Non-local Muscle Fatigue Developed After Low-load Resistance Exercise Of Knee Extensors

Non-local muscle fatigue (NLMF) is defined as the decline in performance of a particular muscle group remote from the muscles fatigued by exercise. The existence of NLMF remains controversial but has been confirmed after high-intensity isometric contractions of lower-limb muscles. On the other hand, the effect of resistance exercise on NLMF remains largely unknown. PURPOSE: To determine the effect of resistance exercise with different loads and volumes on the performance of non-exercised muscle group. METHODS: Twelve healthy adults (age: 23.9 ± 2.4 years) completed three exercise sessions and a control session (CON). In each exercise session, participants performed 5 sets of knee-extension exercises either with high-load to failure (HL, 80% of one repetition maximum [1RM]), low-load to failure (LL-F, 40% of 1RM), or low-load with volume matched to HL (LL-VM, 40% of 1RM). Before and after the exercise, the maximal voluntary contraction (MVC) torque of knee extensors was measured as an index of local muscle fatigue. To assess NLMF, we performed several measurements of elbow flexor performance, including MVC, rate of torque development, and endurance. In addition, we analyzed electromyographic activity (EMG) and twitch torque of elbow flexors evoked with supramaximal stimulation of the brachial plexus to assess both central and peripheral fatigue. RESULTS: The knee extensor MVC decreased after the exercises, the magnitude of which was greater in LL-F than in HL. Both LL-F and LL-VM but not HL caused NLMF, as evidenced by a significant decrease in elbow flexor MVC (LL-F: -11.5 ± 8.6%, LL-VM: -8.7 ± 7.7%). Similarly, both LL-F and LL-VM induced a significant decrease in EMG during MVC (LL-F: -25.5 ± 27.3%, LL-VM: -19.1 ± 18.2%). The resting twitch torque of elbow flexors was unaffected by knee-extension exercise. CONCLUSION: Low-load resistance exercise of knee extensors leads to fatigue of non-exercised elbow flexors via central but not peripheral mechanisms.