Interrelationships of selected mechanical and biochemical properties of hindlimb extensor muscles following low thoracic cord transection were studied. Kittens were spinalized (Sp) at 2 wk and maintained for 6-12 mo. Some Sp animals were exercised (Sp-E) on a treadmill 25-30 min/day, 5 days/wk. In situ contractile properties of the slow-twitch soleus (SOL) and fast-twitch medical gastrocnemius (MG) muscles of normal (N), Sp, and Sp-E cats were determined. Exercise did not affect most parameters; thus Sp and Sp-E groups are considered collectively. The cross-sectional areas (CSA) of the SOL and MG decreased by 43 and 32%, respectively. Specific tension (tension/CSA) was maintained in the SOL but decreased (P less than 0.05) in the MG. Contraction time (CT) and half-relaxation time were significantly shorter in the SOL but unchanged in the MG. Maximum shortening velocity (Vmax) and myosin ATPase (mumol X mg-1 X min-1) increased (P less than 0.05) in the SOL of both groups and the MG of Sp. Frequency-tension responses of both muscles shifted toward that resembling a "faster" muscle. These results substantiate the existence of relatively independent regulatory mechanisms for Vmax and CT and show that myosin ATPase levels are more closely related to Vmax than CT. Although the changes in the SOL were consistent with the hypothesis that slow fibers are converted to fast, the elevated Vmax and myosin ATPase of the MG suggest that significant changes also occur within a "fast" fiber-type category.