CONTROL VARIABLES AND PROPRIOCEPTIVE FEEDBACK IN FAST SINGLE-JOINT MOVEMENT

Citation
Mf. Levin et al., CONTROL VARIABLES AND PROPRIOCEPTIVE FEEDBACK IN FAST SINGLE-JOINT MOVEMENT, Canadian journal of physiology and pharmacology, 73(2), 1995, pp. 316-330
Citations number
49
Categorie Soggetti
Pharmacology & Pharmacy",Physiology
ISSN journal
00084212
Volume
73
Issue
2
Year of publication
1995
Pages
316 - 330
Database
ISI
SICI code
0008-4212(1995)73:2<316:CVAPFI>2.0.ZU;2-6
Abstract
Sensorimotor mechanisms were studied on the basis of kinematic and ele ctromyographic data as well as the static torque developed by the musc les as a function of joint angle. The latter relationship is known as the torque/angle characteristic. Fast single-joint movement may result from a shift in this characteristic and a change in its slope. Such m ovements were studied at the wrist in 9 normal and 1 deafferented subj ect. After training to flex the wrist to a target, subjects repeated t he same movements but in random test trials movements were opposed by the load generated by linear position feedback to a torque motor. At t he end of the loaded trials, the load was suddenly removed. In the sec ond experiment, subjects made wrist movements to the target that were opposed by the load and, on random test trials, the movements were not loaded. In these test trials, the wrist arrived in a static position outside the target zone. In both experiments, subjects were instructed not to correct errors. The final torque/angle characteristics specifi ed in the movements were reconstructed on the basis of the static wris t positions and torques before and after unloading. Normal subjects ma de movements by shifting the position of the torque/angle characterist ic and by increasing its slope. If subjects indeed maintained the same pattern of control variables (descending commands), the same final po sition of the characteristic would be reproduced from trial to trial r egardless of load perturbations. This assumption of equifinality was t ested by comparing the final position of the wrist in nonloaded moveme nts with that after removal of the load in loaded movements. Equifinal ity was observed in normal subjects. Movements in the deafferented sub ject were also associated with a shift of the torque/angle characteris tic and a change in its slope. However, she was unable to consistently reproduce its final position. In spite of muscle coactivation, her ma ximal stiffness was lower than in normal subjects. In the absence of v ision, the subject made movements with the load by increasing the slop e of the characteristic instead of by shifting its position far enough . Load perturbation affected her final wrist position (inequifinality) , which may reflect the presence of a significant hysteresis of the ch aracteristic as a result of the absence of stretch reflexes. The defic its following deafferentation presumably result from the destruction o f biomechanical and sensorimotor mechanisms including the ability of c ontrol variables to specify the positional frame of reference for affe rent and descending systems.