Predicting Shoulder Force to Prevent Injury: Body Fat Mass Increases Shoulder Stress among Pitchers

The purpose of this study was to examine the relationship between whole body and throwing arm composition and length with throwing shoulder distraction and anterior force. Literature shows a relationship between shoulder force and pain; therefore, as well as between body mass and pain, it was hypothesized that pitchers with greater body fat mass would display increased throwing shoulder force during the acceleration phase of the softball pitch.

Thirty-eight high school softball pitchers from southern United States agreed to participate (1.69 ± 0.07 m, 74.71 ± 16.36 kg, 15.2 ± 1.1 yr). All procedures were completed within one visit. Pitchers underwent a dual-energy x-ray absorptiometry scan then threw fastballs at regulation distance while motion was measured using an electromagnetic motion capture system. Data from the three fastest pitches were averaged and analyzed. Peak shoulder distraction and anterior force during the acceleration phase of the softball pitch were extracted and used as dependent variables in two separate regression analyses. Whole body and throwing arm fat mass and lean mass, as well as throwing arm length were entered as independent variables into both regression equations.

The first regression equation was statistically significant (F(5,37) = 13.28, P < 0.001) and explained approximately 62% of the variance in peak shoulder distraction force (R2 = 0.675, adjusted R2 = 0.624). Increased whole body fat mass (B = 13.76, SE = 5.39, β = 0.91, t = 2.553, P = 0.016) significantly predicted peak throwing shoulder distraction force. The second regression equation predicting peak anterior force was not significant.

Increased fat mass may exacerbate the high force observed at the throwing shoulder and further increase injury susceptibility to pitchers.