Abstract
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.
Highlights
Human motor activity consists of periods of immobility, enlivened by periods of movement
How are these very different postural and dynamic roles addressed by the neuromuscular system? In many species, there is a division of skeletal muscle into tonic and phasic types with profound mechanical differences, for example amphibians [1], insects [2], birds [3] and reptiles [4]
Our results show that human physiological tremor, sometimes seen as little more than a curiosity, can illuminate the way in which muscle carries out its postural and dynamic duties and provide fundamental insights into the nature of posture and movement
Summary
Human motor activity consists of periods of immobility, enlivened by periods of movement How are these very different postural and dynamic roles addressed by the neuromuscular system? Mammals, including humans, do not have specific muscle types for tonic or phasic behaviour Instead, they draw on a common pool of skeletal muscle motor units, invariably recruited in order of increasing motor neuron size, to satisfy both posture and movement [5]. One study on human extensor hallucis brevis muscles showed contraction times with an extreme range of 35–98 ms [6] These differences are rather slight, considering that posture may endure for periods of many seconds or minutes, whereas movements may be over in a fraction of a second. They seem more suited to generating movements of different contraction speeds rather than for the fundamentally opposed roles of movement and posture
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