Authors: H. W. Axelson; K.-E. Hagbarth

Source: The Journal of Physiology, Volume 535, Number 1, August 2001 , pp. 279-288(10)

Publisher: Wiley-Blackwell

Abstract:

• The primary aim of the present study was to explore whether in healthy subjects the muscle contractions required for unrestrained voluntary wrist dorsiflexions are adjusted in strength to thixotropy-dependent variations in the short-range stiffness encountered in measurements of passive torque resistance to imposed wrist dorsiflexions.

• After a period of rest, only the first movement in a series of passive wrist dorsiflexions of moderate amplitude exhibited clear signs of short-range stiffness in the torque response. During analogous types of voluntary movements, the extensor EMG during the first movement after rest showed a steep initial rise of activity, which apparently served to compensate for the short-range stiffness.

• The passive torque resistance to minute repetitive wrist dorsiflexions (within the range of short-range stiffness) was markedly reduced after various types of mechanical agitation. During analogous low-amplitude voluntary wrist dorsiflexions the extensor EMG signals were weaker after than before agitation.

• Mechanical agitation also led to enhancement of passive dorsiflexion movements induced by weak constant torque pulses. In an analogous way, the movement-generating capacity of weak voluntary extensor activations (as determined by EMG recordings) was greatly enhanced by mechanical agitation.

• The signals from a force transducer probe pressed against the wrist flexor tendons - during passive wrist dorsiflexions - revealed short-range stiffness responses which highly resembled those observed in the torque measurements, suggesting that the latter to a large extent emanated from the stretched, relaxed flexor muscles. During repetitive stereotyped voluntary wrist dorsiflexions, a close correspondence was observed between the degree of short-range stiffness as sensed by the wrist flexor tension transducer and the strength of the initial extensor activation required for movement generation.

• The results provide evidence that the central nervous system in its control of voluntary movements takes account of and compensates for the history-dependent degree of inherent short-range stiffness of the muscles antagonistic to the prime movers.

Document Type: Research article

DOI: http://dx.doi.org/10.1111/j.1469-7793.2001.00279.x

Publication date: 2001-08-01

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