Spinal cord injury and contractile properties of the human tibialis anterior

The purpose of this study was to evaluate contractile properties of the tibialis anterior of paralyzed and non-paralyzed subjects. The contractile properties and the fatigability of the tibialis anterior muscle (TA) were tested in 8 spinal cord injured (SCI) and 8 control individuals. The TA was stimulated at frequencies from 10 to 100 Hz to determine a force-frequency curve. A fatigue bout was also performed by stimulating the muscle at 40 Hz every two seconds for three minutes. The SCI muscles produced lower forces overall, but higher forces relative to maximal force at lower frequencies, shifting the force-frequency curve of the SCI group to the left. The half-relaxation time and rate of relaxation at 40 Hz was slower in the SCI muscles than in the control muscles (127 ± 18.4 ms vs. 78 ± 8.7 ms, 6 ± 1.5 kg·s^-1 20 ± 4.1 kg·s^-1 respectively). In addition, force loss and slowing of relaxation during the fatigue protocol were not significantly different between the two groups due to high variability in the SCI group. The TA of the SCI group had slower contractile properties than the control group and fatigability was not significantly different between the SCI and control group. The protocol may be useful to assess training effects during rehabilitation of paralyzed muscle.