Paper 19: Fatigue of the Dynamic Stabilizers of the Medial Elbow in Baseball Pitchers
Authors: Kwiecien Susan , Fink Andrew , Brown Kenneth , McHugh Malachy , Mullaney Michael 
Published: August 2023

Objectives: The flexor carpi ulnaris (FCU) and flexor digitorum superficialis (FDS) are thought to provide dynamic stability to the medial elbow, with a lesser contribution from the pronator teres (PT). The purposes of this study were to: (1) determine whether middle finger (MF) and/or ring finger (RF) flexion strength tests selectively activate the FCU and FDS, and (2) determine if MF or RF fatigue are more sensitive measures of pitching fatigue than grip strength. It was hypothesized that MF and RF flexion strength tests would selectively activate the FCU and FDS and that MF and RF fatigue during a pitching performance would occur earlier and be more pronounced than grip fatigue. Methods: Surface electromyographic (EMG) signals were recorded from the FCU, FDS and PT during hand-held dynamometry testing of MF and RF flexion strength in 10 healthy men (36±12 yr). EMG amplitudes, expressed as percent of maximal voluntary contraction (MVC) based on standard MVC tests for each muscle, were compared between tests and muscles with repeated measures analysis of variance (ANOVA). Field Testing was performed in 18 baseball pitchers (age 17±4 yr) throwing 4 innings of a simulated game (16 pitches per inning plus 5 warm up pitches each inning; 10 fast balls, 4 curve balls, 2 change ups per inning). MF flexion, RF flexion, and grip strength were recorded prior to the game and after each inning. A standard full grip test (FG) and a modified 3 finger grip test (3F) were employed. Fatigue responses for the 4 tests (MF, RF, FG, 3F) were compared between tests with 4x4 (innings by test) repeated measures ANOVA. Additionally, fatigue was classified as marked (>20% loss), moderate (10-20% loss) or minimal (<10% loss). It was estimated that with 18 pitchers there would be 80% power to detect a 10% difference in fatigue between tests (P<0.05). Results: The MF and RF tests activated the FCU (MF 66%, RF 93%) and FDS (MF 74%, RF 61%) more than the PT (MF 35%, RF 23%; muscle effect P=0.008) indicating that both tests targeted the dynamic stabilizers of the medial elbow. In the baseball pitchers MF strength was greater on the dominant versus nondominant hand (7% P=0.039). There was no hand dominance effect for the other three tests (RF P=0.154, FG P=0.790, 3F P=0.897). The fatigue responses differed significantly between tests (P<0.001). After the fourth inning MF fatigue (21%) was greater than RF (7%, P<0.001), FG (5%, P=0.002) and 3F (5%, P<0.001) fatigue. MF fatigue was evident early and was progressive. After the first inning 4 pitchers (22%) had marked MF fatigue and 3 (17%) had moderate MF fatigue. By the end of the fourth inning 10 pitchers (56%) had marked MF fatigue and 6 (33%) had moderate MF fatigue. By contrast, after four innings only 5 pitchers (28%) had marked RF fatigue, and only 3 pitchers (17%) had marked FG or 3F fatigue. Conclusions: The MF flexion test is a valid test of fatigue in the dynamic stabilizers of the medial elbow during a baseball pitching performance. After only 4 innings more than 50% of pitchers had marked MF fatigue and 89% had some evidence of MF fatigue. MF flexion training to improve fatigue resistance is recommended to maintain dynamic stability of the medial elbow during a pitching performance.