The effects of isotonic and isokinetic muscle stretch on the excitability of the spinal alpha motor neurones in patients with muscle spasticity

Abstract

To examine the effect of isotonic (with and without weight bearing) and isokinetic muscle stretch on the excitability of the spinal alpha motor neurones (alphaMN) in patients with spasticity and to establish whether this effect is maintained for at least 24 h after the intervention. A single 20-min session of isotonic muscle stretch (with or without weight bearing) or isokinetic stretch was delivered to the ankle plantar flexors in patients with post-stroke lower limb spasticity and healthy control subjects. The effect of these types of muscle stretch on the excitability of alphaMN was assessed by measuring the latency of the Hoffmann reflex (H-reflex) and the ratio of the amplitude of the maximum H-reflex (H(max)) to that of the maximum action motor potential of the soleus muscle (M(max)). Sixty-six hemiplegic stroke patients and 21 healthy control subjects were recruited and completed the trial. The H(max):M(max) ratio was significantly higher in patients with spasticity than in healthy control subjects. However, there were no statistically significant differences in the H-reflex latency or the change in H(max):M(max) ratio between the baseline values and those recorded immediately after the therapy intervention or 24 h later for each type of muscle stretch. Similarly, there were no significant differences in these variables between the interventions. In the present study neither isotonic muscle stretch (with or without weight bearing) or isokinetic stretch had a statistically significant effect on the excitability of the alphaMN in patients with muscle spasticity. This suggests that the previously reported reduction in spasticity after muscle stretch is because of mechanisms other than the direct effect on alphaMN. However, the lack of a demonstrable benefit of treatment may be due the fact that we examined the effects of a single, rather than repeated treatment cycles.