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 (∼5 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.