Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCB156

Poster Communications

Physiological tremor reveals how thixotropy adapts skeletal muscle for posture and movement

C. Vernooij2,1, R. F. Reynolds1, M. Lakie1

1. School of Sport, Exercise & Rehabilitation Sciences, University of Birmingham, Birmingham, United Kingdom. 2. Institut des Sciences du Mouvement, Aix-Marseille Université and CNRS, Marseille, France.

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 EMG, and its output, measured as tremor (finger acceleration). Both signals were analysed continuously during smooth transitions between posture and movement. Fifteen human volunteers performed a simple tracking task using the middle finger. This involved a gradual transition from static posture to movement, and back again, lasting 120s in total (see figure 1). We recorded vertical finger position and acceleration (tremor), along with extensor EMG. Changes in the frequency and amplitude of these signals were monitored over time using a continuous wavelet transform. The results show that movement caused up to 7-fold increases in tremor amplitude, accompanied by a reduction in frequency (from 18 to 8Hz approx.). These changes correlated primarily with finger velocity, rather than position (r2 ≥ 0.87; p<0.05). By contrast, 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 a useful and hitherto unrecognized insight into skeletal muscle's role in posture and movement (Vernooij et al, 2016).

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