Sakineh B. Akram

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 Parkinsons 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 Parkinsons disease is not due to their slower gait.

Challenging Horizontal Movement of the Body During Sit-to-Stand: Impact on Stability in the Young and Elderly

ABSTRACT 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).

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.

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.

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.

Time to disengage: holding an object influences the execution of rapid compensatory reach-to-grasp reactions for recovery from whole-body instability

Rapid reach-to-grasp (RTG) reactions are important for balance recovery. Despite the benefit of having hands free to regain balance, people do not always release a handheld object. We investigated whether reluctance to release is related to central nervous system (CNS) processing delays that occur when the initial reaction is to drop the object rather than RTG. Young adults sat in a custom-designed chair that tilted backwards. Participants regained balance by reaching to a handle with hands free or while holding onto (1) a chair-fixed object or (2) a SMALL or LARGE free-moving object (unbreakable plastic tubes). EMG was collected from the upper limb to determine onset of reaction. Kinematic data from a digitized wrist marker were used to determine movement time. 9 of 10 participants released the object in every trial. Extensor digitorum onset occurred significantly later than anterior deltoid onset in all conditions. LARGE object release induced further delays in extensor onset while both SMALL and LARGE object release increased response and movement time. Object disengagement led to delays in perturbation-evoked, RTG reactions, particularly in the focal muscle (extensor digitorum) and when the objects’ properties posed greater risk for a failed RTG response. We propose that time required for cognitive disengagement accounts for the observed delays. This study offers a potential explanation for the tendency to avoid disengaging from a handheld object during balance recovery. Results also provide insight into the challenges imposed upon the CNS during temporally urgent movements.