Explaining Task Failure From Sustained Submaximal Isometric Contractions

Abstract

Sustaining a submaximal isometric contraction leads to task failure, defined as the inability to maintain the requested force level. Both neural and muscular mechanisms contribute to the muscle fatigue associated with a sustained contraction performed with the knee extensor muscles, but the origin of task failure has not been clearly identified. PURPOSE: To determine the extent to which the knee extensor muscles themselves are limiting the endurance time (ET) of a sustained submaximal contraction. METHODS: Fourteen subjects (28±1.8 yrs, mean±SE) were asked to sustain an isometric contraction with the knee extensor muscles (dominant leg) at a target force equal to 20% of their maximal voluntary contraction force (MVC) until voluntary exhaustion (ET1). At task failure, the knee extensors were electrically stimulated (40 Hz) for 1 min aiming to develop the same target force that was not possible to develop voluntarily (20% MVC). At the end of the stimulation period, subjects were asked to again voluntarily contract the knee extensors at 20% MVC for as long as possible (ET2). Superimposed on MVC (twitch interpolation technique) and potentiated doublets were recorded before fatigue, after ET1 and after ET2. RESULTS: After task failure of ET1 (246±18s), all subjects developed 20% MVC under electrical stimulation. After the 1 min electrical stimulation period, subjects were able to resume the voluntary task and ET2 lasted 73±7s. MVC was decreased (p<0.05) by 513% after ET1 and by 573% after ET2, whereas maximal voluntary activation level was reduced (from 932% to 873% and to 89±2% after ET1 and ET2, respectively, p<0.01). Potentiated peak doublet was impaired after ET1 (-374%, p<0.001 and after ET2 (-485%, p<0.001). CONCLUSION: Although peripheral impairment is evidenced at task failure (large decrease in potentiated peak doublet), the duration of a sustained submaximal isometric knee extension performed at 20% MVC is not limited by the extensor muscles themselves but rather to mechanisms located above the neuromuscular junction, presumably involving descending motor drive.