Individual differences in knee extensor fatigue induced by sustained mid-level contraction

Identifying the factors that contribute to individual differences in mid-level intensity neuromuscular fatigue is essential to improve performance and effective exercise in training and rehabilitation. This study investigated factors affecting individual differences in knee extensor fatigue induced by sustained isometric mid-level contractions. Twenty-six healthy young men performed a sustained fatiguing contraction to task failure at 40% maximal voluntary contraction (MVC) torque of the knee extensors. The MVC torque (TQMVC), evoked triplet torque (TQTRI), and voluntary activation (VA%) of the knee extensors were determined before and after the fatiguing task. Differences in TQMVC, TQTRI, and VA% before and after the fatiguing task were examined, and the relationship between TQMVC and TQTRI or VA% after the fatiguing task normalized to baseline was examined using Pearson product-moment correlation analysis. TQMVC and TQTRI decreased significantly after the fatiguing task (31.1% decrease for TQMVC, 38.8% decrease for TQTRI, p < 0.001), whereas VA% did not change significantly (4.2% increase for VA%, p = 0.057). After the fatiguing task, the normalized TQMVC was significantly correlated with the normalized TQTRI (r = 0.617, p < 0.001), but not with the normalized VA% (r = 0.348, p = 0.082). Individual differences in knee extensor fatigue induced by sustained mid-level contraction were more affected by peripheral fatigue than by central fatigue. Therefore, focusing on fatigue resistance in peripheral areas is important to improve endurance in moderate-intensity exercises.