Hindlimb immobilization: length-tension and contractile properties of skeletal muscle.

The effect of hindlimb immobilization (IM) on the contractile properties of fast and slow skeletal muscle was studied in rats following various periods of IM ranging from 1 to 42 days; muscle atrophy, muscle, fiber, and sarcomere length, and the length-tension characteristics were determined after 42 days of IM. The slow-twitch soleus (SOL), the fast-twitch extensor digitorum longus (EDL), and the fast-twitch superficial region of the vastus lateralis (SVL) all showed rapid atrophy following the onset of IM, reaching a new reduced steady-state weight by day 21. After 42 days of IM the passive tension (g) and active twitch tension (g/cm2) plotted vs. muscle length (cm) were shifted to the left for the slow-twitch SOL, indicating a decreased extensibility compared with control muscles. The peak tetanic tension of the slow SOL declined to 47% of the control level of 2,893 +/- 125 g/cm2, whereas the fast EDL maintained 72% of its initial force of 4,392 +/- 229 g/cm2, and the fast SVL was unaltered by IM. Peak twitch tension and peak rate of tension development and decline fell rapidly in the slow SOL while remaining relatively unaltered in the fast-twitch muscles. Surprisingly, maximal isotonic shortening velocity was elevated in both fast and slow muscles with IM. These results indicate that IM produces muscle atrophy in fast as well as slow skeletal muscle and, in addition, causes fiber type-specific changes in the contractile properties.