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
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.