Shirley Rietdyk

Visual control of limb trajectory over obstacles during locomotion: effect of obstacle height and width

Abstract The focus of these experiments is to describe the changes in limb trajectory when obstacles of different dimensions are encountered at the normal minimal ground clearance position in the step cycle. Kinematics, ground reaction force data, and EMG signals from four ipsilateral limb muscles were collected and analysed while subjects avoided obstacles placed in their path and maintained their normal step length. The results revealed that the trajectory is substantially modulated for height changes but minimally for the width, provided the width of the obstacle does not force subjects to alter their step length. The average clearance of the toe over the obstacle was ≈10 cm, with three subjects increasing the clearance for higher obstacles. The horizontal toe velocity while going over obstacles was reduced more than the hip velocity as a function of obstacle height, while the hip position was further back and closer to the contralateral limb. Subjects landed with an increase in vertical velocity, while reducing the horizontal velocity. These modifications serve to ensure stability and safety of the subjects by minimizing the dangers due to tripping or slipping. The scaling of the trajectory for different heights was not achieved through linear scaling of muscle activity or ground reaction forces. These results provide insights into the visually observable obstacle properties that can influence the locomotor act.

Locomotor Patterns of the Leading and the Trailing Limbs as Solid and Fragile Obstacles are Stepped over: Some Insights into the Role of Vision During Locomotion

The issues explored in this article are the role of exproprioceptive input and the nature of exteroceptive input provided by the visual system in the control of limb elevation as obstacles are stepped over during locomotion. In the first experiment, the differences in limb trajectory of movements over solid and fragile obstacles of similar dimensions were examined. Subjects increased their toe clearance, vertical position of the hip, and the hip vertical velocity when going over a fragile obstacle with the leading limb. This suggests that in addition to visually observable properties of obstacles such as height or width, other properties, such as rigidity or fragility, which may be classified as visually inferred, also influence the limb trajectory. Part of the first and the second experiment was focused on understanding differences in leading limb and trailing limb trajectory over obstacles. The toe clearance of the trailing limb was lower for smaller obstacles. There was no consistent correlation between the toe clearance values of the leading and trailing limbs. The variability in toe clearance was higher for the trailing limb, which is attributable to lack of visual exproprioceptive input about trailing limb movements and to the shorter time available following toe-off to fine-tune the trailing limb trajectory. Because the body center of mass is moving toward the supporting foot when the trailing limb goes over obstacles and the trailing limb foot is moving up, the chances of a trip are minimized and recovery from an unexpected trip are more likely. These results highlight the role of exproprioceptive input provided by the visual system and possible cognitive influences on the limb trajectory as one travels over uneven terrains.

What guides the selection of alternate foot placement during locomotion in humans

Abstract Our goal was to understand the bases for selection of alternate foot placement during locomotion when the normal landing area is undesirable. In this study, a light spot of different shapes and sizes simulated an undesirable landing area. Participants were required to avoid stepping on this spot under different time constraints. Alternate chosen foot placements were categorised into one of eight choices. Results showed that selection of alternate foot placement is systematic. There is a single dominant choice for each combination of light spot and normal landing spot. The dominant choice minimises the displacement of the foot from its normal landing spot (less than half a foot length). If several response choices satisfy this criterion, three selection strategies are used to guide foot placement: placing the foot in the plane of progression, choosing to take a longer step over a shorter step and selecting a medial rather than lateral foot placement. All these alternate foot-placement choices require minimal changes to the ongoing locomotor muscle activity, pose minimal threat to dynamic stability, allow for quick initiation of change in ongoing movement and ensure that the locomotor task runs without interruption. Thus, alternate foot-placement choices are dependent not only on visual input about the location, size and shape of the undesirable surface, but also on the relationship between the characteristics of the undesirable surface and the normal landing area.

Age-related changes in balance control system: initiation of stepping

The balance control system of a group of healthy and fit, young and elderly subjects was studied during the initiation of stepping in one of three directions: forward, sideways, and backwards in response to a light cue. The performance of these movements requires shifting support from two to one foot, moving the centre of mass outside the initial base of support and creating a new support configuration. By recording and analysing the vertical ground reaction force beneath the subjects stepping foot, we were able to examine the two phases prior to limb lift-off for stepping: reaction time and weight transfer time. Both reaction time and weight transfer time increased with age: The elderly subjects had a proportionately larger increase in weight transfer time compared to the reaction time. The peak force generated showed an age by stepping direction effect: the elderly had a significantly lower peak force for the forwards stepping compared to the younger subjects. The larger increase in weight transfer results in a slower stepping response. Since a stepping task is often recruited to avoid a fall, the increase in response execution time can have undesirable consequences.

Visual exteroceptive information provided during obstacle crossing did not modify the lower limb trajectory.

The roles of visual exteroception (information regarding environmental characteristics) and exproprioception (the relation of body segments to the environment) during gait adaptation are not fully understood. The purpose of this study was to determine how visual exteroception regarding obstacle characteristics provided during obstacle crossing modified foot elevation and placement with and without lower limb-obstacle visual exproprioception (manipulated with goggles). Visual exteroceptive information was provided by an obstacle cue - a second obstacle identical to the obstacle that was stepped over - which was visible during crossing. Ten subjects walked over obstacles under four visual conditions: full vision with no obstacle height cue, full vision with an obstacle height cue, goggles with no obstacle height cue and goggles with an obstacle height cue. Obstacle heights were 2, 10, 20 and 30 cm. The presence of goggles increased horizontal distance (distance between foot and obstacle at foot placement), toe clearance and toe clearance variability. The presence of the obstacle height cue did not alter horizontal distance, toe clearance or toe clearance variability. These observations strengthen the argument that it is the visual exproprioceptive information, not visual exteroceptive information, that is used on-line to fine tune the lower limb trajectory during obstacle avoidance.

The Waterloo vision and mobility study: postural control strategies in subjects with ARM

Binocular vision function and standing balance control was assessed in 16 subjects with age-related maculopathy (ARM) (mean age 73.9 +/- 7.4 years) and 19 controls (mean age 69.1 +/- 5.5 years). Balance control was assessed using the center of pressure signal from force plate data. It was quantified using the root mean square (RMS) error of the amplitude, sampled over a 1 min period. This was evaluated during normal standing, and while the input from the kinesthetic and/or visual systems were disrupted. The two subject groups showed significantly different RMS values across conditions (P < 0.005). The differences in RMS between ARM and control groups were only significant when the input to the kinesthetic sensory system was disrupted. Our results suggest that in the normal standing condition, the kinesthetic and vestibular systems compensated for the lack of useful information from the visual system in ARM subjects. When kinesthetic system input was disrupted, balance control of the ARM group was significantly poorer than the control. There was a weak correlation between postural control and contrast sensitivity. Various strategies for preventing falls in elderly patients with low vision are suggested.

Multiple timescales in postural dynamics associated with vision and a secondary task are revealed by wavelet analysis

Discrete wavelet analysis is used to resolve the center of pressure time series data into several timescale components, providing new insights into postural control. Healthy young and elderly participants stood quietly with their eyes open or closed and either performed a secondary task or stood quietly. Without vision, both younger and older participants had reduced energy in the long timescales, supporting the concept that vision is used to control low frequency postural sway. Furthermore, energy was increased at timescales corresponding to closed-loop (somatosensory and vestibular) and open-loop mechanisms, consistent with the idea of a shift from visual control to other control mechanisms. However, a relatively greater increase was observed for older adults. With a secondary task a similar pattern was observed—increased energy at the short and moderate timescales, decreased energy at long timescales. The possibility of a common strategy—at the timescale level—in response to postural perturbations is considered.

Task-Dependent Postural Control Throughout The Lifespan

Routine activities performed while standing and walking require the ability to appropriately and continuously modulate postural movements as a function of a concurrent task. Changes in task-dependent postural control contribute to the emergence, maturation, and decline of complex motor skills and stability throughout the lifespan.

Waterloo vision and mobility study: gait adaptations to altered surfaces in individuals with age-related maculopathy

Walking is an extremely complex task that can become seriously challenged if one of the sensory systems which provides input to the motor system is compromised. The present study evaluated gait adaptations to altered surface characteristics and high and low ambient light conditions by subjects with age-related maculopathy (ARM). Twenty subjects with ARM and 20 control subjects walked along a 6 m path, along which they met 1 of 3 altered surfaces (compliant, uneven, or shiny). Kinematic data and ground reaction forces information were analyzed to discern gait adaptation strategies used by the ARM group. Ten trials on each surface were collected under both high and low ambient light levels. The ARM subjects were found to be generally more cautious when walking on the altered surfaces. For example, they walked more slowly, with a longer swing time. However, gait adaptations in the ARM group were not merely scaled versions of normal gait but were adjustments to adapt to environmental changes. Gait was modified to avoid tripping over a surface edge, to prevent slipping at heel contact, and to balance during stance. These adaptations enabled subjects to maintain safe mobility when walking in a challenging environment.

Context-dependent reflex control: some insights into the role of balance

Abstract Recent research suggests that the balance requirements of a task dictate the reflexive response. However, these observations were inferred indirectly from either different tasks or different phases of the same task. This study directly tested the hypothesis of balance-dependent control during recovery from an unexpected trip. The subjects were tripped in two different support conditions: unilimb support (provided by the stance limb) or trilimb support (provided by the stance limb and both arms placed on adjacent parallel bars). The subjects exhibited anticipatory changes: they biased the body center of mass toward the support limb in the mediolateral direction and elevated the swing limb higher when there was a possibility of being tripped. The electromyographic (EMG) latencies were not influenced by the threat to equilibrium. The magnitudes of the EMG reflexive response to the trip were clearly modulated as a function of the threat to stability, not in a simple manner, but rather in a complex manner, which optimized the recovery strategy. It is evident that the overriding concern, equilibrium control during locomotion, has a dominant influence on reflex modulation.