Resolving kinematic redundancy in target-reaching movements with and without external constraint

In the context of target-reaching movements that involve the arms, trunk and legs, we have delineated rules for apportioning motions amongst body segments, which would be valid for a range of target locations that require forward bending. We further attempted to determine whether the rules are altered when motion is restricted at the knee, obliging a re-apportioning of segment motions. For each participant moving with unrestricted joints to nine target locations, principal component analysis of the changes in orientation (i.e., excursions) of six chosen segments revealed that their coupling can be described by two effective degrees of freedom (DoFs), whose weighted combinations account for the segmental excursions. Investigating the similarities and differences among individuals, we found that a set of two effective DoFs could account for the segment excursions among the group of participants who flex their knees significantly, and another set of two for the group who do not. Comparing the motions with and without the knee joints braced, we found that for each individual participant a set of two (or in some cases three) effective DoFs derived from the unrestricted segment excursions could account well for the altered segment excursions when the participant reached for the targets with the knees restricted. Our findings imply that the redundancy of kinematic DoFs can be resolved by reliance on a small number of couplings of segmental excursions, and, in light of the robustness of these couplings against mechanical restriction of joint motion, suggest a neural rather than mechanical origin for them.