Load type influences motor unit recruitment in biceps brachii during a sustained contraction

Abstract

Twenty subjects participated in four experiments designed to compare time to task failure and motor-unit recruitment threshold during contractions sustained at 15% of maximum as the elbow flexor muscles either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). Subcutaneous branched bipolar electrodes were used to record single motor unit activity from the biceps brachii muscle during ramp contractions performed before and at 50 and 90% of the time to failure for the position task during both fatiguing contractions. The time to task failure was briefer for the position task than for the force task (P=0.0002). Thirty and 29 motor units were isolated during the force and position tasks, respectively. The recruitment threshold declined by 48 and 30% (P=0.0001) during the position task for motor units with an initial recruitment threshold below and above the target force, respectively, whereas no significant change in recruitment threshold was observed during the force task. Changes in recruitment threshold were associated with a decrease in the mean discharge rate (-16%), an increase in discharge rate variability (+40%), and a prolongation of the first two interspike intervals (+29 and +13%). These data indicate that there were faster changes in motor unit recruitment and rate coding during the position task than the force task despite a similar net muscle torque during both tasks. Moreover, the results suggest that the differential synaptic input observed during the position task influences most of the motor unit pool.

FIG. 1.

Experimental protocol and motor unit recordings. A: representative data for averaged EMG (aEMG) and force for 1 subject when performing the position task to 90% of the time to failure. The maximal voluntary contractions (MVCs) have been removed to improve the clarity of the figure. The arrows on the force trace indicate the recruitment threshold for 1 motor unit from the short head of the biceps brachii with an initial recruitment threshold of 56% MVC, which decreased to 31% MVC at the end of the sustained contraction. B–D: expended time scale during the ramp contractions showing the force (top trace), subcutaneous EMG recording with the 1st 5 action potentials of the motor unit indicated by the arrows in A, and instantaneous discharge rate (bottom trace) before the sustained contraction (B), and at 50 (C) and 90% (D) of the time to failure. Note the decline in motor unit recruitment threshold and instantaneous discharge rate for the 2 1st interspike intervals, whereas the slope of force development remained similar between the different time points.

FIG. 2.

Change in EMG amplitude and force fluctuations during the 2 fatiguing contractions. The average rectified aEMG for the elbow flexor muscles (collapsed across the 3 elbow flexor muscles and expressed as means ± SE; A) and the average CV for force (B) during the force (•) and position tasks (○) sustained to failure. The values have been averaged over 30-s intervals for each task at 5 time points that correspond to the relative times at the start and 25, 50, 75, and 100% of time to task failure. *P < 0.05 compared with initial values; †P < 0.05 compared with force task values at the same relative time point.

FIG. 3.

Change in EMG amplitude during the 2 contractions sustained to 90% of the time to failure for the position task. The average rectified aEMG for the elbow flexor muscles (collapsed across the 3 elbow flexor muscles and expressed as means ± SE) during the force (•) and position tasks (○) sustained to 90% of the time to failure for position task. The values have been averaged over 30-s intervals for each task at four time points that correspond to the relative times at the start and 25, 75, and 90% of time to task failure. The values for the 50% time point have been averaged over 15-s intervals at the end of the 1st part of the contraction and at the beginning of the 2nd part. *P < 0.05 compared with initial values; †P < 0.05 compared with force task values at the same relative time point.

FIG. 4.

Change in recruitment threshold of individual motor units during the force and position tasks. Recruitment threshold of single motor units in biceps brachii with an initial recruitment threshold above the target force of 15% MVC force (A and B) and with an initial recruitment threshold below the target force (C and D) during the force (left) and position tasks (right).

FIG. 5.

Change in recruitment threshold during the force and position tasks. Changes in average recruitment (left) and derecruitment threshold (right) of single motor units with an initial recruitment threshold either above (top) or below (bottom) the target force during the force (•) and position tasks (○) before and at 50 and 90% of the time to failure for the position task (means ± SE). *P < 0.05 compared with initial values.

FIG. 6.

Change in mean discharge rate and CV for interspike interval at recruitment and derecruitment for the force and position tasks sustained to 90% of the time to failure for the position task. Top: the average mean discharge rate (means ± SE) for the 1st 4 interspike intervals at recruitment (left) and the last 4 interspike intervals at derecruitment (right) for the force (•) and position tasks (○) sustained to 90% of the time to failure for position task. Bottom: the average CV for interspike interval (means ± SE) for the 1st 4 interspike intervals at recruitment (left) and the last 4 interspike intervals at derecruitment (right) for the force (•) and position tasks (○) sustained to 90% of the time to failure for position task. *P < 0.05 compared with initial values; †P < 0.05 compared with force task values at the same relative time point.

FIG. 7.

Relations between the relative changes in recruitment threshold and in discharge characteristics of motor units at the end of the force and position tasks sustained to 90% of the time to failure for the position task. The change in recruitment threshold of motor units isolated during the force (•) and the position (○) tasks was associated with changes in the mean discharge rate (P = 0.02; A), discharge variability (P = 0.02; B), and duration of the 1st interspike interval (P = 0.008; C). Changes are expressed as a percentage of the initial values.