Isometric tests to evaluate upper and lower extremity functioning in people with multiple sclerosis: reliability and validity

Background: Both upper and lower extremity motor symptoms are common in people with multiple sclerosis (PwMS) and there is a need to develop objective, reliable, and valid outcome measures. The aim of this study was to evaluate the reliability and external validity of the standard and novel isometric tests in the assessment of neuromuscular functioning in both upper (grip force; GF) and lower (knee extensors; KE) extremities in PwMS. Methods: Twenty-nine relapsing-remitting PwMS (Expanded Disability Status Scale (EDSS)<6) completed isometric and functional tests in upper (grip force) and lower (knee extension) extremity in two separate visits. Isometric testing included maximum force (maxF), maximum rate of force development (maxRFD), and our recently developed novel brief force pulse protocol (BFP). The dependent variables of BFP included rate of force development and relaxation scaling factors (RFD-SF and RFR-SF), which quantifies an individual's ability to scale the rates of force development and relaxation with the magnitude of force pulse produced. PwMS also completed functional tests of upper (9-hole peg (9HPT), finger tapping (FTT)) and lower extremity (25-ft walk test (T25WT), timed up and go (TUG), 5-time sit-to-stand (5StS), and Multiple Sclerosis Spasticity Scale (MSSS-88)). Results: Most isometric outcome measures had high reliability (ICCs>0.87 and CVs<12%). In GF, both RFD-SF and RFR-SF had significant associations with 9HPT and FTT (r's between 0.49-0.55, p<0.05). In KE, while maxF, maxRFD, and RFD-SF were moderately correlated to some of the functional tests, the strongest correlations were observed for the RFR-SF (T25FW, r=0.71; TUG, r=0.60; 5StS, r=0.47; MSSS-88, r=0.60, and EDSS, r=0.71). Multiple linear regression analysis indicated that RFD-SF is the only predictor for 9HPT and RFR-SF is the only predictor of walking speed among the studied variables. Conclusions: BFP protocol provides highly reliable and relevant outcome measures to evaluate both upper and lower extremity functioning in PwMS. Specifically, the ability to relax muscle forces quickly after a quick force production highly contributes to walking speed in PwMS.