Three sets of low-intensity resistance exercises with slow movement and tonic force generation cause more muscular fatigue.

Purpose: There are no previous reports investigating the effects of different set numbers in low-intensity resistance exercise with slow movement and tonic force generation (LST) on muscle fatigue using surface electromyography (sEMG). This study aimed to examine muscle fatigue induced by one set and three sets of LST and to compare the impact of set differences on muscle activity by comparing LST with traditional high-intensity resistance exercise (TRAD).

Methods: Twenty-three healthy male students participated in this study. After 1RM testing in each leg was completed, participants were randomly assigned to either a group performing one set of exercises (n = 10) or a group performing three sets (n = 13). Each participant performed the LST protocol (50% 1RM) and the TRAD protocol (80% 1RM) with single leg extension until failure. The LST protocol consisted of a 3-s concentric, a 1-s isometric, and a 3-s eccentric phase. In contrast, the TRAD protocol consisted of a 1-s concentric, a 1-s eccentric, and a 1-s rest phase. For the three-set group, a 2-min rest interval was provided between sets. The outcome measures included maximal isometric knee extension torque (MVC) before and after exercise, root mean square (RMS), and mean power frequency (MPF) values recorded during the exercises.

Results: No significant differences in MVC were observed between the type of exercise or the number of sets. Similarly, no significant differences in the RMS during the exercise were observed across exercise types or number of sets. On the other hand, with the significant interaction of MPF (p = 0.001, η_p² = 0.399), there was a significant difference in three sets of LST compared to one set of LST (p = 0.012, d = 1.16) and three sets of TRAD (p < 0.001, d = 0.93).

Conclusion: Our findings suggest that performing three sets of LST induces significant muscle fatigue. Therefore, we speculate that performing three sets of LST may lead to the accumulation of metabolic stress and thereby cause muscle fatigue.