Influence of complete spinal cord injury on skeletal muscle mechanics within the first 6 months of injury

In this study we examined the influence of complete spinal cord injury (SCI ) on the mechanical characteristics of skeletal muscle in vivo within 6 mon ths of the injury. Surface electrical stimulation (ES) was applied to the l eft m, quadriceps femoris of patients at 6, 11 and 24 weeks after injury. S urface ES was also applied to seven able-bodied controls (AB) at two time p oints 18 weeks apart. ES consisted of 2 bouts of 20, 1-s isometric contract ions with 2 s and 2 min of rest between contractions and bouts, respectivel y. The time from 20- 80% of peak torque (rise time) and the half relaxation time (1/2 RT) were determined for the first and for the last few contracti ons. Force loss over repeat contractions was greater in SCI than AB (27% vs 95%; P = 0.0001), and did not change over the 18-week period. Rise time di d not change over repeat contractions, was not different between groups, an d nor did it change over the 18-week period (range: 150-172 ms). 1/2 RT sho wed several group differences. Overall, 1/2 RT was longer at the beginning of ES in SCI than AB [mean (SE) 133 (15) ms vs 90 (6) ms, P = 0.037]. Slowi ng of relaxation time with force loss over repeat contractions was found in SCI at 24 weeks after injury [167 (18) ms, P = 0.016], but not at 6 [128 ( 14)ms] or 11 [145 (12) ms] weeks after injury. AB, in contrast, showed prol onged relaxation times, with force loss at both time points [115 (10) ms an d 113 (11) ms; P = 0.0001]. The results indicate that SCI alters the relaxa tion but not contractile properties of mixed skeletal muscle within the fir st 24 weeks of injury. Altered calcium handling and contraction-induced fib er injury are suggested to explain the slower relaxation time per se, and t he prolonged relaxation with force loss observed after SCI.