Author(s): PARK, J.1, SONG, J.1, CHOI, Y.1, TANABE, H.2, Institution: SEOUL NATIONAL UNIVERSITY, Country: KOREA, SOUTH, Abstract-ID: 1914

INTRODUCTION:
Muscle fatigue, which is recognized by a reduction in the ability to perform physical actions, is influenced by several physiological factors. These physiological processes begin to manifest themselves before any noticeable loss of strength occurs during submaximal effort. Thus, muscle fatigue can be viewed as a cumulative effect that occurs after the onset of sustained physical activity, rather than as a point of task failure when muscles become exhausted. This study specifically focused on the cumulative effect of muscle fatigue by examining changes in muscle excitation and oxygen saturation through the integration of surface electromyogram (sEMG) and near-infrared spectroscopy (NIRS) measurements,
METHODS:
Nine male volunteers performed repetitive, sustained isometric knee extension tasks at 50% of their maximal voluntary contraction to induce muscle fatigue. The tasks were performed sequentially until the participants could no longer produce the target torque. Knee extension torque, sEMG, and vastus lateralis NIRS data were collected simultaneously. Data from trials in which a consistent target torque was maintained were used for analysis, excluding data from the last trial in which a reduction in torque occurred due to fatigue. Coefficient of variation (CV) of torque, integrated EMG (iEMG), and median frequency (MDF) of the sEMG signal were calculated. A number of NIRS-derived tissue oxygen saturation index (TSI) parameters were analyzed, including the magnitude of oxygen desaturation (ΔTSIMIN) and recovery (ΔTSIRECOV), and the rate of desaturation in the initial contraction period (TSISLOPE).
RESULTS:
An increase in iEMG and a decrease in MDF were observed across trials, followed by an increase in torque variability (CV of torque), although the average magnitude of knee extension torque remained constant. The NIRS measurements showed changes in TSI parameters, reflecting increased metabolic demand and decreased oxygen delivery in the fatigued muscle. In addition, sEMG variables began to change early in response to fatigue accumulation, whereas TSI parameters showed significant changes closer to the task failure phase. Significant relationships were also found between changes in torque, sEMG, and NIRS variables due to the development of muscle fatigue.
CONCLUSION:
Our findings provide a comprehensive understanding of the development of muscle fatigue and highlight the interplay between mechanical, electrical, and metabolic responses during submaximal exercise. The reduction in force generation capacity due to muscle fatigue is reflected in the sEMG signal and manifests as an increase in motor variability. NIRS parameters representing the magnitude of oxygen desaturation and recovery could serve as reliable indicators of fatigue progression, including prediction of potential muscle exhaustion, which is critical for injury prevention.