Portable fixed dynamometry enables home-based, reliable assessment of muscle strength in patients with amyotrophic lateral sclerosis: a pilot study

OBJECTIVE: To determine the feasibility, reliability, and sensitivity of remotely monitoring muscle strength loss of knee extensors using a novel portable fixed dynamometer (PFD) in patients with amyotrophic lateral sclerosis (ALS).

METHODS: We conducted a pilot study with a newly developed device to measure knee extension strength. Patients performed unsupervised PFD measurements, biweekly, for 6 months at home. We evaluated feasibility using adherence and a device-specific questionnaire. Reliability was assessed by (1) comparing unsupervised and supervised measurements to identify systematic bias, and (2) comparing consecutive unsupervised measurements to determine test-retest reliability expressed as intraclass correlation coefficient (ICC) and standard error of measurement (SEM). Sensitivity to detect longitudinal change was described using linear mixed-effects models.

RESULTS: We enrolled 18 patients with ALS. Adherence was 86%, where all patients found that the device suitable to measure muscle strength at home; 4 patients (24%) found the measurements burdensome. The correlation between (un)supervised measurements was excellent (Pearson's r 0.97, 95%CI; 0.94 - 0.99) and no systematic bias was present (mean difference 0.13, 95%CI; -2.22 - 2.48, p  = 0.91). Unsupervised measurements had excellent test-retest reliability with an average ICC of 0.97 (95%CI: 0.94 - 0.99) and SEM of 5.8% (95%CI: 4.8 - 7.0). Muscle strength declined monthly by 1.9 %predicted points (95%CI; -3.0 to -0.9, p  = 0.001).

CONCLUSIONS: Using the PFD, it proved feasible to perform knee extension strength measurements at home which were reliable and sensitive for detecting muscle strength loss. Larger studies are warranted to compare the device with conventional outcomes.