Between-day reliability of common muscle fatigue measures during a repeated upper limb fatigue protocol

Neuromuscular fatigue manifestation is of interest in both basic neurophysiological and applied (e.g. sporting, ergonomics) contexts. A unique challenge in fatigue research is that experimental sessions often need to be collected across many days to allow for adequate recovery. The purpose of this study was to examine the between-day reliability of several surface electromyography- and strength-based fatigue measures in response to a repeated fatigue protocol. Twenty participants (10 M, 10 F) performed an isometric elbow flexion fatigue protocol on three different days. The reliability of commonly used amplitude- and frequency-based myoelectric and performance-based indicators of fatigue were assessed using traditional reliability assessment methods. Baseline MVC strength (N) demonstrated excellent between-day reliability (ICCA, 1: 0.96, 95%CI[0.92, 0.98]) with good absolute reliability (SEM: 5.10%, MD95: 14.1%). The absolute reliability of all slope-based fatigue measures was low. However, %MVC Slope (ICC: 0.67, [49, .82]), %MnPF Slope (ICC: 0.75, [.60, .87]), and endurance time (ICC: 0.60, [0.39, 0.77]) had poor/moderate to good relative reliability. Baseline MVC strength was shown to be very repeatable between days. Caution is recommended when using slope-based fatigue measures for cyclic repetitive upper limb tasks, as slope-based measures of muscle fatigue were shown to have low between-session reliability.