Spinal mechanisms contribute to differences in the time to failure of submaximal fatiguing contractions performed with different loads

This study compared the mechanisms that limit the time to failure of a sustained submaximal contraction at 20% of maximum when the elbow flexors either supported an inertial load ( position task) or exerted an equivalent constant torque against a rigid restraint ( force task). The surface electromyogram ( EMG), the motor-evoked potential ( MEP) in response to transcranial magnetic stimulation ( TMS) of the motor cortex, and the Hoffmann reflex ( H-reflex) and maximal M-wave ( Mmax) elicited by electrical stimulation of the brachial plexus were recorded in biceps brachii during the two tasks. Although the time to failure for the position task was only 44% of that for the force task, the rate of increase of the average EMG ( aEMG; % initial MVC) and MEP area (% Mmax) did not differ significantly during the two tasks. At task failure, however, the increases in normalized aEMG and MEP area were significantly ( P <0.05) greater for the force task ( 36.4 and 219.9%) than for the position task ( 22.4 and 141.7%). Furthermore, the superimposed mechanical twitch (% initial MVC), evoked by TMS during a brief MVC of the elbow flexors immediately after task failure, was increased similarly in both tasks. Although the normalized H-reflex area (% Mmax) decreased during the two fatiguing contractions, the reduction was more rapid and greater during the position task ( 59.8%) compared with the force task ( 34.7%). Taken together, the results suggest that spinal mechanisms were a major determinant of the briefer time to failure for the position task.