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 months of the injury. Surface electrical stimulation (ES) was applied to the left m, quadriceps femoris of patients at 6, 11 and 24 weeks after injury. Surface ES was also applied to seven able-bodied controls (AB) at two time points 18 weeks apart. ES consisted of 2 bouts of 20, 1-s isometric contractions with 2 s and 2 min of rest between contractions and bouts, respectively. 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 contractions. 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 did not change over repeat contractions, was not different between groups, and nor did it change over the 18-week period (range: 150-172 ms). 1/2 RT showed 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]. Slowing 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 prolonged relaxation times, with force loss at both time points [115 (10) ms and 113 (11) ms; P = 0.0001]. The results indicate that SCI alters the relaxation but not contractile properties of mixed skeletal muscle within the first 24 weeks of injury. Altered calcium handling and contraction-induced fiber injury are suggested to explain the slower relaxation time per se, and the prolonged relaxation with force loss observed after SCI.