The effect of muscle fatigue on the stretch reflex and related mechanical changes in aging and Parkinsonism

Abstract

Parkinson’s disease (PD) is the second most prevalent, chronic and neurodegenerative movement disorder in elderly people. Often people complain about fatigue and weakness. Despite its apparent frequency and functional importance, fatigue has not been thoroughly investigated in PD patients. Few studies have evaluated the impact of neuromuscular fatigue on the stretch reflex and inconsistent results exist in the literature. This study attempted to directly correlate the reflex activity with parallel biomechanical measures at the elbow joint from young, old and patient groups. A broadband perturbation (0-20Hz) at rest and 15% of contraction and narrowband perturbations with three frequencies (0.1Hz, 0.5Hz and 1 Hz) were used. The surface electromyogram (EMG) signals from biceps, joint angle (position), and torque signals were recorded. Cross correlation analyses were performed between these signals to obtain the measures of reflex gain and torque angle gain (stiffness). The fatigue was induced where the subjects contracted for one minute followed by 30 seconds rest period for a total duration of 15 min. The stretch reflex gain was reduced in all groups after the fatigue. Age had a significant effect as the reflex gain was reduced due to fatigue in the older group than in younger group. However, the effect of fatigue on reflex gain was not significantly different in patients for broadband perturbations, which suggest that the reflex gain was reduced due to an age factor and not due to the disease factor. The torque angle gain also did not vary between controls and patients that might be due to the dominance of inertia that masked the quantification of stiffness. Reflex gain was observed to significantly increase with frequency for narrowband perturbations. Stiffness increased in the patient group with time, but not for the control group. The frequencies 0.5 Hz and 1 Hz showed a strong correlation which suggests that the stiffness present in the patient group was related to enhanced reflex activity while no correlation was found in age-matched controls. The torque angle gain showed correlation with UPDRS measures and had higher estimates at low frequencies. Thus, narrowband perturbation is suited to measure the mechanical changes in PD patients.