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 degrees to 5 degrees of DF, were randomly imposed at 5 degrees /s, 10 degrees /s, 20 degrees /s, 40 degrees /s, 60 degrees /s, 120 degrees /s and 180 degrees /s using an isokinetic dynamometer (Kin-Com(TM)). 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 (ANOVA); Scheffé post-hoc procedure) significance (P < 0.01) at a lower velocity for the CTLs (40 degrees /s) group than for the SCI group (60 degrees /s). Second, significantly (t-test; P < 0.001) larger net increments of RT (RT at each velocity minus RT at 5 degrees /s) were found for the CTLs at 180 degrees /s than for the SCI group whose RT plateaued at 60 degrees /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.