Inability to increase the neural drive to muscle is associated with task failure during submaximal contractions

We investigated changes in motor unit (MU) behavior and vasti-muscle contractile properties during sustained submaximal fatiguing contractions with a new time-domain tracking technique to understand the mechanisms responsible for task failure. Sixteen participants performed a nonfatiguing 15-s isometric knee extension at 50% of the maximum voluntary (MVC) torque, followed by a 30% MVC sustained contraction until exhaustion. Two grids of 64 surface electromyography electrodes were placed over vastus medialis and lateralis. Signals were decomposed into MU discharge times and the MUs from the 30% MVC sustained contraction were followed until task failure by overlapping decomposition intervals. These MUs were then tracked between 50% and 30% MVC. During the sustained fatiguing contraction, MUs of the two muscles decreased their discharge rate until ~40% of the endurance time, referred to as the reversal time, and then increased their discharge rate until task failure. This reversal in firing behavior predicted total endurance time and was matched by opposite changes in twitch force (increase followed by a decrease). Despite the later increase in MU firing rates, peak discharge rates at task failure did not reach the frequency attained during a nonfatiguing 50% MVC contraction. These results show that changes in MU firing properties are influenced by adjustments in contractile properties during the course of the contraction, allowing the identification of two phases. Nevertheless, the contraction cannot be sustained, possibly because of progressive motoneuron inhibition/decreased excitability, as the later increase in firing rate saturates at a much lower frequency compared with a higher-force nonfatiguing contraction. NEW & NOTEWORTHY Motor unit firing and contractile properties during a submaximal contraction until failure were assessed with a new tracking technique. Two distinct phases in firing behavior were observed, which compensated for changes in twitch area and predicted time to failure. However, the late increase in firing rate was below the rates attained in absence of fatigue, which points to an inability of the central nervous system to sufficiently increase the neural drive to muscle with fatigue.