The effects of immobilization, after lower leg fracture, on the contractile properties of human triceps surae.

The contractile properties of the triceps surae were evaluated in 11 patients after unilateral fracture of the lower leg and subsequent immobilization for 135 +/- 68 days. Calf muscle cross-sectional area (plus bone: CSA) was assessed from anthropometric measurement. It was shown that the injured leg had a faster time to peak tension and increased half-relaxation time (1/2 RT); twitch force (Pt) was reduced by 25%. Evoked maximal tetanic tensions (Po) at 10 and 20 Hz were reduced by 51% and 46% respectively compared with the uninjured leg. The force of a maximal voluntary contraction (MVC) was also reduced, by 50%, but calf circumference and CSA were only 5% and 16% respectively lower in the injured leg. It was concluded that the changes in contractile speed may indicate a relatively greater atrophy of slow (type I) muscle fibres. The relationship between CSA and tension generation in the injured limb was shown to be poor after immobilization and during recovery. Anthropometric estimation of CSA does not appear to reflect the degree of muscle wasting, as indicated by reduced tension development after immobilization.