Quantitative interpretation of lumbar muscle myoelectric signals during rapid cyclic attempted trunk flexions and extensions.

The quantitative relationship between lumbar myoelectric signals (MES) and rapidly varying isometric trunk muscle forces was investigated. Ten young adult males were asked to cycle harmonically between attempted trunk flexion and attempted trunk extension in an upright position at rates of 0.33, 0.67 and 1.0 Hz to peak efforts of 20, 40 and 60% of maximum voluntary exertion levels. The forces voluntarily exerted against a load cell were measured and used along with acquired kinematic data to calculate the time course of the net sagittal moment at the level of the third lumbar vertebra during task performances. A 22 muscle double linear programming biomechanical model was used to predict the lumbar trunk muscle contraction forces from the calculated moments. Rectified and bidirectionally low-pass filtered myoelectric activities were acquired at the L3 level from four abdominal muscles and four back muscles. The processed MES were found to be well correlated (r > 0.90) with predicted muscle forces when the MES were time-shifted to account for electromechanical delay as well as the dynamic phase shift between muscle electrical activity and contraction force. Mean time shifts that maximized the linear MES-force relationship ranged from 111 to 218 ms, were greater for the trunk extensors than the trunk flexors and generally exhibited lateral symmetry. The corresponding approximate phase angles averaged 20 degrees at the slowest rate and 50 degrees at the fastest rate. MES-force phase angles decreased as effort level was increased indicating that the dynamic MES-force relationship is nonlinear. These results illustrate the importance of accounting for the phase lag between muscle electrical activity and force when using MES to quantify muscle loads during rapidly varying exertions.