet had similar biomechanical stresses on the upper extremity when compared with pitching full-effort from the mound. Despite lower velocity, this similar stress illustrates the mechanical disadvantage of throwing from the flat ground in a stationary position. No increase in shoulder or elbow stress was seen with increasing distances from the flat ground, likely because the pitchers began using a “crow-hop” for the longer distances, facilitating the throw with their lower extremity. The mechanical advantage of throwing from a mound or using the crow-hop may be protective during rehabilitation and training throws, generating lower humeral internal rotational torque, lower elbow valgus load, and more throwing efficiency.