The length–tension relationship of human dorsiflexor and plantarflexor muscles after spinal cord injury

Study design:A cross-sectional design.Objectives:To examine the length–tension relationship of dorsiflexion (DF) and plantarflexion (PF) muscle groups in seven individuals with chronic spinal cord injury (SCI; C2–T7; age 43±10.1 years) and compare it with a group of age and sex-matched able-bodied (AB) controls.Setting:McMaster University, Hamilton, ON, Canada.Methods:Isometric single twitch properties, 0.5-s tetanic contractions (SCI) and maximal voluntary contractions (AB) were measured at nine joint angles from 20° DF to 20° PF.Results:In the DF muscles, peak-evoked twitch (PT) torque occurred at 20° PF for SCI (3.4±1.1 N m) and AB (3.8±1.4 N m) with no difference in peak torque between groups, whereas peak summated force occurred at 10° PF in AB and 20° PF in SCI (P<0.01). In the PF muscles, PT torque occurred at 15° DF in AB (18.6±2.6 N m) and at 5° DF (6.8±3.3 N m; P<0.01) in SCI, and peak-summated force occurred at 15° DF in AB. The SCI group did not show any change in PF peak-summated force with varying joint angles. Rates of contraction and relaxation were not different between the two groups.Conclusions:The results suggest a significant change in the length–tension relationship of the PF muscles after SCI, but no change in the DF muscle group. Rehabilitation programs should focus on maintaining PF muscle length in order to optimize muscle strength and function after SCI.