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 degrees DF to 20 degrees PF. RESULTS: In the DF muscles, peak-evoked twitch (PT) torque occurred at 20 degrees 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 degrees PF in AB and 20 degrees PF in SCI (P<0.01). In the PF muscles, PT torque occurred at 15 degrees DF in AB (18.6+/-2.6 N m) and at 5 degrees DF (6.8+/-3.3 N m; P<0.01) in SCI, and peak-summated force occurred at 15 degrees 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.