Examining the test–retest reliability of commonly used neuromuscular, morphological, and functional measures in aging adults

The present study aimed at determining the test-retest reliability of commonly used measures in a sample of middle-aged and older participants who presented with a wide range of functional abilities. Participants were middle-aged (40-55 years) and older adults (> 60 years) with- and without mobility limitations, tested both before and after a 4-week period (n = 43). Measures included maximal dynamic (1-RM) and isometric strength, surface EMG (sEMG), peak power, ultrasound-derived muscle size and quality, as well as the performance on the 30-s sit-to-stand, habitual and maximal gait speed, timed up-and-go, stair climb, and 6-min walk tests. Reliability was assessed by means of the intraclass correlation coefficient (ICC), the standard error of measurement (SEM), the minimal detectable change (MDC), and the coefficient of variation (CV). Both dynamic and isometric strength measures displayed excellent reliability (ICCs = 0.96-0.99; CV = 2.2%-7%), whereas muscle size and quality (ICCs = 0.88-0.98; CV = 3.3%-8.7%), functional performance (ICCs = 0.78-0.92; CV = 4.2%-6.8%) and peak power (ICCs = 0.76-0.98; CV = 6.6%-12.8%) results showed good to excellent reliability. Peak power at 80-90% 1‑RM and sEMG measures, on the other hand, showed larger absolute error (CV = 14.4%-18.3% and CV = 14.3%-19.8%, respectively), despite good relative reliability (ICCs = 0.85-0.86). Further results include the comparison between the three subsets of participants included. Our main analysis suggests that most of these measures are sufficiently reliable, even when the two tests are performed a month apart from each other. Our study also supports the notion that, provided that the same equipment and procedures are used, the test-retest reliability of the measures is mostly comparable between the subsets of participants investigated.