Weakening of the Active Muscle Restraint Against Valgus Loading During Repeated Baseball Pitching: Direct Evidence of Fatigue in Dynamic Valgus Stabilizers of the Elbow Joint in Collegiate Baseball Pitchers

Background: Although research has shown the effect of repeated pitching on throwing arm strength, no studies have examined the weakening in the overall ability of dynamic stabilizers to restrain the medial elbow against valgus loading. Fatigue in the dynamic stabilizers increases reliance on the ulnar collateral ligament (UCL) to withstand the valgus loads during pitching. Hypothesis: It was hypothesized that (1) the muscular varus strength weakens as the pitch count accumulates and (2) the muscular varus strength becomes insufficient to overcome the valgus loads in later innings. Study Design: Descriptive laboratory study. Methods: A total of 28 collegiate baseball pitchers threw 7 innings of 15 pitches in a bullpen. Seven genlocked cameras were used to record pitching performances, and the elbow valgus load was determined. Muscular varus strength was measured before the first inning and after completing the fourth and seventh innings using an isokinetic dynamometer system and an ultrasound device. A mixed-effect model was used to examine changes over the innings in the valgus load and varus strength, and t tests were used to compare the varus strengths and the largest valgus load in corresponding innings. Results: Neither the largest valgus load of the inning nor the mean valgus load of the inning significantly changed across the innings, whereas the varus strength was reduced significantly after completing the fourth and seventh innings ( P < .001). The varus strength was significantly greater than the mean valgus load in the first inning ( P < .008) but not in the later innings. Conclusion/Clinical Relevance: Pitchers’ medial elbow muscular varus strength decreased by a mean of 10% after 7 innings of 15 pitches, while motion-dependent valgus load remained fairly constant throughout the 7 innings, which could indicate increased reliance on the UCL to withstand the valgus loads in later innings.