The chemomechanical coupling mechanism in striated muscle contraction was examined by changing the nucleotide substrate from ATP to CTP. Maximum shortening velocity [extrapolation to zero force from force-velocity relation (V_max) and slope of slack test plots (V₀)], maximum isometric force (P_o), power, and the curvature of the force-velocity curve [a/P_o (dimensionless parameter inversely related to the curvature)] were determined during maximum Ca²⁺-activated isotonic contractions of fibers from fast rabbit psoas and slow rat soleus muscles by using 0.2 mM MgATP, 4 mM MgATP, 4 mM MgCTP, or 10 mM MgCTP as the nucleotide substrate. In addition to a decrease in the maximum Ca²⁺-activated force in both fiber types, a change from 4 mM ATP to 10 mM CTP resulted in a decrease in V_max in psoas fibers from 3.26 to 1.87 muscle length/s. In soleus fibers, V_max was reduced from 1.94 to 0.90 muscle length/s by this change in nucleotide. Surprisingly, peak power was unaffected in either fiber type by the change in nucleotide as the result of a three- to fourfold decrease in the curvature of the force-velocity relationship. The results are interpreted in terms of the Huxley model of muscle contraction as an increase in f₁ and g₁ coupled to a decrease in g₂ (where f₁ is the rate of cross-bridge attachment and g₁ and g₂ are rates of detachment) when CTP replaces ATP. This adequately accounts for the observed changes in P_o, a/P_o, and V_max. However, the two-state Huxley model does not explicitly reveal the cross-bridge transitions that determine curvature of the force-velocity relationship. We hypothesize that a nucleotide-sensitive transition among strong-binding cross-bridge states following P_i release, but before the release of the nucleotide diphosphate, underlies the alterations in a/P_o reported here.