Neyroud D, Rüttimann J, Mannion AF, Millet GY, Maffiuletti NA, Kayser B, Place N. Comparison of neuromuscular adjustments associated with sustained isometric contractions of four different muscle groups. J Appl Physiol 114: 1426 -1434, 2013. First published March 7, 2013 doi:10.1152/japplphysiol.01539.2012.-The extent and characteristics of muscle fatigue of different muscle groups when subjected to a similar fatiguing task may differ. Thirteen healthy young men performed sustained contractions at 50% maximal voluntary contraction (MVC) force until task failure, with four different muscle groups, over two sessions. Per session, one upper limb and one lower limb muscle group were tested (knee extensors and thumb adductor, or plantar and elbow flexors). Changes in voluntary activation level and contractile properties were derived from doublet responses evoked during and after MVCs before and after exercise. Time to task failure differed (P Ͻ 0.05) between muscle groups (220 Ϯ 64 s for plantar flexors, 114 Ϯ 27 s for thumb adductor, 77 Ϯ 25 s for knee extensors, and 72 Ϯ 14 s for elbow flexors). MVC force loss immediately after voluntary task failure was similar (Ϫ30 Ϯ 11% for plantar flexors, Ϫ37 Ϯ 13% for thumb adductor, Ϫ34 Ϯ 15% for knee extensors, and Ϫ40 Ϯ 12% for elbow flexors, P Ͼ 0.05). Voluntary activation was decreased for plantar flexors only (from 95 Ϯ 5% to 82 Ϯ 9%, P Ͻ 0.05). Potentiated evoked doublet amplitude was more depressed for upper limb muscles (Ϫ59.3 Ϯ 14.7% for elbow flexors and Ϫ60.1 Ϯ 24.1% for thumb adductor, P Ͻ 0.05) than for knee extensors (Ϫ28 Ϯ 15%, P Ͻ 0.05); no reduction was found in plantar flexors (Ϫ7 Ϯ 12%, P Ͼ 0.05). In conclusion, despite different times to task failure when sustaining an isometric contraction at 50% MVC force for as long as possible, diverse muscle groups present similar loss of MVC force after task failure. Thus the extent of muscle fatigue is not affected by time to task failure, whereas this latter determines the etiology of fatigue. endurance time; fatigue; maximal voluntary contraction; contractile properties; surface electromyography TIME TO TASK FAILURE of a sustained submaximal isometric contraction (i.e., endurance time, ET) depends on physiological factors such as relative force level (30, 35), muscle fiber type composition (16,51,77), muscular activation strategy (6,19,38), muscle size (36), and length (54, 58) but also depends on psychological factors such as motivation, mood, and expectation (24). Whereas the task dependency of ET is well documented (23,25,26), little is known about muscle fatigue characteristics induced by a given task sustained until failure in different muscle groups, in the same individuals. Frey-Law and Avin (28) suggested that no single fatigue model applies to all muscles. Their meta-analysis showed large differences in ET depending on the muscle group considered, especially at low contraction intensities. However, it is unknown whether the extent of maximal voluntary contraction (MVC) force loss immediately after task ...