Daniel M. Corcos

Effect of age and gender in the control of elbow flexion movements.

Abstract In previous studies of rapid elbow movements in young healthy men, characteristic task-dependent changes in the patterns of muscle activation when movement speed or distance was varied have been reported. In the present study, the authors investigated whether age or gender is associated with changes in the patterns of muscle activity previously reported in young men. Arm movements of 10 healthy older and 10 healthy younger participants (5 men and 5 women in each group) were studied. Surface electromyograms (EMGs) from agonist (biceps) and antagonist (triceps) muscles, kinematic and kinetic parameters, as well as anthropometric and strength measures were recorded. All 4 groups of participants showed similar task- (distance or speed) dependent changes in biphasic EMG activity. Similar modulation of the initial rate of rise of the EMG, integrated agonist and antagonist EMG activity, as well as their relative timing were observed in all 4 groups. Those results suggest that older individuals of both genders retain the control strategies for elbow movements used by young individuals. Despite the qualitative similarities in the patterns of muscle activation, the men moved more quickly than the women, and younger participants moved more quickly than older participants. Those performance differences could not be explained in terms of differences in body size and strength alone.

Principles Underlying Single-Joint Movement Strategies

Humans make “rational” decisions in different ways. One is according to a plan that optimizes some consequence of the decision. Such a decision-making strategy is normative, leading to a behavior appropriate for achieving some desired goal. Another way is to use a heuristic or rule based approach that guides behavior on the basis of simpler criteria than those characterized by an “optimal” result. For example, optimal strategies for obtaining wealth from the stock market or the race track are sufficiently subtle that few have found them. On the other hand, “rules of thumb” are many, and perhaps almost as much money is made from the sale of such rules as from their application. In fact, people often apply such rules in the face of objective evidence that they are in fact not optimal.

Organizing Principles Underlying Motor Skill Acquisition

A review of the literature on motor behavior suggests that there have been few attempts to understand motor learning and motor control from a common theoretical perspective. Both fields of study have a long history of research but with little overlap. This is unfortunate since the development of knowledge in each field must ultimately depend on the development of knowledge in the other. For example, many experimental results in the motor control literature are potentially confounded by the prior history of experimental subjects and the degree of learning that took place before and during data collection. Experimental paradigms requiring subjects “not to intervene” (Asatryan and Feldman, 1965; Gottlieb and Agarwal, 1988) usually require considerable subject practice and are seldom possible in untrained subjects.

Effects of STN DBS on Memory Guided Force Control in Parkinson's Disease (June 2007)

This study examined the control of elbow force in nine patients with Parkinsons disease when visual feedback was available and when visual feedback was removed to determine how medication (Meds) and unilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) affect memory guided force control. Patients were examined in each of four treatment conditions: 1) off treatment; 2) Meds; 3) STN DBS; and 4) Meds plus STN DBS. With visual feedback available, there was no difference in force output across treatment conditions. When visual feedback was removed force output drifted under the target in both the off-treatment and the Meds conditions. However, when on STN DBS or Meds plus STN DBS force output drifted above the target. As such, only STN DBS had a significant effect on force output in the vision removed condition. Increased force output when on STN DBS may have occurred due to disruptions in the basal ganglia-thalamo-cortical circuitry. We suggest that modulation of output of the internal segment of the globus pallidus by STN DBS may drive the effect of STN DBS on memory guided force control.

Resolving kinematic redundancy in target-reaching movements with and without external constraint

In the context of target-reaching movements that involve the arms, trunk and legs, we have delineated rules for apportioning motions amongst body segments, which would be valid for a range of target locations that require forward bending. We further attempted to determine whether the rules are altered when motion is restricted at the knee, obliging a re-apportioning of segment motions. For each participant moving with unrestricted joints to nine target locations, principal component analysis of the changes in orientation (i.e., excursions) of six chosen segments revealed that their coupling can be described by two effective degrees of freedom (DoFs), whose weighted combinations account for the segmental excursions. Investigating the similarities and differences among individuals, we found that a set of two effective DoFs could account for the segment excursions among the group of participants who flex their knees significantly, and another set of two for the group who do not. Comparing the motions with and without the knee joints braced, we found that for each individual participant a set of two (or in some cases three) effective DoFs derived from the unrestricted segment excursions could account well for the altered segment excursions when the participant reached for the targets with the knees restricted. Our findings imply that the redundancy of kinematic DoFs can be resolved by reliance on a small number of couplings of segmental excursions, and, in light of the robustness of these couplings against mechanical restriction of joint motion, suggest a neural rather than mechanical origin for them.

Kinetic and kinematic adaptation to anisotropic load

Different investigators have proposed that multi-joint arm movements are planned with respect to either the path of the hand or the forces and torques acting about the moving joints. In this experiment, we examined the kinematic and kinetic response of the motor system when a load was applied to the forearm, which reduced the natural anisotropy of the arm. We asked two questions: (1) when the movement path changes upon the introduction of the novel load, do muscle torques at the shoulder and elbow remain the same as they were before the load was applied? and (2) when the path is restored partially as the novel load is learned, do changes in muscle torque occur preferentially at one or the other joint? Participants performed rapid arm movements to a target with and without the novel load attached to their arm. Changes in hand path and muscle torque profiles were examined immediately after the application of the load and again following 30 practice trials. The introduction of the load increased the curvature of hand paths for each participant and resulted in changes in the magnitude and time course of muscle torque at both joints, although to a greater extent at the shoulder. After practice with the load, hand paths and elbow muscle torques resembled those produced with no load. Muscle torques produced at the shoulder, however, did not return to pre-load patterns. These observations provide support for the idea that movements are initiated by planned muscle torques and that as the movement proceeds muscle torques are regulated in order to produce hand paths that conform approximately to a kinematic plan.

Organizing principles for single joint isometric contractions

The dual-strategy hypothesis of motor control is extended to isometric contractions about the elbow. Subjects made isometric contractions (both pulses and steps) of different amplitudes and rates. The speed-sensitive strategy is used when the task requires the subject to explicitly regulate the rate at which torque is generated. In all other situations, the speed-insensitive strategy is used.<<ETX>>