Length–tension properties of ankle muscles in chronic human spinal cord injury

Contracture, or loss of range of motion (ROM) of a joint, is a common clinical problem in individuals with spinal cord injury (SCI). In order to measure the possible contribution of changes in muscle length to the loss of ankle ROM, the active force vs. angle curves for the tibialis anterior (TA) and gastrocnemius soleus (GS) were measured in 20 participants, 10 with SCI, and 10 gender and age matched, neurologically intact (NI) individuals. Electrical stimuli were applied to the TA and GS motor nerves at incremented angles of the entire ROM of the ankle and the resulting ankle and knee torques were measured using a multi-axis load cell. The muscle forces of the TA and GS were calculated from the torque measurements using estimates of their respective moment arms and the resulting forces were plotted against joint angle. The force–angle relation for the GS at the ankle (GSA) was significantly shifted into plantar flexion in SCI subjects, compared to NI controls (t-test, p<0.001). Similar results were obtained based upon the GS knee (GSK) force–angle measurements (p<0.05). Conversely, no significant shift in the force–angle relation was found for the TA (p=0.138). Differences in the passive ROM were consistent with the force–angle changes. The ROM in the dorsiflexion direction was significantly smaller in SCI subjects compared to NI controls (p<0.05) while the plantar flexion ROM was not significantly different (p=0.114). Based upon these results, we concluded that muscle shortening is an important component of contracture in SCI.