Biomedical subjects

Frederic Danion

Publications and source records attributed to Frederic Danion.

6 recordsLinked to original sources

PubMed2003

Effects of transcranial magnetic stimulation on muscle activation patterns and joint kinematics within a two-joint motor synergy.

We studied the effects of transcranial magnetic stimulation (TMS) on muscle activation patterns and joint kinematics during oscillatory movements of a joint within a two-joint (elbow-wrist) synergy. Healthy young adults (N=8) performed cyclic movements in one of the joints while the other joint was or was not involved into a postural task. TMS was applied unexpectedly at different phases of the movement cycle. TMS-induced motor evoked potentials (MEPs) showed a modulation within the movement cycle, partly related to changes in the background muscle activity. After normalization by the background activity, MEPs were higher in conditions involving maintenance of a posture by one of the joints. MEPs in a flexor-extensor muscle pair could show peaks at the same phase within the movement cycle while background EMG peaks were out of phase. TMS induced a phase shift in the ongoing movement, and the phase shift changed over the next cycles. It stabilized by the fourth cycle and could show both phase advances and phase delays, the latter dominated. The findings are interpreted as supporting two hypotheses: (1) TMS can activate cortical neurons interposed in a transcortical long-latency loop leading to pre-programmed reactions to perturbations; and (2) apparent effects of TMS on the timing of an ongoing cyclic movement are defined by an interaction between its effects on a central 'clock' and those induced by the peripheral mechanical response.

Adult↗

PubMed2003

Approaches to analysis of handwriting as a task of coordinating a redundant motor system.

We consider problems of motor redundancy associated with handwriting using the framework of the uncontrolled manifold (UCM) hypothesis. Recent studies of finger coordination during force production tasks have demonstrated that the UCM-hypothesis provides a fruitful framework for analysis of multi-finger actions. In particular, it has been shown that during relatively fast force changes, finger force variance across trials is structured such that a time pattern of total moment produced by the fingers with respect to a point between the two most lateral fingers involved in the task is stabilized while the time pattern of total force may be destabilized. The findings of selective moment stabilization have been interpreted as being conditioned by the experience with everyday motor tasks that commonly pose more strict requirements to stabilization of total moment than to stabilization of total force. We discuss implications of these findings for certain features of handwriting seen in elderly, children, patients with neurological disorders, and forgers.

Biomechanical Phenomena↗

PubMed2002

Coupling phenomena during asynchronous submaximal two-hand, multi-finger force production tasks in humans.

We studied asynchronous two-hand multi-finger force production at submaximal levels. Subjects produced constant levels of force with one hand (F(REF)), and started ramp force production with the other hand (F(RAMP)) 3 s later. The zero-maximal force-zero ramp lasted 10 s. Visual feedback was provided only for F(REF) during the first 3 s and only for F(RAMP) during the rest of the trial. The initiation of F(RAMP) induced a transient overshoot by F(RAMP) and a drop in F(REF); both scaled with F(REF). Later into the trial, F(REF) dropped slowly, while F(RAMP) undershot maximal force by about 15%. Two regimes of bimanual interaction are hypothesized, 'strong interaction' when an effector is added to a one-hand task, and 'weak interaction' during two-hand force production.

Adult↗

PubMed2002

Finger coordination during discrete and oscillatory force production tasks.

We used the framework of the uncontrolled manifold (UCM) hypothesis to analyze the structure of finger force variability in discrete (ramp) and oscillatory force production tasks performed by the index and middle fingers of the right hand acting in parallel. Subjects performed the tasks at fast and slow rates, with and without a visual template presented on the screen. The variance of finger forces was partitioned into two components, compensated variance (V(COMP)), which did not affect total force, and uncompensated variance (V(UN)), which affected total force. Only minor effects of task (discrete or oscillatory) and of template (with or without) were seen on the variance profiles, leading us to conclude that the basic principles of synergy organization are common across discrete and oscillatory tasks. In contrast, the rate of force production had major effects on the structure of force variance. A modification of Goodman's model of motor variability was used to analyze the dependences V(UN) and V(COMP) on the magnitude of force and on the rate of force production. V(UN) showed a strong relation to the rate of force production and only weak dependence on the magnitude of force. In contrast, V(COMP) showed minimal effects of the rate of force production and strong effects of the force magnitude. The findings are interpreted as demonstrations of a limitation in the ability of the central nervous system to organize a two-finger synergy such that errors in the timing of individual finger force profiles are canceling each other's effects on the total force. In contrast, the synergy is efficiently intercompensating errors related to imprecise setting of force magnitudes of the two fingers.

Adult↗

PubMed2002

Central mechanisms of finger interaction during one- and two-hand force production at distal and proximal phalanges.

In this study we used changes in the relative involvement of different muscle groups during force production at the distal (DT) and proximal (PR) phalanges to test and modify a hypothesis on the central organization of multi-finger control for tasks involving non-homologous elements in the two hands. Ten subjects produced maximal force with different finger combinations. Two symmetrical (PR/PR and DT/DT) and two asymmetrical (PR/DT and DT/PR) combinations of force application sites in the two hands were used. During one-hand tasks, higher forces were produced at the PR site. In multi-finger tasks, total peak force was smaller than the sum of peak forces in single-finger tasks by the involved fingers (force deficit). Force production by some fingers of a hand was accompanied by involuntary force production by other fingers (enslaving). Force deficit and enslaving were both higher at the PR site. Two-hand tasks were accompanied by an additional drop in the force of individual fingers, i.e., bilateral deficit (BD). When symmetrical sites of force production were used in the two hands, BD was lower for symmetrical finger groups than for asymmetrical groups. During tests at asymmetrical sites, BD was higher and did not depend on symmetry of involved finger groups. We conclude that within-a-hand force deficit and enslaving are likely to be of a central, neural origin. An earlier introduced hypothesis has been expanded assuming that excitatory projections to contralateral finger representations exist only for homologous elements (sub-synergies) of a multi-finger force production synergy, while only inhibitory projections connect non-homologous elements.

Adult↗

PubMed2002

Understanding finger coordination through analysis of the structure of force variability.

Most common motor acts involve highly redundant effector systems. Understanding how such systems are controlled by the nervous system is a long-standing scientific challenge. Most proposals for solving this problem are based on the assumption that a particular solution, which optimizes additional constraints, is selected by the nervous system out of the many possible solutions. This study attempts to address this question in the context of coordinating individual finger forces to produce a controlled total force oscillation between 5% and 35% of each subject's maximum force of voluntary contraction, under two different combinations of four fingers. The structure of variability of individual finger forces was evaluated with respect to hypotheses that, at each instance in time, subjects attempt to: (1) stabilize the value of total force and (2) stabilize the total moment created by the fingers about the long axis passing through the forearm and midline of the hand. The results provide evidence that a range of goal-equivalent finger force combinations is generated to stabilize the values of total force and the total moment. The control of total force was specified explicitly by the task. However, it was stabilized only near the time of peak force. In contrast, the total moment was stabilized throughout most of the force cycle. The results lead to the suggestion that successful task performance is achieved, not by selecting a single optimal solution, but by discovering an appropriate control law that selectively stabilizes certain combinations of degrees of freedom relevant to the task while releasing from control other combinations.

Adult↗