Impaired Isometric Force Matching in Upper and Lower Limbs Revealed by Quantitative Motor Assessments in Huntington's Disease.

Background: Assessment of motor symptoms in Huntington's disease (HD) is based on the Unified-HD-Rating-Scale-Total-Motor-Score (UHDRS-TMS). Its categorical and rater-dependent nature reduces the ability to detect subtle changes and often placebo effects have been observed in trials. We have previously shown that impairments in isometric force matching can be detected by quantitative motor (Q-Motor) assessments of tongue protrusion forces (glossomotography) in HD. Objective: We aimed to investigate whether similar impairments in isometric force matching can be detected in tasks assessing hand and foot force coordination and whether correlations with clinical measures and the disease burden score can be found. Methods: Using a pre-calibrated force transducer, the ability of subjects to generate and maintain isometric forces at different target levels displayed on a monitor was assessed. Target forces applied in the hand were 1.5 and 5 Newton [N] and in feet 1, 5, and 10 N. Subjects with HD (n = 31) and age-matched controls (n = 22) were recruited from the HD out-patient clinic. Results: All paradigms distinguished controls from HD. The static coefficient of variability (%) was the most robust measure across all matching tasks. Correlations with clinical measures, such as the UHDRS-TMS, TFC, and the DBS were found. Conclusions: Assessment of hand and foot force matching tasks was feasible and provided quantitative objective measures for severity of motor phenotype in HD. Since both upper and lower extremity motor function are relevant for everyday activities, these measures should be further assessed as candidates for developing functionally meaningful quantitative motor tasks.