In 17 healthy volunteers, we studied movements of the forearm, which included episodes of positioning on the target level. The trajectory of the non-ballistic (relatively slow) movement looked like a double trapezium (flexion of the elbow joint from the state of full extension, 0 deg, positioning on the 50 deg level, further flexion to the limit angle of 100 deg, and a similar reverse sequence). The command trajectory and the trajectory of the realized movement were visualized with movements of cursors on a monitor in time/joint angle coordinates. We compared parameters of the tracking movements (in the presence of visual feedback) and their blindfold reproduction (with the complete absence of visual control). It was found that blindfold reproduction movements differ from sample tracking movements and their reproduction with partial limitation of visual control [16] in higher peak velocities and shorter durations, i.e., a trend toward conversion of such movements into ballistic ones was observed. Under conditions of elimination of visual control, movements that led to positioning were mostly hypermetric, i.e., positioning was usually accompanied by positive systematic errors (whose sign coincided with the direction of the preceding movement phase). The mean intragroup value of the systematic error of the first positioning (after flexion to the target level) was +6.73 ± 1.15 deg, while the respective mean for the second positioning (after extension to the same level) was +4.00 ± 1.31 deg. The nonlinear properties of stretch receptors of muscles whose activity provides the formation of a "proprioceptive" estimate of the joint angle are considered the crucial reason for systematic errors of blindfold positioning.
Healthy adult humans performed elbow movements in a horizontal plane under a small external extending torque (2.1-3.3 Nm). Test movements (TMs) consisted of slow ramp-and-hold flexions in the absence of visual feedback, with the target joint angle to be remembered from a preceding conditioning movement (CM). The CM was produced by matching two beams on the monitor screen: (1) command representing the target position (a straight line); and (2) a signal from the sensor of the elbow joint angle. Two kinds of CM were applied, which had the same target position (50 degrees in most experiments) but differed in initial positions: (1) fully extended joint (0 degrees, P1 CMs); (2) flexed joint (100 degrees, P2 CMs). In a group of 25 subjects, the target in TMs was usually overshot, with the position errors depending on the CMs: 2.7 +/- 0.6 degree (mean +/- SEM) for P1 CMs, and 10.9 +/- 0.7 degree (P < 0.001) for P2 CMs. Vibration of the elbow flexors substantially diminished the difference between the position errors, amounting to--0.31 +/- 0.5 degree and 2.33 +/- 0.6 degrees, respectively. It is suggested that the observed position errors resulted from after-effects in the activity of muscle spindles in agonist and antagonist muscles, but influence of differences in dynamic components of the afferent signals during oppositely directed approaches to the target cannot be excluded.
We studied movements of the forearm within the limits of 0 deg (full extension) -flexion to a 100 deg angle in the elbow joint -a reverse movement with episodes of target positioning at an intermediate target level (50 deg). The standard trajectory and trajectory of the performed movement were visualized by movements of cursors on the screen of a monitor in joint angle vs time coordinates. Systematic errors of blindfold (kinesthetic) positioning (after removal of the visual feedback informing the subject on the characteristics of the performed movement) observed under control conditions and after realization of a fatigue-inducing series of flexions/extensions of the forearm with a high loading were compared. It was found that the development of fatigue evoked no fundamental changes in the pattern of systematic errors of kinesthetic positioning. Both considerable prevailing of positive systematic errors within the examined group, their high interindividual variability, and (in most cases) patterns of signs of errors after reaching the target level by movements of opposite directions typical of the given subject were preserved. Mean intragroup values after the development of fatigue demonstrated some trend toward a decrease, but these changes did not reach the significance level. Possible mechanisms of the influence of muscle fatigue on the process of target positioning of a limb link realized exclusively under proprioceptive control are discussed.
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