Control scheme governing concurrently active human motor units during voluntary contractions

Abstract

SUMMARY1. The electrical activity of up to eight concurrently active motor units has been recorded from the human deltoid and first dorsal interosseous muscles. The resulting composite myoelectric signals have been decomposed into their constituent motor-unit action potential trains using a recently developed technique.2. A computer cross-correlation analysis has been performed on motor-unit firing rate and muscle-force output records obtained from both constant-force and triangular force-varying isometric contractions performed by normal subjects, and three groups of highly trained performers (long-distance swimmers, powerlifters and pianists).3. The temporal relationships between firing rate activity and force output have provided evidence that the deltoid of long-distance swimmers has a significantly higher percentage of slowly fatiguing fibres than that of normal subjects. 4. Results showed that both muscles are incapable of producing a purely isotonic contraction under isometric conditions. Small, possibly compensatory force variations at 1-2 Hz result from a common drive to all active motoneurones in a single muscle pool.5. Rapid force reversals during triangular, force-varying isometric contractions appear to be accomplished through a size-related motor-unit control scheme. All firing rates decline prior to the force peak, but small motor units with slow-twitch responses tend to decrease their firing rates before large, fast-twitch motor units. This mechanism is not visually controlled, and does not depend on force rate in non-ballistic contractions.

Bilateral motor unit synchronization is functionally organized

“…In support of this interpretation, an increase in amplitude and a decrease in median frequency have been found in the contralateral homologous muscle after unilateral muscle fatigue (Morrison et al 2005). Note that common drive refers to a homogeneous input to human MU pools (De Luca et al 1982;De Luca and Erim 1994) and that antagonist cocontraction could be instrumental in maintaining joint stability (Solomonow et al 1988).…”

Aging effects on the control of grip force magnitude: An fMRI study

“…At the neuromuscular level it has been demonstrated that recruitment of motor units is used during hand force production only up to 40-50% MVC (De Luca et al, 1982;Kukulka and Clamann, 1981), suggesting that rate coding is primarily responsible for increasing force output beyond 50% MVC. Rate coding occurs over a narrower range of forces in older adults (Barry et al, 2007).…”

“…It was hypothesized that the older adults would require greater levels of activation within these motor areas to achieve the same relative force levels as the younger group. It was expected that these differences would be greatest at higher levels of force production due to the extra neural drive that is needed to overcome the neuromuscular changes that occur with aging (Barry et al, 2007;De Luca et al, 1982;Kukulka and Clamann, 1981). A voxel-wise whole brain analysis was also carried out to determine all of the locations of age-related changes in brain activation during this force modulation task.…”

Theoretical Model and Experimental Validation of the estimated proportions of common and independent input to motor neurons

“…This phenomenon is often referred to as "short term synchronization" [8] and can be estimated either in the temporal domain using cross-correlation techniques [9] or in the spectral domain by computing coherence functions [10]. The low frequency range in correlation (up to 5 Hz) reflects the usually termed 'common drive' to motor neurons [11] while the range 5-12 is related to the generation of relatively slow movements and tremorogenic oscillations [12]. Higher frequency bands are thought to mainly the interaction between the motor cortex and the spinal pathways [13].…”