Rule for scaling shoulder rotation angles while walking through apertures

Abstract

Background: When an individual is trying to fit into a narrow aperture, the amplitude of shoulder rotations in the yaw dimension is well proportioned to the relative aperture width to body width (referred to as the critical ratio value). Based on this fact, it is generally considered that the central nervous system (CNS) determines the amplitudes of shoulder rotations in response to the ratio value. The present study was designed to determine whether the CNS follows another rule in which a minimal spatial margin is created at the aperture passage; this rule is beneficial particularly when spatial requirements for passage (i.e., the minimum passable width) become wider than the body with an external object.

Methodology/principal findings: Eight young participants walked through narrow apertures of three widths (ratio value = 0.9, 1.0, and 1.1) while holding one of three horizontal bars (short, 1.5 and 2.5 times the body width). The results showed that the amplitude of rotation angles became smaller for the respective ratio value as the bar increased in length. This was clearly inconsistent with the general hypothesis that predicted the same rotation angles for the same ratio value. Instead, the results were better explained with a new hypothesis which predicted that a smaller rotation angle was sufficient to produce a constant spatial margin as the bar-length increased in length.

Conclusion: The results show that, at least under safe circumstances, the CNS is likely to determine the amplitudes of shoulder rotations to ensure the minimal spatial margin being created at one side of the body at the time of crossing. This was new in that the aperture width subtracted from the width of the body (plus object) was taken into account for the visuomotor control of locomotion through apertures.

Figure 1. Two hypotheses tested in the present study (use of critical ratio value and creation of minimum spatial margin).

These two hypotheses predict different results regarding the amplitude of shoulder rotations (a) and the spatial margin created on one side of the body for each size of aperture under each bar-length condition (b).

Figure 2. Calculation of spatial margin created in one side of the body at the time of fitting into an aperture.

The r is one half the width of the body, θ is the amplitude of shoulder rotation, and Dx is the egocentric location of the door edge (i.e., the lateral distance from the center of the body midpoint).

Figure 3. Experimental setup.

(a) A participant holds a long horizontal bar (2.5 times the body width) while holding the ends (left) or the center (right) with both hands. The participant of the photograph has given written informed consent, as outlined in the PLoS consent form, to publication of their photograph. (b) The experimental task of walking through an aperture.

Figure 4. Mean absolute angles of shoulder rotation when crossing the aperture.

(a) Mean absolute angles for each aperture size under each bar-length and bar-holding condition. (b)The same data but with the results under the two bar-holding conditions are averaged are shown to easily check the consistency with the hypotheses.

Figure 5. Mean spatial margin created on one side of the body when crossing the aperture.

(a) Mean spatial margin for each aperture size under each bar-length and bar-holding condition. (b) The same data but with the results under the two bar-holding conditions are averaged are shown to easily check the consistency with the hypotheses.

Figure 6. Results of the number of accidental collisions and the magnitude of deviation of the body midline from the center of the apertures.

(a) Number of accidental collisions totaled for all participants for each size of aperture under each condition for bar length. (b) Mean deviation of the body midpoint from the center when crossing an aperture. Negative values represent leftward deviations.