BACKGROUND: Chronic unloading and restricted activity are distinctly different processes, i.e., unloading completely removes the load on postural muscles, whereas restricted activity allows for loading of postural muscles. There are limited data available on the effects of restricted activity on skeletal muscles. Thus the effects of restricted activity on the properties of the slow soleus and fast plantaris muscles in rats were examined.METHODS: Eight-week-old rats were housed for 21 d in normal-sized (control group) or in small-sized (restricted group) cages.RESULTS: Decreased mRNA levels of peroxisome proliferator-activated receptor γ coactivator-1α (81 and 85% of control values) and reduced succinate dehydrogenase activity (85 and 88% of control values) were observed in the soleus and the plantaris muscles of the restricted group, respectively. Increased mRNA levels of forkhead box-containing protein O1 (128% of control values), decreased muscle weight (74% of control values), and reduced cross-sectional areas of type IIA (89% of control values) and type IIB (80% of control values) fibers were observed in the plantaris muscle of the restricted group.DISCUSSION: Restricted activity decreased the mRNA levels of peroxisome proliferator-activated receptor γ coactivator-1α and increased the mRNA levels of forkhead box-containing protein O1, which are associated with reduced oxidative capacity and atrophy, respectively, in the muscles. The plantaris muscle was more affected by restricted activity than the soleus muscle, most likely reflecting a greater relative change in the normal activity pattern in the fast than slow plantarflexor muscle.Takemura A, Roy RR, Edgerton VR, Ishihara A. Biochemical adaptations in a slow and a fast plantarflexor muscle of rats housed in small cages. Aerosp Med Hum Perform. 2016; 87(5):443–448.