Impaired viscoelastic behaviour of spastic plantarflexors during passive stretch at different velocities

Objective. The aim of this study was to evaluate the effect of velocity on non-reflexly induced resistive torque (RT) responses of the spastic plantarflexors (PFs) of subjects with spinal cord injuries. Design. Descriptive study, transversal. Background. In spastic muscles, non-reflex changes such as increased muscle stiffness, contractures and atrophy have been reported. These changes probably alter muscle tensile properties and viscoelastic behaviour. Methods. Six subjects with chronic (1–3 yr) spinal cord injuries (SCI) and 12 normal controls (CTLs) participated in this study. Passive ankle dorsiflexions (DFs), ranging from −35 ° to 5 ° of DF, were randomly imposed at 5 °/s, 10 °/s, 20 °/s, 40 °/s, 60 °/s, 120 °/s and 180 °/s using an isokinetic dynamometer (Kin-ComTM). Unwanted muscle activity was detected using surface electrodes on the soleus and tibialis anterior muscles. Results. The results show first that RT rose with increasing velocity; the RT increment reached statistical (analysis of variance (image); Scheffé post-hoc procedure) significance (P < 0.01) at a lower velocity for the CTLs (40 °/s) group than for the SCI group (60 °/s). Second, significantly (t-test; P < 0.001) larger net increments of RT (RT at each velocity minus RT at 5 °/s) were found for the CTLs at 180 °/s than for the SCI group whose RT plateaued at 60 °/s. Finally, whereas the RT-velocity relationship was linear (r = 0.94) in the CTLs, that of the SCI group followed a power regression model (r = 0.85). Conclusions. These results show that the spastic PFs of the subjects with SCIs have an impaired velocity-sensitive behaviour, especially at high velocities of stretch where greater resistance is expected.