Poster 161: Musculoskeletal Adaptations to Baseball Pitching Differ Markedly Between Righthand Dominant Versus Lefthand Dominant Pitchers
Authors: Mullaney Michael , Kwiecien Susan , Tyler Timothy , McHugh Malachy 
Published: August 2023

Objectives: Musculoskeletal adaptations on the dominant arm of baseball pitchers are typically (1) a gain in glenohumeral external rotation (ER) range of motion (ROM), (2) a loss of glenohumeral internal rotation (IR) ROM, (3) posterior shoulder tightness, (4) weakness in the empty can test, and (5) increased IR strength. The purpose of this study was to determine if these adaptations differed between righthand dominant (RHD) versus lefthand dominant (LHD) pitchers. Based on recent work it was hypothesized that ROM adaptations would be less apparent in LHD pitchers, but strength adaptations would not differ between RHD and LHD pitchers. Methods: Shoulder ROM and strength were measured in 327 baseball pitchers in preseason screening (91 were tested on more than one season, 418 player-seasons, 322 RHD vs. 96 LHD). There were 157 youth pitchers (age 11-15 yr) 146 adolescent pitchers (age 16-17 yr) and 115 adult pitchers (age 18-24 yr). ER ROM, IR ROM and posterior shoulder ROM (Tyler test) were measured with a digital level. Empty can, ER, IR and scapular retraction strength were measured with a hand-held dynamometer. Differences between the dominant and nondominant arms, between RHD and LHD pitchers, and between age groups, were assessed using mixed model analysis of variance (mean and SD reported). Results: ROM Adaptations RHD pitchers had 11±10° more ER ROM on the dominant versus nondominant arm (P<0.001) but LHD pitchers had no ER ROM difference (0±10°, P=0.685). The difference in ER ROM between RHD and LHD pitchers (P<0.001) was unaffected by age (P=0.829). IR ROM loss on the dominant versus nondominant arm for RHD pitchers (10±11°) was greater (P<0.001) than for LHD pitchers (5±10°). This difference was apparent for youth pitchers (RHD 9±9° vs. LHD 1±9°, P<0.001) and adolescent pitchers (RHD 12±10 vs. LHD 5±11°, P<0.001) but not for adult pitchers (RHD 9±14° vs. LHD 8±10°, P=0.863; RHD vs. LHD by Age Group P=0.040). Total ROM (ER+IR ROM) adaptations also differed between RHD and LHD pitchers (P<0.001) and this effect was more apparent in the adult pitchers (Age Group by RHD vs. LHD P=0.027): RHD adult pitchers had a 4±15° gain in total ROM on the dominant side (P=0.007) while LHD adult pitchers had a 7±10° loss in total ROM on the dominant side (P=0.002). Posterior shoulder tightness on the dominant side was affected by arm dominance and age (P=0.038); for youth and adolescent pitchers there was no difference between RHD and LHD pitchers (youth RHD 6±13° vs. LHD 10±11°, P=0.156; adolescent RHD 5±10° vs. LHD 8±15°) but for adult pitchers, posterior shoulder tightness was greater for RHD versus LHD pitchers (6±8° vs. 1±11°, P=0.032). Strength Adaptations For the empty can test, RHD pitchers were weaker on the dominant versus nondominant arm (5±15%, P<0.001) while there was no difference between the dominant and nondominant arms for LHD pitchers (3±16% stronger on dominant side P=0.100; RHD vs. LHD P<0.001). For IR strength, RHD pitchers were stronger on the dominant arm (7±17%, P<0.001) while there was no difference between the dominant and nondominant arms for LHD pitchers (2±21% P=0.360; RHD vs. LHD P=0.032). By contrast, for scapular retraction strength, LHD pitchers were stronger on the dominant arm (6±19%, P=0.007) while there was no difference between the dominant and nondominant arms for RHD pitchers (0±15% P=0.723; RHD vs. LHD P=0.003). Dominant versus nondominant ER strength was not different between RHD and LHD pitchers (P=0.323). Conclusions: The typical strength and ROM adaptations seen in baseball pitchers were mostly absent, or diminished, in LHD pitchers, but with some age dependent effects for ROM. Adult RHD pitchers had IR ROM loss and posterior shoulder tightness, but paradoxically adult LHD pitchers had IR ROM loss without associated posterior shoulder. The opposite was the case for youth and adolescent pitchers, with LHD pitchers having posterior shoulder tightness in the absence of IR ROM loss. Differences in strength adaptations between RHD and LHD pitchers have not been reported previously. Epidemiological studies examining the role of strength and ROM in injury risk for baseball pitchers have not previously accounted for differences between lefthanded and righthanded pitchers.