Temporal predictive mechanisms modulate motor reaction time during initiation and inhibition of speech and hand movement

Skilled movement is mediated by motor commands executed with extremely fine temporal precision. The question of how the brain incorporates temporal information to perform motor actions has remained unanswered. This study investigated the effect of stimulus temporal predictability on response timing of speech and hand movement. Subjects performed a randomized vowel vocalization or button press task in two counterbalanced blocks in response to temporally-predictable and unpredictable visual cues. Results indicated that speech and hand reaction time was decreased for predictable compared with unpredictable stimuli. This finding suggests that a temporal predictive code is established to capture temporal dynamics of sensory cues in order to produce faster movements in responses to predictable stimuli. In addition, results revealed a main effect of modality, indicating faster hand movement compared with speech. We suggest that this effect is accounted for by the inherent complexity of speech production compared with hand movement. Lastly, we found that movement inhibition was faster than initiation for both hand and speech, suggesting that movement initiation requires a longer processing time to coordinate activities across multiple regions in the brain. These findings provide new insights into the mechanisms of temporal information processing during initiation and inhibition of speech and hand movement.     

Older adults exhibit variable responses in stepping behaviour following unexpected forward perturbations during gait initiation

With the socioeconomic burden associated with falls expected to rise as the average age of the Canadian population increases, research is needed to elucidate the nature of postural responses generated by older adults (OA) following a posture-destabilizing event. This knowledge is even more imperative for novel and difficult tasks, such as gait initiation (GI), a task known to pose a postural threat to stability for OA. A common technique to regain stability following an unexpected perturbation is reactive stepping. A deficiency in the execution of a reactive control strategy following a destabilizing event may be the cause of many unexpected falls in OA. The purpose of this study is to explore age related changes in the nature of these responses during a challenging GI task combined with an unexpected forward perturbation of the support surface. A total of 18 young adults (YA) and 16 OA performed 36 trials containing 20 unexpected perturbations. We calculated step width, length, time and COM velocity in the first unperturbed step and the second perturbed step. Results revealed that, during unperturbed GI, OA had a reduced forward velocity and took shorter, faster steps. Following forward perturbations, OA altered stepping patterns, perhaps to reduce single support duration, via reduced base of support and shorter step length compared to YA. Additionally, OA executed both forward and backwards directed steps however YA only generated forward steps. Regression analyses revealed that reduced forward velocity was predictive of step direction; which is possibly an unfavorable motor control strategy as OA who walk slower generated a posterior directed step immediately following the perturbation. This strategy is of concern as rapid responses by the trail limb are required to recover successfully, and these alterations may be associated with an elevated risk of falls.     

Adaptive perception of changes in affordances for walking on a ship at sea

Ocean waves cause oscillatory motion of ships. Oscillatory ship motion typically is greater in roll (i.e., the ship rolling from side to side) than in pitch (i.e., tipping from front to back). Affordances for walking on a ship at sea should be differentially influenced by ship motion in roll and pitch. When roll exceeds pitch, the maximum walkable distance within a defined path should be greater when walking along the ship’s short, or athwart axis than when walking along its long, or fore-aft axis. When pitch exceeds roll, this relation should be reversed. We asked whether such changes in ship motion would be reflected in judgments of direction-specific affordances for walking. Participants (experienced maritime crewmembers) judged how far they could walk along a narrow path on the ship deck. On different days, sailing conditions were such that the relative magnitude of pitch and roll was reversed. Judgments of direction-specific affordances for walking mirrored these changes in ship motion. The accuracy of judgments was consistent across directions, and across changes in ship motion. We conclude that experienced maritime crewmembers were sensitive to dynamic variations in affordances for walking that were, themselves, a function of dynamic properties of the animal-environment system.     

Age-dependent variability in spatiotemporal gait parameters and the walk-to-run transition

Adolescents tend to exhibit more variability in their gait patterns than adults, suggesting a lack of gait maturity during this period of ongoing musculoskeletal growth and development. However, there is a lack of consensus over the age at which mature gait patterns are achieved and the factors contributing to gait maturation. Therefore, the purpose of this study was to investigate gait control and maturity in adolescents by determining if differences existed between adolescents and adults in a) the amount of spatiotemporal variability of walking and running patterns across a range of speeds, and b) how swiftly gait patterns are adapted to increasing gait speed during the walk-to-run transition. Forty-six adolescents (10–12-year-olds, n = 17; 13–14-year-olds, n = 12; and 15–17-year-olds, n = 17) and 12 young adults completed an incrementally ramped treadmill test (+0.2 km·h⁻¹ every 30 s) to determine the preferred transition speed (PTS) during a walk-to-run transition. Age-related differences in the variability of stride lengths and stride durations were assessed across 4 speeds (self-selected walking speed, PTS − 0.06 m·s⁻¹, PTS + 0.06 m·s⁻¹, PTS + 0.83 m·s⁻¹). Repeated measures ANOVAs (p < 0.05) compared coefficients of variation for these spatiotemporal parameters, while a one-way ANOVA compared the numbers of gait transitions and speed increments used to identify PTS between the adolescent groups and young adults. Compared to adults, 10–12yo exhibited more spatiotemporal variability during all gait conditions, while 13–17yo only exhibited more variability at PTS + 0.06 m·s⁻¹. No age-dependent pattern was observed in PTS values, but 10–12yo completed more gait transitions over more speed increments than 15–17yo and adults. The development of mature gait patterns is thus a progressive process, with walking maturing at an earlier age than running. As 10-12yo were unable to swiftly adapt gait patterns to the changing task demands, their control mechanisms of gait may not have fully matured yet.     

The effects of sub-threshold vibratory noise on visuomotor entrainment during human walking and standing in a virtual reality environment

Humans will naturally synchronize their posture to the motion of a visual surround, but it is unclear if this visuomotor entrainment can be attenuated with an increased sensitivity to somatosensory information. Sub-threshold vibratory noise applied to the Achilles tendons has proven to enhance ankle proprioception through the phenomenon of stochastic resonance. Our purpose was to compare visuomotor entrainment during walking and standing, and to understand how this entrainment might be attenuated by applying sub-threshold vibratory noise over the Achilles tendons. We induced visuomotor entrainment during standing and treadmill walking for ten subjects (24.5 ± 2.9 years) using a speed-matched virtual hallway with continuous mediolateral perturbations at three different frequencies. Vibrotactile motors over the Achilles tendons provided noise (0–400 Hz) with an amplitude set to 90% of each participant’s sensory threshold. Mediolateral sacrum, C7, and head motion was greatly amplified (4–8× on average) at the perturbation frequencies during walking, but was much less pronounced during standing. During walking, individuals with greater mediolateral head motion at the fastest perturbation frequency saw the greatest attenuation of that motion with applied noise. Similarly, during standing, individuals who exhibited greater postural sway (as measured by the center of pressure) also saw the greatest reductions in sway with sub-threshold noise applied in three of our summary metrics. Our results suggest that, at least for healthy young adults, sub-threshold vibratory noise over the Achilles tendons can slightly improve postural control during disruptive mediolateral visual perturbations, but the applied noise does not substantially attenuate visuomotor entrainment during walking or standing.     

Locomotor pattern and mechanical exchanges during collective load transport

While the locomotor behavior of humans walking alone, loaded or unloaded, has been extensively studied, the locomotor behavior of humans transporting a load collectively is very poorly documented in the biomechanics literature. Yet, collective carriage is a task commonly performed in sport (CrossFit), military and health care (carriage of an injured person) activities and is a task that raises growing interest in robotics (Cobots). The primary aim of our research was to test the hypothesis that the mechanical cost of locomotion is comparable when two individuals are transporting an object collectively and when they are walking alone. To test this, the movements of ten pairs of individuals walking side by side, separately or while transporting collectively an object, were recorded with a three-dimensional motion analysis system (Vicon©). Our results show a similar pattern in the periodic displacement of the center of mass and in mechanical costs, between individuals walking alone and individuals carrying a load collectively. Moreover, a better pendulum-like behavior was found in the sagittal plane and in 3D for the pairs of individuals carrying an object, which suggests that the saving in mechanical exchanges is higher when two individuals are carrying an object collectively than when they are walking alone. The values of the parameters measured in our experiment could be used as a benchmark for the implementation of collective carriage tasks in robotics.     

The effect of stopping before turning on the direct observational measure of whole body turning bias

Turning bias, the preferential tendency to turn toward a given direction has been reported in both rodents and human participants. The observational gait method of determining turning bias in humans requires a stop prior to turning. This study removed the stop and hypothesised that turning bias would remain the same between stop and non-stop conditions if bias was solely under the control of neurochemical asymmetries. The results showed that statistically turning bias remained the same (to the left) regardless of method used but there was no agreement between the methods thus rejecting the hypothesis. It is likely that when not stopping biomechanical factors related to gait when turning influence the direction of turn rather than solely neurochemical asymmetries.     

Gait variability in people with neurological disorders: A systematic review and meta-analysis

There has been growing evidence showing gait variability provides unique information about gait characteristics in neurological disorders. This study systemically reviewed and quantitatively synthesized (via meta-analysis) existing evidence on gait variability in various neurological diseases, including Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), cerebellar ataxia (CA), Huntington’s disease (HD), multiple sclerosis (MS), and Parkinson’s disease (PD). Keyword search were conducted in PubMed, Web of science, Cumulative Index to Nursing and Allied Health Literature, and Cochrane Library. Meta-analysis was performed to estimate the pooled effect size for gait variability for each neurological group. Meta-regression was performed to compare gait variability across multiple groups with neurological diseases. Gait variability of 777 patients with AD, ALS, CA, HD, MS, or PD participating in 25 studies was included in meta-analysis. All pathological groups had increased amount of gait variability and loss of fractal structure of gait dynamics compared to healthy controls, and gait variability differentiated distinctive neurological conditions. The HD groups had the highest alterations in gait variability among all pathological groups, whereas the PD, AD and MS groups had the lowest. Interventions that aim to improve gait function in patients with neurological disorders should consider the heterogeneous relationship between gait variability and neurological conditions.     

Subtasks affecting step-length asymmetry in post-stroke hemiparetic walking

This study was performed to investigate whether components from trunk progression (TP) and step length were related to step length asymmetry in walking in patients with hemiparesis. Gait analysis was performed for participants with hemiparesis and healthy controls. The distance between the pelvis and foot in the anterior-posterior axis was calculated at initial-contact. Step length was partitioned into anterior foot placement (AFP) and posterior foot placement (PFP). TP was partitioned into anterior trunk progression (ATP) and posterior trunk progression (PTP). The TP pattern and step length pattern were defined to represent intra-TP and intra-step spatial balance, respectively. Of 29 participants with hemiparesis, nine participants showed longer paretic step length, eight participants showed symmetric step length, and 12 participants showed shorter paretic step length. For the hemiparesis group, linear regression analysis showed that ATP asymmetry, AFP asymmetry, and TP patterns had significant predictability regarding step length asymmetry. Prolonged paretic ATP and shortened paretic AFP was the predominant pattern in the hemiparesis group, even in participants with symmetric step length. However, some participants showed same direction of ATP and AFP asymmetry. These findings indicate the following: (1) ATP asymmetries should be observed to determine individual characteristics of step length asymmetry, and (2) TP patterns can provide complementary information for non-paretic limb compensation.