Extraction of individual muscle mechanical action from endpoint force

Abstract

Most motor tasks require the simultaneous coordination of multiple muscles. That coordination is poorly understood in part because there is no noninvasive means of isolating a single muscle's contribution to the resultant endpoint force. The contribution of a single motor unit to isometric tasks can, however, be characterized using the spike-triggered averaging (STA) technique, applied to a single motor unit's spike train. We propose that a technique analogous to STA, which we call electromyogram (EMG)-weighted averaging (EWA), can be applied to surface EMGs to extract muscle mechanical action from the natural endpoint force fluctuations generated during steady isometric contraction. We demonstrate this technique on simultaneous recordings of fingertip force and surface EMG from the first dorsal interosseous (FDI) and extensor indicis (EI) of humans. The EWA direction was approximately the same across a wide range of fingertip force directions, and the average EWA direction was consistent with mechanical action direction of these muscles estimated from cadaveric and imaging data: the EWA directions were 193 +/- 2 degrees for the FDI and 71 +/- 5 degrees for the EI (95% confidence). EWA transient behavior also appears to capture temporal characteristics of muscle force fluctuations with peak force time and general waveform shape similar to that of the associated spike-triggered averages from single motor units. The EWA may provide a means of empirically characterizing the complex transformation between muscle force and endpoint force without the need for invasive electrode recordings or complex anatomical measurements of musculoskeletal geometry.

Fig.1.

Conceptual comparison of spike-triggered averaging (STA) and electromyogram (EMG)-weighted averaging (EWA). A: STA is typically performed by sorting motor unit spike trains from fine-wire EMG recordings, whereas EWA (B) is performed on a rectified surface EMG signal composed of electrical potentials from many motor units. C: STA typically uses spike trains from individual motor units as a measure of activity, and these spike trains could be combined into a single spike train representing all spikes from all motor units. D: EWA uses rectified surface EMG as a measure of activity, and increases in surface EMG will be correlated with increased activity in the superposed spike train (inset). E: both STA and EWA cross-correlate their activity measure with force fluctuations to produce STA (F) or EWA (G) waveforms. We expect a high degree of similarity between the average STA waveform and the EWA waveform because the surface EMG may be highly correlated with the superposed spike train.

Fig.2.

Representative EWA. A: a sample trial is shown in the time domain, where the subject exerted primarily a metacarpophalangeal (MCP) extension force, which produced EMG activity in both the 1st dorsal interosseous (FDI) and extensor indicis (EI) muscles. B: cross-correlation of rectified surface EMG and force components yielded EWA waveforms peaking at time lag (between EMG and force) of 50 ms. The EWA time to peak is shown as a vertical line. C: plots of EWA components against each other shows EWA trajectories in task space, approximately directed along the mechanical action of each muscle. ×, the point along the trajectory when the EWA magnitude peaked.

Fig.3.

EMG activity, EWA direction, and EWA time to peak for all trials. A: polar plots of normalized EMG activity as a function of task direction for the FDI and EI with each dot representing a trial. The FDI was primarily active in the left half-plane between extension and flexion and the EI in the upper half-plane between adduction and abduction. B: polar histograms of EWA direction for the FDI and the EI for trials in the half-plane where each muscle was active, showing that regardless of task direction, the EWA direction was focused near the direction of mechanical action for both muscles. C: histograms of EWA time to peak for the FDI and EI for trials in the half-plane where each muscle was active. Data shown are for all trials and all subjects.

Fig.4.

EWA-based thresholding and EWA shifts as a function of task direction. A: the EWA direction is essentially random when the muscle as little activity and becomes focused along the muscle's direction of action only when the EMG activity is greater. EMG threshold is determined so that the SD of EWA direction across above-threshold trials (●) is ∼25°. Below-threshold trials are shown as light points. B: a plot of EWA direction vs. task direction for above-threshold (●) and below-threshold () trials. A linear fit (—) to the above-threshold trials illustrates small but predictable shifts in EWA direction with changes in task direction.

Fig.5.

Sensitivity analysis of EWA. Using the Fuglevand model of muscle force production and EMG, we examined the sensitivity of EWA to several parameters. We found that changes in EWA magnitude were very similar to changes in the magnitude of the average STA as the muscle force was increased from 0 to 100% maximum voluntary contraction (MVC). A: the surface-recorded action potential duration is shown relative to 20 ms. The firing rate of every 10th motor unit in the model is shown as a function of muscle force level with ↓ indicating the force level at which all motor units have been recruited. The EWA and average STA magnitude first increased and then decreased with the peak indicating the percentage MVC at which all motor units were recruited. B: we found that this effect disappeared when the surface-recorded motor unit action potential was lengthened to 20 ms. C: if the range of muscle force over which motor units were recruited was compressed, the peak EWA magnitude reflected this compressed recruitment. D: we found the peak in EWA magnitude as a function of percentage MVC indicated the upper limit of motor unit recruitment in the presence of physiological levels of motor unit synchronization. E: if the surface-recorded action potential durations were not assumed to be equal, but rather to be proportional to the motor unit force generating capacity, the peak EWA magnitude still indicated the upper limit of motor unit recruitment.