Alterations in human muscle and central control mechanisms

Abstract

Research has shown that skeletal muscle, despite showing a high degree of specialisation, has a remarkable ability to modify its properties. Understanding these changes is important for optimal response to therapeutic intervention. These studies investigated alterations in neuromuscular performance of quadriceps femoris muscle in normal subjects, before investigating changes in muscle and neural mechanisms in the first six months following stroke. Muscle function studies were conducted to monitor the effects of selected patterns of long term electrical stimulation (PI - uniform 8 Hz, P2 - mixed frequency, and P3 random high and low frequency) on quadriceps femoris of 21 healthy subjects. Stimulated muscles showed significant increases in strength, fatigue resistance and relaxation times after 3 weeks and in force-frequency output after 6 weeks. Significant changes were observed in the stimulated muscles in Groups P2 and P3 indicating that a mixed or random pattern of activation induced greater changes than a uniform 8 Hz pattern. These studies together with soleus H reflexes were used to study concurrent changes in quadriceps femoris and la spinal reflex pathways of stroke patients and age-matched controls (n=10). One month following stroke, both paretic and non-paretic muscles were more fatiguable and weaker than the muscles of controls and disynaptic and presynaptic inhibition were reduced in the paretic limbs. In subsequent months, the paretic muscles regained strength and inhibitory effects were restored. Subjective analysis suggested that different patterns of recovery related to walking ability at six months. In patients taking less than 15s to walk 10m (Group 1 n=5), the paretic muscles became significantly stronger and less fatiguable over time whereas the muscles of patients who took longer to walk 10m (Group 2 n=5) remained weaker and more fatiguable. Reciprocal inhibition was regained by 1 month in Group 1 and by 6 months in Group 2. These findings provide insight into long-term recovery and rationale for therapeutic intervention following stroke