Human motor compensations for thixotropy-dependent changes in resting wrist joint position after large joint movements

Aim: Resting tension of relaxed skeletal muscle fibres held at a given length varies with the immediate previous history of length changes and contractions. The primary aim of this study was to explore the motor control consequences of this history-dependency in healthy subjects. Methods: Angular position and passive torque were recorded from the intact wrist joint. Integrated surface electromyography (IEMG) was recorded from wrist extensor and flexor muscles. Results: In relaxed subjects, wrist joint position was displaced towards dorsiflexion after a single high-amplitude dorsiflexion movement combined with a strong flexor/extensor co-contraction (dorsiflexion conditioning), whereas after volarflexion conditioning there was a shift towards volarflexion. These after-effects could be abruptly cancelled by short periods (similar to5 s) of rapid flapping hand movements or forceful isometric co-contractions, findings indicative of muscle thixotropy. The IEMG-evaluated motor after-effects were as follows. A slowly subsiding wrist flexor contraction was needed to restore and maintain the original resting wrist position after dorsiflexion conditioning whereas a slowly subsiding extensor contraction was needed for the same goal after volarflexion conditioning. Furthermore, ongoing wrist extensor IEMG activity required to actively hold the wrist in a moderate dorsiflexed position or to resist a constant volar torque at resting position was temporarily reduced after dorsiflexion conditioning and enhanced (not significantly) after volarflexion conditioning. Conclusion: The results provide evidence that during voluntary maintenance of a desired wrist Joint position the motor commands to the position-holding muscles are unconsciously adjusted to compensate for thixotropy-dependent variations in the resting tension of the muscles.