Muscles of ky/ky homozygote mice exhibit neonatal muscle fiber necrosis and regeneration with subsequent motor nerve sprouting and development of a prominent kyphoscoliosis from approximately 100 days onward. Soleus and extensor digitorum longus (EDL) muscles from ky mice weighted < 50% of control muscles from age-matched NMRI mice. Maximal tetanic force was more reduced in soleus than in EDL. In EDL, the velocity constant of the force-velocity relation, maximal velocity, twitch time-to-peak, and isomyosin content were normal at all ages. The early mechanical changes seen in ky soleus muscles (47 day) were not accompanied by significant alterations in isomyosin or myosin heavy- and light-chain composition, since ky and NMRI expressed slow-twitch native myosin 2 (SM2, type I fibers) and intermediate-twitch native myosin (IM, type IIa fibers). Adult ky soleus (172 day) showed wholesale loss of IM and sole expression of SM2. This is sufficient to account for the markedly slowing of the force-velocity relation and the twitches observed in adult ky soleus. We propose that since shifts in muscle type only occurred in soleus, this reflects the persistent requirement to withstand the force of gravity.