We examined the sensitivity of internal tibial forces and moments during running to different subtalar/ankle moment constraints in a static optimization routine. Seventeen participants ran at 2.20, 3.33, and 4.17 ms-1 while force and motion data were collected. Ankle joint contact force was estimated using inverse-dynamics-based static optimization. Three sets of joint moment constraints were tested. All sets included the flexion-extension and abduction-adduction moments at the hip and the flexion-extension moment at the knee, but differed in the constraints used at the subtalar/ankle: 1) flexion-extension at the ankle (Sag), 2) flexion-extension and inversion-eversion at ankle (Sag+Front), and 3) flexion-extension at the ankle and supination-pronation at the subtalar (Sag+SubT). Internal tibial forces and moments were quantified at the distal one-third of the tibia, by ensuring static equilibrium with applied forces and moments. No interaction was observed between running speed and constraint for internal tibial forces or moments. Sag+SubT resulted in larger internal mediolateral force (+41%), frontal (+79%), and transverse (+29%) plane moments, compared to Sag and Sag+Front. Internal axial force was greatest in Sag+Front, compared to Sag and Sag+SubT (+37%). Faster running speeds resulted in greater internal tibial forces and moments in all directions (=+6%). Internal tibial forces and moments at the distal one-third of the tibia were sensitive to the subtalar and ankle joint moment constraints used in the static optimization routine, independent of running speed.

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