38,287 research outputs found
Quantitative evaluation of the electromyographic responses to multidirectional load perturbations of the human arm
No full text1. Force perturbations consisting of a random train of pulses were applied to the forearms of human subjects, the direction of the force being varied +/- 60 degrees from vertical in the sagittal plane in different trials. 2. Both forearm and upper arm were free to move, and the perturbations resulted in angular motion and torque at both joints. By varying the direction of the force, different combinations of these variables could be obtained. 3. Average angular motion and net torque at the shoulder and elbow joints and electromyographic activity of shoulder and elbow muscles due to a single pulse of force were computed by cross-correlation methods. 4. The pattern of responses in biceps, brachio-radialis, and anterior deltoid was not related uniquely to angular motion at the shoulder or elbow joints. Furthermore, the responses appeared to consist of two distinct components, an "early" one with a latency less than 40 ms and a "late" one with a latency of approximately 80 ms. 5. The average amplitude of the early response was best correlated with the average change in angular velocities, whereas that of the late one was best correlated with average changes in torque resulting from the perturbation. The data are consistent with the hypothesis that the two components have different anatomical substrates and that they have different functional implications for the stabilization of the limb in the face of perturbations
Coordination of arm and wrist motion during a reaching task
No full textAn analysis of arm movements involving forward projection of the hand in order to reach for and grasp a target at different orientations is presented. The reaching movements required shoulder flexion, elbow extension, and wrist pronation or supination. The relation between elbow and shoulder instantaneous angular position proved to be consistent from trial to trial of each task, independent of movement speed. Further, this relation was not influenced by the presence or absence of a concomitant wrist rotation. During the deceleratory phase of the movement, the slope of elbow angular velocity to shoulder angular velocity was constant and independent of target orientation. Wrist motion was instead highly variable in timing, course, and duration. Supinatory movements tended to be fractionated. On average, the duration of wrist movements was shorter than that of shoulder and elbow motions. The pattern of biceps EMG activity during supinatory and pronatory movements was different. Since motion at the shoulder and elbow was virtually identical in the two cases, net flexor torque at the elbow was also little different. It is concluded that other elbow flexors and extensors also exhibit a task-dependent patterning of activity so as to produce the same net torque. The results are discussed in the context of the internal constraints present during the movements that we examined. These constraints are the inertial coupling between shoulder and elbow motion and those which derive from the bifunctional nature of many of the muscles participating in the movement
Invariant characteristics of a pointing movement in man
No full textSimple arm movements involving forward projection of the hand toward a target were studied by measuring simultaneous wrist position in three-dimensional space and changes in elbow angle. An attempt was made to identify those features of the movement which exhibit invariant characteristics under the hypothesis that such invariances may reflect the operations by which central processes participate in the organization of the movement. The first such invariance to be identified was that the trajectory in space is independent of movement speed. Secondly, the movement can be viewed as consisting of two phases, an acceleratory phase and a deceleratory one, with the movement during the acceleratory phase being so organized as to maintain the ratio of elbow angular velocity to shoulder angular velocity invariant with respect to target location in the deceleratory phase. It is suggested that proprioceptive information is used to control the movement and that the latter invariance may result from a negative feedback of force involving tendon organ afferents
Modification of trajectory of a pointing movement in response to a change in target location
No full text1. Arm movements toward a target involving motion at the shoulder and elbow joints and restricted to the sagittal plane were investigated. During some movements, target location changed suddenly, thus requiring an intentional correction of the trajectory by the human subject. 2. The reaction time to correct the trajectory was comparable to the reaction time to initiate the movement. 3. Coordination of arm and shoulder movements in this task was achieved by means of a reduction of the number of degrees of freedom of the movement. Such a simplification of the task took two forms. 4. Angular acceleration at the elbow and shoulder were linearly related to each other in the deceleratory phase of the movement, when the trajectory had to be corrected as well as when no such correction was required. 5. When the correction required an increase or decrease in flexor torque at the shoulder and elbow, electromyographic (EMG) activity in anterior deltoid and biceps increased or decreased simultaneously. When the correction demanded more activity in triceps and deltoid, these muscles were activated sequentially instead. It is concluded that rapid corrections of a movement involve the production of stereotyped patterns of activity in shoulder and elbow muscles
Simulation studies on the control of posture and movement in a multi-jointed limb
No full textSimulation studies were performed to evaluate the effectiveness of different control schemes in stabilizing a multi-jointed limb (human arm) in response to force perturbations. The mechanical properties of the arm were modeled as a linear visco-elastic system and the effectiveness of negative feedback of angular position and torque was evaluated. The effectiveness of a given amount of position feedback depended strongly on the initial position of the arm and on the perturbation, while torque feedback was much more consistently effective in damping the motion of the limb
The visual localization of the center of mass of compact asymmetric two-dimensional shapes
No full textBehavior of the stretch reflex in a multi-jointed limb
No full textWhen an external perturbation acts on a segment of a multi-jointed limb, angular motion results in all limb segments which are inertially linked to the one which is perturbed. In such a situation, different hypotheses on the functional role of the stretch reflex lead to different predictions on reflex responses by the stretched muscles. These predictions were examined experimentally and it was found that the EMG responses of elbow muscles evoked by forces applied to either the upper arm or the forearm were not uniquely related to changes in elbow angular position. They were instead correlated with the direction of changes in the net elbow torque resulting from the perturbation
EMG responses to load perturbations of the upper limb: effect of dynamic coupling between shoulder and elbow motion
No full textLoad perturbations were applied to the arm of human subjects under conditions where both limb segments (upper arm and forearm) were free to move. The perturbations consisted of pulses of torque 50 ms in duration and of pseudo-random sequences of such pulses. They were applied to either the forearm or the upper arm. Under all conditions, the perturbations resulted in angular motion at the shoulder and elbow joints and evoked consistent responses in muscles acting about these joints (biceps, triceps, anterior and posterior deltoid). Activity in biceps and triceps was not related simply to angular motion at the elbow joint. For example, activation of biceps could be evoked during elbow flexion (by applying a torque perturbation at the shoulder) as well as during elbow extension (by applying a torque perturbation at the elbow). The effect of varying degrees of dynamic coupling between upper arm and forearm on EMG responses was investigated by applying torque perturbations to the upper arm over a wide range of elbow angles. When the forearm is extended, such a perturbation induces a greater amount of elbow flexion than when the forearm is in a flexed position. The results of these experiments showed that the larger was the amount of flexion of the forearm induced by the perturbation, the larger was the activation of biceps. The results are incompatible with the notion of a negative feedback of total muscle length as being responsible for the EMG activity following the load perturbations. It is suggested that the EMG responses can best be interpreted functionally in terms of parameters more global than muscle length. Among such global parameters, changes in net torque at a joint resulting from the perturbation gave the best correlation with the pattern of EMG activities observed
An assessment of the existence of muscle synergies during load perturbations and intentional movements of the human arm
No full textA cross-correlation analysis was performed on EMG activities in elbow and shoulder flexors evoked by force perturbations acting in different directions on the forearm and during intentionally generated movements with the purpose of characterizing the temporal relationships between patterns of activation of different muscles. Qualitatively it was found that the shape of the cross-correlation function differed from one experimental condition to the next. A principal component analysis permitted a quantitative assessment of this point. In general it was found that two principal components could account for the data. Furthermore when the cross-correlograms are represented in principal component space, there was no clustering of the data points. Several possible definitions of 'muscle synergies' are discussed from the perspective of this finding. It is concluded that the most restrictive definition is incompatible with the data. Less restrictive definitions, while compatible with the data, do not lead to a simplification of the control problem
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