Physiological tremor reveals how thixotropy adapts skeletal muscle for posture and movement.
People and animals can move freely, but they must also be able to stay still. How do skeletal muscles economically produce both movement and posture? Humans are well known to have motor units with relatively homogeneous mechanical properties. Thixotropic muscle properties can provide a solution by providing a temporary stiffening of all skeletal muscles in postural conditions. This stiffening is alleviated almost instantly when muscles start to move. In this paper, we probe this behaviour. We monitor both the neural input to a muscle, measured here as extensor muscle electromyography (EMG), and its output, measured as tremor (finger acceleration). Both signals were analysed continuously as the subject made smooth transitions between posture and movement. The results showed that there were marked changes in tremor which systematically increased in size and decreased in frequency as the subject moved faster. By contrast, the EMG changed little and reflected muscle force requirement rather than movement speed. The altered tremor reflects naturally occurring thixotropic changes in muscle behaviour. Our results suggest that physiological tremor provides useful and hitherto unrecognized insights into skeletal muscle's role in posture and movement.
School of Sport, Exercise and Rehabilitation Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK; Institut des Sciences du Mouvement E.J. Marey (UMR 7287), Aix-Marseille Université and CNRS, 163 Avenue de Luminy, CP 910, Marseille 13009, France.
Carlijn A
CA
Vernooij
School of Sport, Exercise and Rehabilitation Sciences , University of Birmingham , Edgbaston, Birmingham B15 2TT , UK.
Raymond F
RF
Reynolds
School of Sport, Exercise and Rehabilitation Sciences , University of Birmingham , Edgbaston, Birmingham B15 2TT , UK.
Martin
M
Lakie
electromyography; mechanical resonance; muscle; physiological tremor; posture; thixotropy