The influence of contraction amplitude and firing history on spike-triggered averaged trapezius motor unit potentials.

The spike-triggered averaged (STA) technique was used to examine trapezius motor unit potentials and their dependence on contraction amplitude and firing history. Individual motor unit firings were identified by a fine-wire intramuscular electrode, while STA-derived potentials were extracted from the simultaneously recorded surface electromyographic (SEMG) signal. Amplitude-controlled contractions and contractions with typing tasks and mental stress were carried out. STA potentials were mostly derived from 20 s intervals of firing. Motor unit synchrony was estimated by peristimulus time histograms (PSTHs). An association between SEMG amplitude and STA-derived motor unit potentials was found: motor unit area showed a four-fold increase when SEMG amplitude increased from 1.5 to 10.5% of the root mean square-detected SEMG signal at maximal voluntary contraction (%EMG(max)). Low- and higher threshold motor unit potentials, all with recruitment thresholds <10% EMG(max), had similar area at the same contraction amplitude. A significant increase in the STA-derived potentials was observed after 3 min of constant-amplitude contractions; however, this difference was reduced after 10 min and no longer present after 30 min of contraction. Motor unit synchrony accounted for, on average, 2.8% additional firings within 2 ms of the triggering motor unit. We conclude that the increase in STA-derived potentials with contraction amplitude is, to a major extent, due to motor unit synchrony, limiting the applicability of this method in postural muscles presenting wide motor unit potentials. The similar area of motor units at same SEMG amplitude may indicate that trapezius motor units recruited below 10% EMG(max) are of similar size and thus not organized according to the Henneman size principle.

Firing patterns of low-threshold trapezius motor units in feedback-controlled contractions and vocational motor activities.

The study aimed to examine firing patterns of low-threshold trapezius motor units, with attention given to motor unit recruitment threshold. Different motor tasks were explored: shoulder elevation, arm movement in typing, and the motor response to mental stress. Contraction amplitudes in the range from 1 to 10% of the surface electromyographic (SEMG) signal at maximal voluntary contraction (1-10% EMGmax) were studied, representing the range of trapezius muscle activity commonly observed in daily living. Single motor unit activity was recorded by a quadrifilar fine-wire electrode. A surface electrode simultaneously recorded the SEMG signal. Low-threshold motor units showed a small increase in mean firing rate, from 10.5 to 12.5 pulses per second ( p<0.01), in constant-amplitude contractions when SEMG amplitude increased from <2% to >4% EMGmax. After the first few minutes, firing rates were similar for all motor units in a contraction, despite different recruitment thresholds. Firing rates of motor units with threshold <2% EMGmax were the same in constant-amplitude contractions, contractions with vocational arm movement, and contractions with imposed stress for SEMG amplitude at the same level. High-frequency firing patterns were observed in dynamic contractions, limited to bursts of up to a second duration. We conclude that low-threshold trapezius motor units have similar, stable firing rates in sustained contractions, independent of task and recruitment threshold, but with a small increase for increasing contraction amplitude.

Motor unit recruitment and derecruitment induced by brief increase in contraction amplitude of the human trapezius muscle.

The activity pattern of low-threshold human trapezius motor units was examined in response to brief, voluntary increases in contraction amplitude ('EMG pulse') superimposed on a constant contraction at 4-7 % of the surface electromyographic (EMG) response at maximal voluntary contraction (4-7 % EMGmax). EMG pulses at 15-20 % EMGmax were superimposed every minute on contractions of 5, 10, or 30 min duration. A quadrifilar fine-wire electrode recorded single motor unit activity and a surface electrode recorded simultaneously the surface EMG signal. Low-threshold motor units recruited at the start of the contraction were observed to stop firing while motor units of higher recruitment threshold stayed active. Derecruitment of a motor unit coincided with the end of an EMG pulse. The lowest-threshold motor units showed only brief silent periods. Some motor units with recruitment threshold up to 5 % EMGmax higher than the constant contraction level were recruited during an EMG pulse and kept firing throughout the contraction. Following an EMG pulse, there was a marked reduction in motor unit firing rates upon return of the surface EMG signal to the constant contraction level, outlasting the EMG pulse by 4 s on average. The reduction in firing rates may serve as a trigger to induce derecruitment. We speculate that the silent periods following derecruitment may be due to deactivation of non-inactivating inward current ('plateau potentials'). The firing behaviour of trapezius motor units in these experiments may thus illustrate a mechanism and a control strategy to reduce fatigue of motor units with sustained activity patterns.