Extraction of Individual Muscle Mechanical Action From Endpoint Force

Abstract

Kutch JJ, Kuo AD, Rymer WZ. Extraction of individual muscle mechanical action from endpoint force. J Neurophysiol 103: 3535-3546, 2010. First published April 14, 2010 doi:10.1152/jn.00956.2009. 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 Ϯ 2°for the FDI and 71 Ϯ 5°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.

Index finger position fluctuations reflect multi-muscle coordination

“…These results indicate that the fluctuations in the index finger movements were distributed not only in the abduction-adduction direction but also in the extension-flexion direction. The index finger movements in various directions in the current study were consistent with those in previous studies in which the isometric forces in several muscles across the MCP joint were demonstrated by fluctuations in abduction-adduction/extension-flexion (Kutch et al, 2008(Kutch et al, , 2010.…”

“…Biomechanical (Valero-Cuevas et al, 1998) and neurophysiological approaches (Kutch et al, 2008(Kutch et al, , 2010 demonstrated that the preferred direction of the first dorsal interosseus (FDI) muscle is not complete abduction direction but slightly flexion. FDI can help with that, but it comes at a cost flexion force generated by its flexion moment arm (An et al, 1983).…”

“…In order to identify the physiological mechanisms underlying the endpoint force or acceleration fluctuations of the index finger, the multi-directional motor output should be investigated because of the huge degree of freedom of the index finger (Kutch et al, 2008(Kutch et al, , 2010. If the ability to control of precise movements of the index finger is evaluated, then the various directions of index finger movements have to be comprehended.…”

Estimation of musculoskeletal models from in situ measurements of muscle action in the rat hindlimb

“…Note also that although the experiments described here involved extensive dissections to isolate individual muscles, other less invasive methods of estimating muscle actions have been developed (Kutch et al, 2010). Such less invasive methods might be used in combination with the computational analyses described in this study to construct musculoskeletal models.…”

Hysteresis Properties of EMG Activity of the Shoulder Belt and Shoulder Muscles at the Development of Isometric Efforts by the Human Arm

“…In another work of the same research group [18], force contributions of separate muscles to the effort developed by the index finger tip were estimated in a parallel manner with recording of integral EMGs of the muscles involved. In this case, the probable hysteresis phenomenon in EMG activity of the muscles involved in generation of isometric efforts, which was observed in our study, was also not taken into account.…”