Quantifying dynamic and postural balance difficulty during gait perturbations using stabilizing/destabilizing forces.

Intensity of balance exercises used to reduce fall risk is often poorly quantified. The study aimed to test whether balance difficulty can be rated during gait perturbations against balance difficulty during gait without perturbation, using the stabilizing/destabilizing forces. These forces represent the difficulty to maintain balance as the theoretical forces necessary to cancel body velocity and to set the body into an unstable posture, respectively. Ten healthy subjects walked on a split-belt treadmill, that also generated perturbations. Kinetic and kinematic data were collected during gait at comfortable and fast speeds without perturbation, and in five trials at comfortable speed with perturbations. Perturbations consisted of increasing or decreasing the speed of one belt to three different levels in each direction in a random order during the stance phase of 12 random steps per trial. The difficulty of maintaining balance was measured during the perturbation and the three following recovery steps. Compared to comfortable speed, higher stabilizing and lower destabilizing forces indicated higher balance difficulty during the perturbation step for faster-belt perturbations, and recovery steps for slower-belt perturbations. This was also associated with the center of mass shifted forward, and moving faster, and with the center of pressure closer to the forward limit of the base of support. Difficulty increased proportionally with the intensity of perturbation and was significantly higher for the more intense perturbations than at fast speed. Thus, the stabilizing/destabilizing forces seem adequate to evaluate balance difficulty during gait perturbations and could be used to determine the optimal difficulty for balance rehabilitation.

Action-perception coupling in kinesthesia: a new approach.

According to recent findings, intentional motor actions are controlled by resetting the referent position, R, at which neuromuscular elements, including reflexes, begin to act. It is suggested that somatosensory afferents inform the brain about the deviation (P) of body segments from the centrally set referent position. To perceive the actual position (Q) of body segments and form the position sense (PS), the central and afferent signals are combined: Q=R+P. In previous studies, the R has been shown to remain invariant during involuntary changes in the wrist position elicited by sudden unloading of muscles, suggesting that only the afferent component is responsible for the PS during this reflex. In contrast, the central PS component, R, is predominantly responsible for PS during intentional motion in isotonic conditions. We tested the hypothesis that the R and P are interchangeable PS components such that involuntary changes in wrist position elicited by the unloading reflex can easily be reproduced by making intentional changes in wrist position in isotonic conditions, in the absence of vision. The PS rule also suggests that PS is independent of sense of effort, which was tested by asking subjects to reproduce elbow joint angles under different constant loads. We also tested the hypothesis that tendon vibration may elicit motion that may not be perceived by subjects (no-motion illusion). These hypotheses were confirmed in three experiments. It is concluded that the R and P are additive components of PS and that, contrary to the conventional view, PS is independent of the sense of effort or efference copy. The PS rule also explains kinesthetic illusions and the phantom limb phenomenon. This study advances the understanding of action-perception coupling in kinesthesia.

Corticospinal control strategies underlying voluntary and involuntary wrist movements.

The difference between voluntary and involuntary motor actions has been recognized since ancient times, but the nature of this difference remains unclear. We compared corticospinal influences at wrist positions established before and after voluntary motion with those established before and after involuntary motion elicited by sudden removal of a load (the unloading reflex). To minimize the effect of motoneuronal excitability on the evaluation of corticospinal influences, motor potentials from transcranial magnetic stimulation of the wrist motor cortex area were evoked during an EMG silent period produced by brief muscle shortening. The motoneuronal excitability was thus equalized at different wrist positions. Results showed that the unloading reflex was generated in the presence of a corticospinal drive, rather than autonomously by the spinal cord. Although the tonic EMG levels were substantially different, the corticospinal influences remained the same at the pre- and post-unloading wrist positions. These influences however changed when subjects voluntarily moved the wrist to another position. Previous studies showed that the corticospinal system sets the referent position (R) at which neuromuscular posture-stabilizing mechanisms begin to act. In self-initiated actions, the corticospinal system shifts the R to relay these mechanisms to a new posture, thus converting them from mechanisms resisting to those assisting motion. This solves the classical posture-movement problem. In contrast, by maintaining the R value constant, the corticospinal system relies on these posture-stabilizing mechanisms to allow involuntary responses to occur after unloading. Thus, central control strategies underlying the two types of motor actions are fundamentally different.

Modulation of anticipatory postural adjustments in the anticipation-coincidence task.

This study was designed to (a) verify whether the time available for movement preparation and execution modulates anticipatory postural adjustments/focal movement coordination and (b) determine to what extent the coordination in an anticipation-coincidence (AC) timing task is specific. Ten subjects performed an arm-raising movement from standing position in the reaction time, self-initiated (SI), and AC conditions. In the latter condition, subjects had to synchronize movement initiation or the end of the movement to the passage of a visual mobile on a target. In AC trials, time to contact (TC), which is the time before the mobile reached the target, was varied (720, 1,200, 3,000 ms). Electromyography, kinetic, and kinematics data were collected. Results showed that the coordination patterns were modified by TC, the velocity of the mobile, and the condition in which the movement was executed. It also showed that the behavior in the AC condition came closest to the 1 observed in SI condition when TC increased. These results support the existence of different control modes triggered by the temporal pressure.

Postural and focal inhibition of voluntary movements prepared under various temporal constraints.

Large disturbances arising from the moving segments (focal movement) are commonly counteracted by anticipatory postural adjustments (APAs). The aim of this study was to investigate how APAs - focal movement coordination changes under temporal constraint. Ten subjects were instructed to perform an arm raising movement in the reactive (simple reaction time) and predictive (anticipation-coincidence) tasks. A stop paradigm was applied to reveal the coordination. On some unexpected trials, a stop signal indicated to inhibit the movement; it occurred randomly at different delays (SOA) relative to the go signal in the reactive task, and at different delays prior to the focal response initiation in the predictive task. Focal movement was measured using contact switch, accelerometer and EMG from the anterior deltoid. APAs were quantified using centre of pressure displacement and EMG from three postural muscles. The inhibition rates as a function of the SOA produce psychometric functions where the bi-serial points allow the moment of the motor "command release" to be estimated. Repeated measures ANOVAs showed that APAs and focal movement were closely timed in the reactive task but distinct in a predictive task. Data were discussed according to two different models of coordination: (1) hierarchical model where APAs and focal movement are the results of a single motor command; (2) parallel model implying two independent motor commands. The data clearly favor the parallel model when the temporal constraint is low. The stop paradigm appears as a promising technique to explore APAs - focal movement coordination.

Modulation of anticipatory postural adjustments in a complex task under different temporal constraints.

The aim of this experiment was to explore the behavioral effects of various temporal pressures on the anticipatory postural adjustments (APAs) in a complex task. Eighteen handball players performed a handball direct throw in three conditions of temporal pressure: (1) a reactive condition (RC), the throw was initiated as quickly as possible following a visual stimulus; (2) an anticipation-coincidence condition (AC), by synchronizing the impact of the ball with the passage of a visual mobile on a target; and (3) a self-initiated (SI) throw. The whole-body postural oscillation and the acceleration of the wrist were measured before and during the throwing action. Results showed that the delays between the onsets of the postural and focal activities were significantly different between RC and both the SI and the AC conditions. Movement time, time to peaks (negative and positive), are shorter in the RC, intermediate in the AC, and longer in the SI condition. Variability was significantly larger in AC in comparison with RC and SI. These results support the existence of different control modes triggered by the temporal pressure; they demonstrate that these control modes can be generalized to complex intentional movements such as the throwing skill and to an anticipation-coincidence situation.