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Human locomotion is an inherently complex activity that requires the coordination and control of neurophysiological and biomechanical degrees of freedom across various spatiotemporal scales. Locomotor patterns must constantly be altered in the face of changing environmental or task demands, such as heterogeneous terrains or obstacles. Variability in stride times occurring at short time scales (e.g., 5-10 strides) is statistically correlated to larger fluctuations occurring over longer time scales (e.g., 50-100 strides). This relationship, known as fractal dynamics, is thought to represent the adaptive capacity of the locomotor system. However, this has not been tested empirically. Thus, the purpose of this study was to determine if stride time fractality during steady state walking associated with the ability of individuals to adapt their gait patterns when locomotor speed and symmetry are altered. Fifteen healthy adults walked on a split-belt treadmill at preferred speed, half of preferred speed, and with one leg at preferred speed and the other at half speed (2:1 ratio asymmetric walking). The asymmetric belt speed condition induced gait asymmetries that required adaptation of locomotor patterns. The slow speed manipulation was chosen in order to determine the impact of gait speed on stride time fractal dynamics. Detrended fluctuation analysis was used to quantify the correlation structure, i.e., fractality, of stride times. Cross-correlation analysis was used to measure the deviation from intended anti-phasing between legs as a measure of gait adaptation. Results revealed no association between unperturbed walking fractal dynamics and gait adaptability performance. However, there was a quadratic relationship between perturbed, asymmetric walking fractal dynamics and adaptive performance during split-belt walking, whereby individuals who exhibited fractal scaling exponents that deviated from 1/f performed the poorest. Compared to steady state preferred walking speed, fractal dynamics increased closer to 1/f when participants were exposed to asymmetric walking. These findings suggest there may not be a relationship between unperturbed preferred or slow speed walking fractal dynamics and gait adaptability. However, the emergent relationship between asymmetric walking fractal dynamics and limb phase adaptation may represent a functional reorganization of the locomotor system (i.e., improved interactivity between degrees of freedom within the system) to be better suited to attenuate externally generated perturbations at various spatiotemporal scales. [ABSTRACT FROM AUTHOR]

Preferred walking speed (PWS) reflects the integrated performance of the relevant physiological sub-systems, including energy expenditure. It remains unclear whether the PWS during over-ground walking is chosen to optimize one's balance control because studies on the effects of speed on the body's balance control have been limited. The current study aimed to bridge the gap by quantifying the effects of the walking speed on the body's center of mass (COM) motion relative to the center of pressure (COP) in terms of the changes and directness of the COM-COP inclination angle (IA) and its rate of change (RCIA). Data of the COM and COP were measured from fifteen young healthy males at three walking speeds including PWS using a motion capture system. The values of IAs and RCIAs at key gait events and their average values over gait phases were compared between speeds using one-way repeated measures ANOVA. With increasing walking speed, most of the IA and RCIA related variables were significantly increased (p<0.05) but not for those of the frontal IA. Significant quadratic trends (p<0.05) with highest directness at PWS were found in IA during single-limb support, and in RCIA during single-limb and double-limb support. The results suggest that walking at PWS corresponded to the COM-COP control maximizing the directness of the RCIAs over the gait cycle, a compromise between the effects of walking speed and the speed of weight transfer. The data of IA and RCIA at PWS may be used in future assessment of balance control ability in people with different levels of balance impairments. [ABSTRACT FROM AUTHOR]

Reported differences between children and adults with respect to COM horizontal and vertical position to maintain dynamic stability during running deceleration suggest that this relationship may not be as important in children. This study challenged the current dynamic stability paradigm by determining the features of whole body posture that predicted forward velocity and momentum of running gait termination in adults and children. Sixteen adults and 15 children ran as fast as possible and stopped at pre-determined location. Separate regression analyses determined whether COM posterior and vertical positions and functional limb length (distance between COM and stance foot) predicted velocity and momentum for adults and children. COM posterior position was the strongest predictor of forward velocity and momentum in both groups supporting the previously established relationship during slower tasks. COM vertical position also predicted momentum in children, not adults. Higher COM position in children was related to greater momentum; consistent with previously reported differences between children and adults in COM position across running deceleration. COM vertical position was related to momentum but not velocity in children suggesting that strategies used to terminate running may be driven by demands imposed not just by velocity, but also the mass being decelerated. [ABSTRACT FROM AUTHOR]

Interlimb temporal synchrony and spatial symmetry of centre of pressure (COP) displacements may be vital contributors to standing balance control. In previous work among stroke survivors, low-frequency COP displacements (< 0.4 Hz) were proposed to arise from centre of mass (COM) dynamics, or from proactive exploratory processes. COP displacements among higher frequencies (>0.4 Hz), in contrast, have been attributed to corrective balance responses to internal perturbations. The present study extends this work to explore age-related alterations in such stability control processes during standing balance. The combined COP displacements from both limbs (COPnet) in addition to individual-limb COP timeseries were calculated from synchronous force platform data obtained from 19 younger adults and 19 older adults during a 60 s trial of quiet standing. The discrete wavelet transform was used to decompose the anteroposterior and mediolateral COPnet, in addition to the individual-limb timeseries, into low-frequency and high-frequency bandwidths. Root-mean-squared (RMS) amplitudes of high- and low-frequency COPnet displacements were calculated. The cross-correlation coefficient was used to assess the extent of between-limb temporal synchronization, while the ratio of individual-limb RMS amplitudes was used to assess between-limb spatial symmetry within each high- and low-frequency bandwidth. We observed greater high-frequency anteroposterior COPnet displacements among older adults, without age related differences in the lower frequency bandwidth or in the mediolateral direction. Further, older adults exhibited greater high-frequency anteroposterior between-limb synchronization, without age-related differences in the low frequency bandwidth, or among any of the spatial symmetry variables. The present age-related alterations in COPnet could represent a conservative strategy to ensure stability, whereby age-related challenges in stability maintenance during standing are offset by greater demands on stability control. Further, increased high frequency between-limb temporal synchronization among older adults may suggest a loss of adaptability in balance corrective responses during standing. [ABSTRACT FROM AUTHOR]

Past research has revealed that an individual’s rhythmic limb movements become spontaneously entrained to an environmental rhythm if visual information about the rhythm is available and its frequency is near that of the individual’s movements. Research has also demonstrated that if the eyes track an environmental stimulus, the spontaneous entrainment to the rhythm is strengthened. One hypothesis explaining this enhancement of spontaneous entrainment is that the limb movements and eye movements are linked through a neuromuscular coupling or synergy. Another is that eye-tracking facilitates the pick up of important coordinating information. Experiment 1 investigated the first hypothesis by evaluating whether any rhythmic movement of the eyes would facilitate spontaneous entrainment. Experiments 2 and 3 (respectively) explored whether eye-tracking strengthens spontaneous entrainment by allowing the pickup of trajectory direction change information or allowing an increase in the amount of information to be picked-up. Results suggest that the eye-tracking enhancement of spontaneous entrainment is a consequence of increasing the amount of information available to be picked-up. [ABSTRACT FROM AUTHOR]

Highlights: [•] The NIRK is performed relying on the movement of all lower limb segments. [•] The final part of the NIRK presents a more in-phase action of hip and knee. [•] A reduced angular momentum was observed in the final part of the NIRK. [•] The NIRK presents a reduced angular momentum about the vertical axis. [Copyright &y& Elsevier]

Abstract: An experiment was designed to determine if the addition of a load altered the effector transfer profile observed in earlier experiments using multi-element movement sequences. The acquisition task required participants to move a horizontal lever (with 0.567kg load) to 16 sequentially projected targets. One group practiced the movement sequence with their right (dominant) limb and another group practiced with their left (non-dominant) limb. Approximately 24h after completion of the acquisition session both groups were administered test blocks (0kg, 0.567kg, and 1.134kg) using their practiced and unpracticed limbs. Decreased and increased loads had minimal effect on test performance. The results indicated that the group trained with their left limb were able to perform the right limb tests as well as the group that trained with the right limb. However, the group that trained with their right limb were significantly slower performing the tests with the left limb than the group that practiced with their left limb. Importantly, the left acquisition limb group maintained the pattern of element durations used during practice on the various tests including transfer to the dominant limb. However, the pattern of element durations for the right acquisition limb group on the left limb transfer tests was altered such that the production of only the fastest produced elements were disrupted. These results suggest that one of the reasons for poor sequence performance when transferring from the right to left is because the sequence structure developed during acquisition and used on the tests lacked access to the appropriate commands or the controller lacked the ability to implement codes that effectively manage the movement dynamics. [Copyright &y& Elsevier]

Intrinsic coordination patterns exist between limbs such that 1) coordination at these states is inherently stable, 2) any other pattern requires learning to produce, and 3) this learning is subject to interference from a systemic bias towards intrinsic patterns. The dynamics that govern intrapersonal interlimb coordination also govern interpersonal coordination. However, intrapersonal coordination exhibits greater coupling strength and thus more stable intrinsic dynamics than interpersonal coordination. Because the strength of intrinsic coordination tendencies has consequences for learning coordination patterns, the differences in coupling strength between intra- and interpersonal coordination should impact the ability to perform new coordination patterns via greater or less interference from intrinsic dynamics. This was investigated by measuring participants' performance as they learned a new coordination pattern alone (intrapersonal) or in pairs (interpersonal). Participants were implicitly tasked with learning the pattern as they separately controlled the vertical and horizontal position of an on-screen cursor to trace a circling target. We observed better performance of dyads on first trial and steeper learning trajectories for individuals. Overall, these results indicate that individuals experienced greater interference from stronger intrinsic coordination dynamics during early learning but could overcome this interference and achieve similar performance to that of dyads with very little practice. [ABSTRACT FROM AUTHOR]