PubMed · 162651
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K M Backhouse. 1975. Aspects of hand function and dysfunction.. https://doi.org/10.1093/occmed%2F25.4.116
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A model of a pre-planned single joint movements performed without feedback is considered. Modifications of this movement result from transformation of a trajectory pattern f(t) in space and time. The control system adjusts the movement to concrete external conditions specifying values of the transform parameters before the movement performance. The pre-planned movement is considered to be simple one, if the transform can be approximated by an affine transform of the movement space and time. In this case, the trajectory of the movement is x(t) = Af (t/tau + s) + p, were A and l/tau are space and time scales, s and p are translations. The variability of movements is described by time profiles of variances and covariances of the trajectory x(t), velocity v(t), and acceleration a(t). It is assumed that the variability is defined only by parameters variations. From this assumption follows the main finding of this work: the variability time profiles can be expanded on a special system of basic functions corresponding to established movement parameters. Particularly, basic functions of variance time profiles, reflecting spatial and temporal scaling, are x2(t) and t2v2(t) for trajectory, v2(t) and (v(t)+t.a(t))2 for velocity, and a2(t) and (2a(t)+t.j(t))2, where j(t) = d3x(t)/dt3, for acceleration. The variability of a model of a reaching movement was studied analytically. The model predicts certain peculiarities of the form of time profiles (e.g., the variance time profile of velocity is bi-modal, the one of acceleration is tri-modal, etc.). Experimental measurements confirmed predictions. Their consistence allows them to be considered invariant properties of reaching movement. A conclusion can be made, that reaching movement belongs to the type of simple pre-planned movements. For a more complex movement, time profiles of variability are also measured and explained by the model of movements of this type. Thus, a movement can be attributed to the type of simple pre-planned ones by testing its variability.
1. Subjects' ability to detect movements imposed at the elbow during active flexion was measured. Movements of three different angular velocities (0.04, 0.4 and 4.4 deg/s) were applied to the arm while subjects maintained one of two force levels of active flexion. The threshold magnitudes for detection of the direction of imposed movement were found. 2. All thresholds were very small. At the lowest velocity of movement the average threshold was 0.13 deg and no subject had a threshold of greater than 0.3 deg. This contrasts with thresholds of over 2 deg measured in a previous study when the muscles about the joint were relaxed. Thresholds decreased further with increasing velocity of movement. 3. No difference was found between the two levels of contraction of the elbow flexors. However, extension (stretch of the contracting muscle) and flexion thresholds were calculated separately, and smaller extensions than flexions could be detected. 4. These findings indicate that conscious detection of imposed movements is greatly enhanced during active muscle contraction. Movements which cause unloading of the contracting agonist, as well as movements which result in stretch, are more easily detected than when the muscle is contracting. The discussion focuses on possible mechanisms for this enhancement.
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