Investigation of isometric strength and control of the upper extremities in multiple sclerosis

Isometric force assessment can provide insights into strength and motor control in patients with neurological disabilities. This study investigated the connection between isometric strength and control in nine multiple sclerosis (MS) patients and four healthy subjects using a compact isometric setup. The participants carried out isometric assessment tasks in both upper extremities in six directions. Strength was measured through maximum voluntary force/torque (MVF/T), while control ability was measured by applying a constant force/torque of 25% of MVF/T. Isometric control was quantified using coefficient of variation, force directing ability, sample-entropy and spectral bandwidth. The MS patients were also assessed using two impairment measures (Motricity Index and hand-grip strength), and two activity measures (Action Research Arm Test and Nine Hole Peg Test). The results indicate that isometric strength and control (measured by spectral bandwidth) were correlated in most directions. Among the four control measures, spectral bandwidth – a measure introduced in this study – was found to be strongly related to the force/torque regularity as measured by sample-entropy. Isometric strength and spectral bandwidth for all directions were well correlated with the impairment measures, but their correlation with the activity scales was moderate and direction-dependent. Overall the results show potential for using the isometric setup and protocol for assessment in MS population.