Kreter, N. | Search bioRxiv

bioRxiv2022

The Choice of Reference Frame Alters Interpretations of Non-Linear Gait

IntroductionHumans regularly follow non-linear trajectories, such as turning, during everyday ambulation. However, globally-defined and locally-defined reference frames fall out of alignment during non- linear locomotion, which complicates spatiotemporal and biomechanical analyses of gait. Thus, the choice of the locally-defined reference frame is an important methodological consideration. This study investigated how different definitions of reference frame change the results and interpretations of common gait measures. MethodsNine healthy adults completed two walking trials around a circular track. Kinematic data were collected via motion capture and used to calculate step length, step width, anteroposterior margin of stability, and mediolateral margin of stability using three different locally-defined reference frames: walkway-fixed, body-fixed, and trajectory-fixed. Linear-mixed effects models compared the effect of reference frame on each gait measure, and the effect of reference frame on conclusions about a known effect of turning gait - asymmetrical stepping patterns. ResultsAll four gait measures differed significantly across the three reference frames. A significant interaction of reference frame and step type (i.e. inside vs outside step) on all four gait measures (p < 0.001) indicated conclusions about asymmetry differed based on the choice of reference frame. ConclusionThe choice of reference frame will change the calculated gait measures and may alter the conclusions of studies investigating non-linear gait. Care should be taken when comparing studies that used different reference frames, as results cannot be easily harmonized. Future studies of non-linear gait need to justify and detail their choice of reference frame.

bioengineering↗

bioRxiv2022

The effects of physical and temporal certainty on locomotion with discrete underfoot perturbations

BackgroundAmbulation over complex terrain requires active control of foot placement to maintain a normal kinematic relationship between the center of mass and base of support. Recent investigations have suggested that foot placement location may be selected to anticipate shifts to the underfoot center of pressure. However, it is unclear whether temporal affordance and physical certainty contribute to the selection of a perturbation-specific anticipatory strategy. This study investigates anticipatory and reactive locomotor strategies for repeated underfoot perturbations with varying levels of temporal certainty, temporal affordance, and physical certainty. MethodsThirteen healthy adults walked with random underfoot perturbations from a mechanized shoe. Temporal certainty was challenged by presenting the perturbations with or without warning. Temporal affordance was challenged by adjusting the timing of a warning tone before the perturbation. Physical certainty was challenged with conditions that included only eversion perturbations, only inversion perturbations, or both eversion and inversion perturbations. Linear-mixed effects models assessed the effect of each condition on the percent change of margin of stability and step width, respectively. ResultsFor temporally uncertain perturbations and perturbations with one stride or less of affordance, we observed few changes to step width or margin of stability. As affordance increased to two strides, participants adopted a wider step width in anticipation of the perturbation (p = 0.001). Physical certainty had little effect on gait for the step of the perturbation, but participants recovered normal gait sooner when the physical nature of the perturbation was predictable (p < 0.001). DiscussionDespite having information about the timing and magnitude of upcoming perturbations, individuals do not develop perturbation specific feedforward strategies but instead rely on feedback control to recover normal gait after a perturbation. However, physical certainty appears to improve the efficiency of the feedback controller and allows individuals to recover normal gait sooner.

neuroscience↗