Role of peripheral vision in rapid perturbation-evoked reach-to-grasp reactions.

Onset and execution of compensatory reaches are faster than the most rapid voluntary reaches. With onset latencies near 100 ms, it is proposed that initial control of compensatory reaches cannot rely on visual information obtained after perturbation onset; rather, they rely on a visuospatial map acquired prior to instability. In natural conditions, it is not practical to direct gaze toward every potential support surface in preparation for a perturbation, suggesting that peripheral vision may be uniquely important. This study aimed to determine whether visuospatial mapping achieved using only peripheral visual information could be used to control reach-to-grasp reactions. Participants sat in an unstable chair. Whole body perturbations were used to evoke rapid reach-to-grasp reactions. A handle was positioned at midline or to the right of the participant. Gaze was directed toward the center or right to view the handle in peripheral or central visual fields. Electromyographic and kinematic data were recorded. Peripheral information acquired prior to perturbation was sufficient for successful execution of reach-to-grasp without delay. Differences in reach kinematics, however, did exist between vision conditions (e.g., maximum lateral wrist displacement and magnitude of hand overshoot relative to the handle were greater for peripheral vs. central vision). Handle location led to target-specific differences in initial muscle recruitment revealing information acquired prior to perturbation were used to guide initial limb trajectory. Results reveal the capacity to rely on a visuospatial map constructed from peripheral visual information for compensatory reaching but also highlight limitations leading to more conservative reach trajectories.

Parkinson's disease and segmental coordination during turning: I. Standing turns.

OBJECTIVE: Many of the falls among people with Parkinson's disease (PD) occur during sudden, on-the-spot turning which requires systematic reorientation of axial segments towards the new direction. We examined whether a disturbance in the coordination of segmental reorientation is an important cause of turning difficulty in individuals with PD and is altered by dopaminergic medication. METHODS: The sequence and timing of segmental reorientation during 45° and 90° on-the-spot turns was examined in fourteen individuals with PD while "off" and "on" medication and nineteen healthy controls (HC). RESULTS: Regardless of the magnitude of the turn, HC reoriented their head, shoulder, and pelvis simultaneously followed by mediolateral foot displacement. PD patients displayed temporal coordination patterns similar to the HC. PD however, reduced the velocity and early magnitude of reorientation of each body segment which were both slightly improved by dopaminergic medication. CONCLUSION: Our finding that the HC and PD patients turn en bloc when the turn is predictable and there are no time constraints shows that the strategy of en bloc turning is not wrong if the movement parameters are unconstrained. However, in real life situations, which usually require quick and unpredictable turns, the en bloc strategy may be unsafe and more likely to result in falls. While in such situations HC are able to change the strategy from en bloc to sequential segmental turning, PD patients may not be able to do so and continue to turn en bloc.

Parkinson's disease and segmental coordination during turning: II. Walking turns.

OBJECTIVE: Individuals with Parkinson's disease (PD) show poorer balance and greater incidence of falls while turning. We investigated whether a disturbance in timing and sequence of reorientation of body segments is a potential cause of turning difficulty in PD and is altered by levodopa. METHODS: The sequence and timing of segmental reorientation during 45° and 90° walking turns were recorded in nineteen healthy controls and fourteen individuals with PD "off" and "on" medication. RESULTS: Both healthy elderly and PD patients "off" medication displayed a top-down sequence of segment reorientation, but differed with respect to the delay time between segments: PD "off" medication displayed a shorter delay between the onset of head and shoulder reorientation and longer delays for pelvis and foot reorientation. Furthermore, for all segments the peak angular velocities were lower for PD patients than healthy controls, with greater difference between the two groups during larger turns. While for both groups the velocity and magnitude of rotation of all segments were greater during larger turns, the relative timing of reorientation of segments remained the same during small and large turns. Medication had no significant effect on the timing and sequence of reorientation of segments and caused only a small and non-significant increase to segment velocities. CONCLUSION: This study further characterized the turning performance of individuals with PD. Our findings have clinical applications and therapeutic value for PD patients with difficulty turning. Understanding the specific deficiencies of turning performance of PD patients allows the therapists to opt for the most effective rehabilitation techniques.

Does the movement matter?: determinants of the latency of temporally urgent motor reactions.

BACKGROUND: Extremely rapid movements are frequently executed in response to novel, potentially threatening stimuli. The mechanism by which these sophisticated responses are generated is a topic of debate. The current study investigates: 1) the importance of stimulus-response congruence in rapid responses and 2) the relationship between the autonomic nervous system (ANS) and response time. METHODS: Sixteen participants were seated in a chair that could tilt backwards 13°. Participants were instructed to react as fast as possible in response to either an auditory cue (AUD) or balance perturbation (chair tilt) (PERT) and completed one of three different tasks: reach-to-grasp a fixed handle (FIXED), reach-to-grasp a free moving handle (FREE) or plantar flex the left foot (FOOT). Electromyography and electrodermal activity were recorded. RESULTS: For all tasks, muscle onset latency was shorter and muscle response amplitude was greater following the PERT cue compared to the AUD cue. In contrast, there were no differences in onset latency between motor response conditions. Electrodermal response amplitude was greater in the FIXED and FREE conditions than in the FOOT condition. DISCUSSION: Even in situations where the stimulus was incongruent with the response, muscle onset latencies were evoked faster following the perturbation. The response latencies were determined by stimulus characteristics and the most rapid responses were not reliant on stimulus-response congruence. It remains unclear how it is possible to achieve such rapid response latencies to whole body perturbations but we speculate there may exist similar pathways that are uniquely facilitated by a stimulus dependent ANS response.

Challenging horizontal movement of the body during sit-to-stand: impact on stability in the young and elderly.

There are 3 significant challenges to sit-to-stand: (a) bringing the center of mass forward, (b) vertically raising the center of mass from the sitting to standing position, and (c) transition from a relatively large and stable base of support in sitting to a considerably smaller base of support when standing. The authors explored the challenges to stability control following sit-to-stand when the requirement for horizontal movement of the center of mass was influenced by foot position and their potential effect on the preceding phases of sit-to-stand. Eleven healthy young and 11 healthy elderly individuals performed the sit-to-stand with their feet further away and closer to the chair. Kinetic and kinematic data were recorded. Regardless of foot position, challenges in stability were greater in elderly participants than young participants despite their similar movement time and shear forces. The greater instability in elderly participants, despite their comparable movement characteristics, emphasizes the importance of stability control following sit-to-stand performance. For both young and elderly participants, the sit-to-stand duration and the shear forces were greater in the far condition. However, foot position did not affect the stability measures (i.e., duration of the stabilization phase and the total center of pressure path during the 1st second of the stabilization phase).

Coordination of segments reorientation during on-the-spot turns in healthy older adults in eyes-open and eyes-closed conditions.

Turning has frequent occurrence in everyday activities. Despite the prevalence of turning in everyday life and the challenge it poses to older adults, there is far less known about the multisegmental control of turning than the control of standing and straight walking, especially in elderly individuals. The purpose of this study was to examine the timing and sequence of segments reorientation in healthy older adults during 90° on-the-spot turns. The role of vision on segments coordination was also examined by testing the participants in eyes-open and eyes-closed conditions. When turning on-the-spot, healthy elderly reoriented their head, shoulder and pelvis simultaneously, followed by foot displacement. This was a robust behavior not affected by visual condition. Axial segments turned slower and more synchronously when vision was not available. While all segments started to turn together in both visual conditions, head turned faster and reached its peak velocity earlier than shoulder and pelvis. However, the difference in segmental velocity and the time to reach the peak velocity was smaller in eyes-closed than eyes-open condition. Without vision, the functional importance of a faster head turn is diminished. Participants may have adopted a tighter control of segments to simplify the control of movement by reducing the degrees of freedom.

Effect of walking velocity on segment coordination during pre-planned turns in healthy older adults.

BACKGROUND: Despite the prevalence of turning in daily activities and the challenges it poses to mobility-impaired individuals, far less is known about the multi-segmental control of turning than the control of straight walking. Gait slows with aging and neurological disorders such as Parkinson's disease and falls in these populations frequently occur when turning. Nevertheless, the influence of walking velocity on the complex inter-segmental coordination of the head, trunk and lower limbs during turning has not been examined. The purpose of this study was to examine the effect of walking velocity on the coordination of segment reorientation during turns embedded in locomotion in healthy older adults. METHODS: Nineteen healthy older adults volunteered to participate. Participants made a 45 degrees or 90 degrees turn to their right while walking either at their natural self-selected speed or slower or faster than their natural speed. We quantified the timing and sequence of segments reorientation during the turns. RESULTS: There was a top-down temporal sequence in initiation of segments reorientation during turning, i.e., head turned first, followed by shoulder, pelvis, and mediolateral foot displacement. Furthermore, results indicate that the top-down temporal sequence in segments reorientation during turning was a robust behavior which was not affected by the walking velocity or magnitude of the turn. CONCLUSIONS: Walking velocity does not affect segment coordination during pre-planned turns in healthy elderly. Therefore, we conclude that changes in coordination of segments reorientation during pre-planned turns in individuals with neurological disorders such as Parkinson's disease is not due to their slower gait.

Turning behavior in healthy older adults: Is there a preference for step versus spin turns?

This research examined the prevalence of step turns and spin turns during turning while walking in healthy older adults. The potential effect of magnitude of the turn and walking velocity on the prevalence of the step and spin turns were also investigated by examining the participants' performance as they made 45 degrees and 90 degrees turns while walking at three different velocities. Results showed that healthy older adults in our study preferred spin turns while walking either slower or faster than their natural walking speed. Only during 90 degrees turns while walking fast the participants showed a preference for step turns over spin turns. Spin turns are less stable and have a greater biomechanical cost than step turns. The high incidence of spin turns in older adults may contribute to the higher risk of falling in this population.

Stilt walking: how do we learn those first steps?

This study examined how young healthy adults learn stilt walking. Ten healthy male university students attended two sessions of testing held on two consecutive days. In each session participants performed three blocks of 10 stilt-walking trials. Angular movements of head and trunk and the spatial and temporal gait parameters were recorded. When walking on stilts young adults improved their gait velocity through modifications of step parameters while maintaining trunk movements close to that observed during normal over-ground walking. Participants improved their performance by increasing their step frequency and step length and reducing the double support percentage of the gait cycle. Stilts are often used for drywall installation, painting over-the-head areas and raising workers above the ground without the burden of erecting scaffolding. This research examines the locomotor adaptation as young healthy adults learn the complex motor task of stilt walking; a task that is frequently used in the construction industry.

Cognitive demands of postural control during continuous rotational perturbations of the support surface.

This research explored the effect of the level of balance challenge on the cognitive demands of dynamic balance. The postural task was maintaining balance while standing on a rotating platform that moved about the axis of the ankle joint in the pitch plane. Different frequencies and amplitudes of perturbation were employed to introduce different levels of balance challenge. The cognitive task was a silent word identification task. Results showed no significant difference in the participants' performance on the cognitive task in any of the dual-task conditions in comparison with their performance in the "cognitive only" condition. Furthermore, regardless of the level of balance challenge concurrent performance of the cognitive task did not affect the balance control strategies adopted by our participants. The lack of interference between the cognitive task and the present postural task might be due to the fact that the rotational perturbations induced very small Center of Mass (COM) displacements at all frequencies which were under the automatic control.

Balance control during continuous rotational perturbations of the support surface.

This research investigated the effects of continuous rotational perturbations of the support surface on postural control strategies adopted to maintain upright stance. Four different sinusoidal rotations of the support surface were employed: 0.5 Hz, at 2 degrees; 1.0 Hz at 1 degrees; 1.5 Hz at 4 degrees; and 2 Hz at 3 degrees. Thereby two different velocities of perturbation were obtained: 3.1 degrees s(-1) for 0.5 and 1 Hz, and 18.9 degrees s(-1) for 1.5 and 2 Hz. Results indicate that for the frequencies tested, the effect of the perturbation was attenuated. The amplitude of the body's center of mass (COM) displacement was reduced by adopting a multi-segmental strategy which employed anti-phase ankle and hip joint motion. Our results suggest that at least a two-link model of human stance is required to explain responses when the support surface is rotating.