The purpose of the study was to examine the influence of the difference between the recruitment threshold of a motor unit and the target force of the sustained contraction on the discharge of the motor unit at recruitment. The discharge characteristics of 53 motor units in biceps brachii were recorded after being recruited during a sustained contraction. Some motor units (n = 22) discharged action potentials tonically after being recruited, whereas others (n = 31) discharged intermittent trains of action potentials. The two groups of motor units were distinguished by the difference between the recruitment threshold of the motor unit and the target force for the sustained contraction: tonic, 5.9 ± 2.5%; intermittent, 10.7 ± 2.9%. Discharge rate for the tonic units decreased progressively (13.9 ± 2.7 to 11.7 ± 2.6 pulses s −1 ; P = 0.04) during the 99 ± 111 s contraction. Train rate, train duration and average discharge rate for the intermittent motor units did not change across 211 ± 153 s of intermittent discharge. The initial discharge rate at recruitment during the sustained contraction was lower for the intermittent motor units (11.0 ± 3.3 pulses s −1 ) than the tonic motor units (13.7 ± 3.3 pulses s −1 ; P = 0.005), and the coefficient of variation for interspike interval was higher for the intermittent motor units (34.6 ± 12.3%) than the tonic motor units (21.2 ± 9.4%) at recruitment (P = 0.001) and remained elevated for discharge duration (34.6 ± 9.2% versus 19.1 ± 11.7%, P < 0.001). In an additional experiment, 12 motor units were recorded at two different target forces below recruitment threshold (5.7 ± 1.9% and 10.5 ± 2.4%). Each motor unit exhibited the two discharge patterns (tonic and intermittent) as observed for the 53 motor units. The results suggest that newly recruited motor units with recruitment thresholds closer to the target force experienced less synaptic noise at the time of recruitment that resulted in them discharging action potentials at more regular and greater rates than motor units with recruitment thresholds further from the target force.

Bursts of activity in the surface electromyogram (EMG) during a sustained contraction have been interpreted as corresponding to the transient recruitment of motor units, but this association has never been confirmed. The current study compared the timing of trains of action potentials discharged by single motor units during a sustained contraction with the bursts of activity detected in the surface EMG signal. The 20 motor units from 6 subjects [recruitment threshold, 35.3 +/- 11.3% maximal voluntary contraction (MVC) force] that were detected with fine wire electrodes discharged 2-9 trains of action potentials (7.2 +/- 5.6 s in duration) when recruited during a contraction that was sustained at a force below its recruitment threshold (target force, 25.4 +/- 10.6% MVC force). High-pass filtering the bipolar surface EMG signal improved its correlation with the single motor unit signal. An algorithm applied to the surface EMG was able to detect 75% of the trains of motor unit action potentials. The results indicate that bursts of activity in the surface EMG during a constant-force contraction correspond to the transient recruitment of higher-threshold motor units in healthy individuals, and these results could assist in the diagnosis and design of treatment in individuals who demonstrate deficits in motor unit activation.