Sustained intermittent and continuous isometric muscle actions induce different levels of fatigue for the same force-time integral

Introduction: The aim was to compare the fatiguing effects induced by continuous (CONT) and intermittent (INT) tasks of an identical duration by equating the force-time integral.

Method: ː Fourteen men performed two sequences (4 blocks of 378s) of contractions (CONT or INT) of the quadriceps femoris. The CONT contraction of 378s corresponded to 10% of the maximum voluntary muscle contraction (MVC). The INT 3s contractions corresponded to 70% of the MVC, separated by rest periods of 18s (a 21s cycle) and repeated 18 times (21s x 18) for 378s. Hence, the force-time integral was identical for the both contraction tasks (378s × 10% MVC = 37.8; 3s x 18 repetitions x 70% MVC = 37.8).

Results: ː The MVC and central activation ratio were measured before, during and after the sequence of muscle contractions. For a same force-time integral done during an identical duration of activity, the CONT contractions engendered earlier and greater decrease in motor output and voluntary activation than the INT contractions.

Conclusion: Continuous muscle actions are more fatiguing than intermittent muscle actions. They could therefore be relevant to develop resistance to muscle fatigue but would be not suitable for undertaking long or sustained workloads.