Surface electromyography-based assessment of upper-limb muscle strength, endurance, and muscle quality during graded handgrip tasks in healthy adults.

Background: Muscle strength, endurance, and muscle quality are important determinants of upper-limb function. Although surface electromyography (sEMG) characterizes muscle activation and fatigue, its integration with bioelectrical impedance analysis (BIA) for muscle quality assessment remains limited. Objective: This study investigated neuromuscular responses during graded handgrip contractions and evaluated an exploratory muscle quality (MQ) index integrating sEMG and BIA measures in healthy adults. Methods: Fifty-three healthy adults (35 males and 18 females) performed maximal voluntary contraction (MVC) followed by graded handgrip contractions at 0%, 20%, 40%, 60%, 80%, and 100% MVC. Surface EMG was recorded from the biceps brachii, triceps brachii, brachioradialis, and flexor carpi ulnaris. Root mean square (RMS) and frequency-domain features were extracted, with median frequency (fmedian) selected as the representative frequency-domain outcome for nonparametric analyses and linear mixed-effects analysis. The MQ index integrated grip strength, muscle mass, body fat, and RMS and was evaluated using Spearman correlation. Results: RMS increased with contraction intensity, whereas fmedian increased to a peak at 60% MVC and then gradually declined with the lowest values at 100% MVC. Significant fixed effect and interaction indicated that neuromuscular responses varied by gender, muscle, contraction stage, and muscle mass. The MQ index correlated moderately with right-arm muscle mass and phase angle but weakly with the conventional muscle quality index. Conclusion: Integrating sEMG and BIA provides complementary information on muscle activation, fatigue, and tissue characteristics beyond conventional strength-based measures, supporting assessment of upper-limb neuromuscular function and muscle quality.