How do people choose an action to satisfy a goal from among the actions that are afforded by the environment? In 3 experiments the action modes used by actors to reach for a block placed at various distances from them were observed. In each experiment, when actors were not restricted in how they could reach for the object, the transition from their reaching using only arm extension to a mode of reaching in which they used the upper torso to lean forward occurred at closer distances than each actor's absolute critical boundary, beyond which the former action was no longer afforded. In Experiments 2 and 3 actors' seated posture was varied so that the effect of postural dynamics on the distance at which actors actually chose to make the transition between action modes, the preferred critical boundary, could be examined. The results are consistent with the proposal that the preferred critical boundary reflects the relative comfort of available modes of reaching.

In a study of “comfortable” head/neck posture in the absence of a visual target for 24 seated subjects, mean head tilt (Ear-Eye Line) angle was 7.7° above horizontal, and mean head/neck posture (C7-tragus against vertical) was 43.7°. Using these and other studies' findings as reference points for “neutral,” studies examining posture at different computer monitor heights were reviewed: eye-level monitors resulted in head/neck extension.

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