Locomotor adjustments when navigating through apertures.

During everyday locomotion we encounter a range of obstacles which require specific motor responses, for example a narrow aperture forces us to rotate the shoulders to pass through. Research has demonstrated that the decision to rotate the shoulders is body scaled (Warren & Whang, 1987) and that the visuo-motor system generates a rotation proportional to aperture size (Higuchi, Cinelli, Greig, & Patla, 2006). The current study considered how shoulder angle and movement speed are tailored to aperture size in nine adults. Aperture sizes were classified into shoulder/aperture ratios (SA ratio), including two for which participants had to rotate (0.9/1.1) and two for which participants could pass freely (1.5/1.7). During the initial approach phase (first 3s), shoulder rotation and movement speed were invariant across SA ratio. Later in the movement, angle of shoulder rotation and the magnitude and timing of the reduction in speed were proportional to SA ratio. The timing of the reduction in speed was progressively later in the movement as SA ratio increased, suggesting early adjustments of movement, such as the timing of the reduction in speed are tightly tuned to the ratio between aperture size and shoulder width, even when no later body adjustments are needed.