Postural control during gait termination and prehension

BACKGROUND

Challenges to postural stability emerge in the transition from locomotion to a standing posture as during gait termination, often accompanied by another task (e.g., opening a door), which may complicate control. However, less is known about postural control during terminating gait while engaged in a secondary manual task.

RESEARCH QUESTION

What are the changes in postural control when terminating gait with and without a prehension task?

METHODS

In a cross-sectional design, 15 healthy young adults (M=8, F=7; 27±2 years; 69±13 kg; 171±8 cm) underwent both a single task gait termination (GTO) and dual task (gait termination plus reaching; GTR). Postural Time-to-Contact (TtC) was measured using Center of Pressure (CoP) and the sternum position in anterior-posterior (AP) and medial-lateral (ML) directions over two different phases: preparatory phase and stabilization phase. Five successful trials were recorded to obtain a mean TtC. For statistical analysis of TtC, a two-tailed paired t-test was used (p =.05) as normality was satisfied.

RESULTS

For the preparatory phase, there were no differences for the CoP, but TtC of the sternum position in AP was shorter in GTR than GTO (p =.001). Meanwhile, for the stabilization phase, TtCs of both the CoP and sternum position were longer in GTR in both AP and ML directions (p's <.001).

SIGNIFICANCE

We suggest that for the preparatory phase, the shorter TtC of the sternum position with intact TtC of the CoP in GTR indicates that healthy young individuals are flexible, in that they smoothly integrate CoP control with the upper body demands required to also perform the prehension task. Meanwhile, for the stabilization phase, the longer TtC in dual termination and prehension task indicates that the perturbation imposed by the prehension movement did not result in reduced stabilization when returning to an upright posture.

Dynamic stability in runners with and without plantar fasciitis

BACKGROUND

Plantar fasciitis (PF) is a common overuse injury experienced by runners. PF may decrease the ability of the plantar fascia to create tension and reduce stability of the foot. Stability of the foot is necessary for whole-body dynamic stability during running which consists of cyclical periods of single leg stance. Given that a major risk factor for running-related injury is previous injury, evaluating dynamic stability in runners with PF, runners with resolved PF, and healthy runners may elucidate differences between these individuals and clarify risk for secondary injury in these groups.

RESEARCH QUESTION

Is dynamic stability reduced in runners with PF and runners with resolved PF compared to healthy runners?

METHODS

Thirty runners were recruited for this retrospective comparative study based on mileage and injury status: current PF (PF), resolved PF (RPF), or healthy (CON). Kinematic and kinetic data were collected during running and dynamic stability was determined by time-to-contact (TtC) analysis for early, mid, and late stance to the anterior, posterior, medial, and lateral boundary of the foot. Dynamic stability was compared between groups one-way ANOVAs (α = 0.05) and Tukey post-hoc tests conducted when appropriate. Cohen's d effect sizes (d) were reported for all TtC comparisons (small = 0.20, medium = 0.50, large = 0.80).

RESULTS

TtC values were shorter in PF compared to the other groups to all boundaries during mid-stance. TtC was significantly greater in PF compared to RPF to the anterior boundary during late stance.

SIGNIFICANCE

Shorter TtC observed in PF compared to the other groups during midstance may indicate reduced dynamic stability during the most stable portion of running which may lead to increased injury risk.

Collision avoidance behaviors of recently concussed female rugby players with an approaching person

BACKGROUND

Successful collision avoidance requires individuals to use readily available perceptual information to make decisions and act within their action capabilities. Female varsity rugby players use time-to-collision information to get closer to an obstacle before deviating; however, following a sport-related concussion (SRC), individuals may not be able to use the same strategies due to poorer action boundary perception. The current study examined the effects of a SRC on collision avoidance with an approaching person.

METHODS

Non-concussed female varsity rugby athletes (ATH, n = 10) and recently concussed but asymptomatic teammates (CONC, n = 3) were instructed to walk at a comfortable pace and avoid colliding with an approaching confederate. Time-to-collision (TTC), rate of medial-lateral (ML) avoidance, and ML spatial requirement were calculated and analyzed using mixed repeated measures ANOVAs.

FINDINGS

There were no physical characteristic differences between groups (p < 0.05) and no collisions occurred on any of the trials. However, CONC displayed larger TTC than ATH (p = 0.03), indicative of a more cautious avoidance strategy.

INTERPRETATION

While rugby players are trained to avoid opponents, a more cautious strategy following a SRC may reflect visuomotor detraining and changes in embodiment. This suggests that despite being asymptomatic, there is a reduced sense of agency during dynamic perception-action tasks and persistent alterations between pre-concussion and post-concussion self.

Mediolateral postural stability when carrying asymmetric loads during stair negotiation

The purpose of this study was to assess postural stability in the medial-lateral (ML) direction when carrying unilateral and bilateral loads during stair negotiation. Twenty-four healthy young adults were instructed to ascend and descend a three step staircase under three load conditions: no load, 20% body mass (BM) bilateral load, and 20% BM unilateral load. A modified time-to-contact (TTC) method was proposed to evaluate postural stability during stair negotiation. Carrying unilateral loads required more rapid postural adjustments as evidenced by lower minimum ML TTC and ML TTC percentage as compared bilateral loads and no load during stair descent. In addition, lower ML TTC and TTC percentage were found for loaded limb stance for stair descent. Taken together, unilateral loads and the loaded leg during stair descent are of concern when considering postural stability during load carriage. These results illustrate differing postural control challenges for stair ascent and descent during load carriage.

Road crossing decisions in real and virtual environments: A comparative study on simulator validity

Virtual reality (VR) is a valuable tool for the assessment of human perception and behavior in a risk-free environment. Investigators should, however, ensure that the used virtual environment is validated in accordance with the experiment's intended research question since behavior in virtual environments has been shown to differ to behavior in real environments. This article presents the street crossing decisions of 30 participants who were facing an approaching vehicle and had to decide at what moment it was no longer safe to cross, applying the step-back method. The participants executed the task in a real environment and also within a highly immersive VR setup involving a head-mounted display (HMD). The results indicate significant differences between the two settings regarding the participants' behaviors. The time-to-contact of approaching vehicles was significantly lower for crossing decisions in the virtual environment than for crossing decisions in the real one. Additionally, it was demonstrated that participants based their crossing decisions in the real environment on the temporal distance of the approaching vehicle (i.e., time-to-contact), whereas the crossing decisions in the virtual environment seemed to depend on the vehicle's spatial distance, neglecting the vehicle's velocity. Furthermore, a deeper analysis suggests that crossing decisions were not affected by factors such as the participant's gender or the order in which they faced the real and the virtual environment.

Auditory pitch glides influence time-to-contact judgements of visual stimuli

A common experimental task used to study the accuracy of estimating when a moving object arrives at a designated location is the time-to-contact (TTC) task. The previous studies have shown evidence that sound motion cues influence TTC estimates of a visual moving object. However, the extent to which sound can influence TTC of visual targets still remains unclear. Some studies on the crossmodal correspondence between pitch and speed suggest that descending pitch sounds are associated with faster speeds compared to ascending pitch sounds due to an internal model of gravity. Other studies have shown an opposite pitch-speed mapping (i.e., ascending pitch associated with faster speeds) and no influence of gravity heuristics. Here, we explored whether auditory pitch glides, a continuous pure tone sound either ascending or descending in pitch, influence TTC estimates of a vertically moving visual target and if any observed effects are consistent with a gravity-centered or gravity-unrelated pitch-speed mapping. Subjects estimated when a disc moving either upward or downward at a constant speed reached a visual landmark after the disc disappeared behind an occluder under three conditions: with an accompanying ascending pitch glide, with a descending pitch glide, or with no sound. Overall, subjects underestimated TTC with ascending pitch glides and overestimated TTC with descending pitch glides, compared to the no-sound condition. These biases in TTC were consistent in both disc motion directions. These results suggest that subjects adopted a gravity-unrelated pitch-speed mapping where ascending pitch is associated with faster speeds and descending pitch associated with slower speeds.

Intuitive physics of gravitational motion as shown by perceptual judgment and prediction-motion tasks

In Experiment 1, we explored participants' perceptual knowledge of vertical fall by presenting them with virtually simulated polystyrene or wooden spheres falling to the ground from about two meters high. Participants rated the perceived naturalness of the motion. Besides the implied mass of the sphere, we manipulated the motion pattern (i.e., uniform acceleration vs. uniform velocity), and the magnitude of acceleration or velocity. Results show that relatively low values of acceleration or velocity were judged as natural for the polystyrene sphere, whereas relatively high values of acceleration or velocity were judged as natural for the wooden sphere. In Experiment 2, the same stimuli of Experiment 1 were used, but the sphere disappeared behind an invisible occluder at some point of its trajectory. Participants were asked to predict the time-to-contact (TTC) of the sphere with the ground by pressing a key at the exact time of impact of the lower edge of the sphere with the floor of the room. Results show that the estimated TTC for the simulated wooden sphere was slightly but consistently smaller than the estimated TTC for the simulated polystyrene sphere. The influence of the implied mass on participants' responses might be the manifestation of two processes, namely an explicit 'heavy-fast, light-slow' heuristic, and/or an implicit, automatic association between mass and falling speed.

The effects of sport specific training of rugby players on avoidance behaviours during a head-on collision course with an approaching person

Individuals use visual information to plan and adapt movements to dynamically changing environments. This information is used to accurately determine when and where they may come in contact with an object. More specifically, individuals may determine the time prior to contacting an object, known as time-to-contact (TTC). Sport provides a scenario where athletes compete in dynamically changing environments and must interact with opposing players. The effects of sport-specific training on obstacle avoidance is highly controversial and research suggests the quantifiable differences in behaviours compared to non-athletes is highly context specific. The current study aimed to examine if sport-specific training has an impact on the avoidance behaviours of rugby players during a head-on collision course with an approaching person. Female rugby players (N = 10, x- = 20 ± 0.94 years) and non-athletes (N = 10, x- = 21.9 ± 1.6 years) were instructed to walk along a 10 m path towards a goal located along the midline. A female confederate initially positioned along the midline 180° from the participant walked towards the participant to one of four predetermined final positions: 1) along the midline in the participants' starting position; 2) stopped along the midline 2.5 m from her starting position; 3) to the left of the participants' starting position; and 4) to the right of the participants' starting position. Results revealed when the path of the confederate was uncertain, individuals used a consistent TTC to determine when to change their path. The rugby players in the current study were found to avoid significantly later (i.e. smaller TTC) than non-athletes. However, following a change in path, sport-specific training did not impact the avoidance behaviours of the groups, but rather the environment was the regulating factor. When the path of the confederate was uncertain, individuals did not use a single avoidance strategy, instead considered the fit between their individual characteristics (i.e., action capabilities) and components of the environment (i.e. path of the confederate and task constraints). Athletes who are specifically trained to pass through spaces and avoid obstacles (i.e., rugby backs) may consider their action capabilities in conjunction with their visual information to determine time of avoidance.

Carrying asymmetric loads while walking on an uneven surface

BACKGROUND

Individuals often carry asymmetric loads over challenging surfaces such as uneven or irregular terrain, which may require a higher demand for postural control than walking on an even surface.

RESEARCH QUESTION

The purpose of this study was to assess postural stability in the medial-lateral (ML) direction while carrying unilateral versus bilateral loads when walking on even versus uneven surfaces.

METHODS

Nineteen healthy young adults walked on even and uneven surface treadmills under three load conditions: no load, 20% body weight (BW) bilateral load, and 20% BW unilateral load. A Pedar in-shoe pressure system (Novel, Munich, Germany) was used to evaluate center of pressure (COP)-based parameters.

RESULTS

Carrying 20% BW bilateral or unilateral loads significantly increased double support ratio. In addition, carrying a 20% BW unilateral load significantly increased coefficient of variation (CV) of double support ratio, CV of ML COP excursion, and CV of ML COP velocity. Walking on an uneven surface significantly increased double support ratio, ML COP excursion, ML COP velocity, and CV of double support ratio. When carrying a 20% BW unilateral load, unloaded limb stance had significantly increased double support ratio and ML COP velocity, although it appears that the loaded limb may be used to make step-by-step adjustments as evidenced by the higher CV of ML COP velocity.

SIGNIFICANCE

Unilateral load carriage, walking on uneven surfaces, and unloaded leg stance are of particular concern when considering postural stability.

Asymmetrical time-to-contact error with two moving objects persists across different vertical separations

When human observers estimate the time-to-contact (TTC) of more than one object there is an asymmetric pattern of error consistent with prioritizing the lead object at the expense of the trail object. Here, we examined TTC estimation in a prediction motion task where two objects moved along horizontal trajectories (5 or 7.5 °/s) that had different vertical separation, and thus placed specific demands on visuospatial attention. Results showed that participants were able to accurately judge arrival order, irrespective of vertical separation, in all but two conditions where the object trajectories crossed close to the arrival location. Constant error was significantly higher for the object that trailed, as opposed to led, by 250 or 500 ms. Asymmetry in constant error between the lead and trail object was not influenced by vertical separation, and was also evident across a range of arrival times. However, while the lag between the two consecutive TTC estimations was scaled to the actual difference in object arrival times, lag did increase with vertical separation. Taken together, our results confirm that TTC estimation of two moving objects in the prediction motion task suffers from an asymmetrical interference, which is likely related to factors that influence attentional allocation.

Prospective dynamic balance control during the swing phase of walking: stability boundaries and time-to-contact analysis

This study examined the prospective control of the swing phase in young healthy adults while walking at preferred speed over unobstructed ground and during obstacle clearance. Three aspects of swing were examined: (1) the relation of the body Center of Mass (CoM) to the stability boundaries at the base of support; (2) a dynamic time-to-contact analysis of the CoM and swing foot to these boundaries; and (3) the role of head movements in the prospective control of gait and field of view assessment. The time-to-contact analysis of CoM and swing foot showed less stable swing dynamics in the trail foot compared to the lead foot in the approach to the unstable equilibrium, with the CoM leading the swing foot and crossing the anterior stability boundary before the swing foot. Compensations in temporal coupling occurred in the trail limb during the late swing phase. Time-to-contact analysis of head movement showed stronger prospective control of the lead foot, while fixation of the field of view occurred earlier in swing and was closer to the body in the obstacle condition compared to unobstructed walking. The dynamic time-to-contact analysis offers a new approach to assessing the unstable swing phase of walking in different populations.