Baseball and Softball Pitchers are Distinct Within-Subject Controlled Models for Exploring Proximal Femur Adaptation to Physical Activity

Within-subject controlled models in individuals who preferentially load one side of the body enable efficient exploration of the skeletal benefits of physical activity. There is no established model of physical activity-induced side-to-side differences (i.e., asymmetry) at the proximal femur. Proximal femur asymmetry was assessed via dual-energy X-ray absorptiometry in male jumping athletes (JMP, n = 16), male baseball pitchers (BB, n = 21), female fast-pitch softball pitchers (SB, n = 22), and controls (CON, n = 42). The jumping leg was the dominant leg in JMP, whereas in BB, SB and CON the dominant leg was contralateral to the dominant/throwing arm. BB and SB had 5.5% (95% CI 3.9–7.0%) and 6.5% (95% CI 4.8–8.2%) dominant-to-nondominant leg differences for total hip areal bone mineral density (aBMD), with the asymmetry being greater than both CON and JMP (p < 0.05). BB and SB also possessed dominant-to-nondominant leg differences in femoral neck and trochanteric aBMD (p < 0.001). SB had 9.7% (95% CI 6.4–13.0%) dominant-to-nondominant leg differences in femoral neck bone mineral content, which was larger than any other group (p ≤ 0.006). At the narrow neck, SB had large (> 8%) dominant-to-nondominant leg differences in cross-sectional area, cross-sectional moment of inertia and section modulus, which were larger than any other group (p ≤ 0.02). Male baseball and female softball pitchers are distinct within-subject controlled models for exploring adaptation of the proximal femur to physical activity. They exhibit adaptation in their dominant/landing leg (i.e., leg contralateral to the throwing arm), but the pattern differs with softball pitchers exhibiting greater femoral neck adaptation.