Evaluating the energetics of entrainment in a human-machine coupled oscillator system.

During locomotion, humans sometimes entrain (i.e. synchronize) their steps to external oscillations: e.g. swaying bridges, tandem walking, bouncy harnesses, vibrating treadmills, exoskeletons. Previous studies have discussed the role of nonlinear oscillators (e.g. central pattern generators) in facilitating entrainment. However, the energetics of such interactions are unknown. Given substantial evidence that humans prioritize economy during locomotion, we tested whether reduced metabolic expenditure is associated with human entrainment to vertical force oscillations, where frequency and amplitude were prescribed via a custom mechatronics system during walking. Although metabolic cost was not significantly reduced during entrainment, individuals expended less energy when the oscillation forces did net positive work on the body and roughly selected phase relationships that maximize positive work. It is possible that individuals use mechanical cues to infer energy cost and inform effective gait strategies. If so, an accurate prediction may rely on the relative stability of interactions with the environment. Our results suggest that entrainment occurs over a wide range of oscillation parameters, though not as a direct priority for minimizing metabolic cost. Instead, entrainment may act to stabilize interactions with the environment, thus increasing predictability for the effective implementation of internal models that guide energy minimization.

Do infants avoid a traversable slope leading into deep water?

Ramps used to access swimming pools are designed with a shallow slope that affords easy access for all including infants. Locomotor experience has been linked to infants' avoidance of falling into the water from drop-offs; however, the effect of such experience on infants' behavior when a slope is offered to access the water has not been addressed. Forty-three crawling infants (Mage  = 10.63 ± 1.91 months; Mcrawling  = 2.38 ± 1.77 months) and 34 walking infants (Mage  = 14.90 ± 2.18 months; Mwalking  = 2.59 ± 1.56 months) were tested on a new Water Slope paradigm, a sloped surface (10°) leading to deep water. No association between infants' avoidance of submersion and locomotor experience was found. Comparison with the results of infants' behavior on the water cliff revealed that a greater proportion of infants reached the submersion point on the water slope than fell into the water cliff. Collectively, these results indicate a high degree of specificity in which locomotor experience teaches infants about risky situations. Importantly, sloped access to deep water appears to increase the risk of infants moving into the water thereby making them more vulnerable to drowning.

Experienced crawlers avoid real and water drop-offs, even when they are walking.

Crawling experience was recently linked to crawling and walking infants' avoidance of falling on real and water cliffs, whereas walking experience had no effect on walkers' avoidance behavior (Burnay et al., 2021). In the current study, the behavior of 25 infants was analyzed on the Real Cliff/Water Cliff apparatus using a longitudinal study design. Infants were tested as experienced crawlers (Mcrawling = 2.93 months, SD = 1.07), novice walkers (Mwalking = 0.68 months, SD = 0.29), and experienced walkers (Mwalking = 4.90 months, SD = 0.92). Infants avoided falling on both cliffs when tested as experienced crawlers and their behavior was not different when tested as novice or experienced walkers. These findings confirmed the effect of crawling experience on crawling and walking infants' avoidance of falls from heights and into water and the transfer of perceptual learning from crawling to walking postures.

The effect of specific locomotor experiences on infants' avoidance behaviour on real and water cliffs.

Infants' avoidance of drop-offs has been described as an affordance learning that is not transferable between different locomotor postures. In addition, there is evidence that infants perceive and act similarly around real and water cliffs. This cross-sectional study investigated the effects of specific locomotor experiences on infants' avoidance behaviour using the Real Cliff/Water Cliff paradigm. The experiments included 102 infants, 58 crawling, but pre-walking, infants (Mage  = 11.57 months, SD = 1.65) with crawling experience ranging between 0.03 and 7.4 months (M = 2.16, SD = 1.71) and 44 walking infants (Mage  = 14.82 months, SD = 1.99), with walking experience ranging between 0.13 and 5.2 months (M = 1.86, SD = 1.28). The association between crawling experience and crawlers' avoidance of the real and water cliffs was confirmed. Importantly, crawling and total self-produced locomotor experience, and not walking experience, were associated with walkers' avoidance behaviour on both cliffs. These results suggest that some degree of perceptual learning acquired through crawling experience was developmentally transferred to the walking posture. A longer duration of crawling experience facilitates a more rapid recalibration to the new walking capability. In addition, there was no difference in infants' avoidance of falling on the real and the water cliff. However, infants explored the water cliff more than the real cliff, revealing more enticement to examine bodies of water than for drop-offs. A video abstract of this article can be viewed at https://youtu.be/23LXIGiLhHI.

Form in the context of function: Fundamentals of an energy effective striding walk, the role of the plantigrade foot and its expected size.

What morphological and functional factors allow for the unique and characteristic upright striding walk of the hominin lineage? Predictive models of locomotion that arise from considering mechanisms of energy loss indicate that collision-like losses at the transition between stance limbs are important determinants of bipedal gait. Theoretical predictions argue that these collisional losses can be reduced by having "functional extra legs" which are physically the heel and the toe part of a single anatomical foot. The ideal spacing for these "functional legs" are up to a quarter of a stride length, depending on the model employed. We evaluate the foot in the context of the dynamics of a bipedal system and compare predictions of optimal foot size against empirical data from modern humans, the Laetoli footprint trackways, and chimpanzees walking bipedally. The dynamics-based modeling approach provides substantial insight into how, and why, walking works as it does, even though current models are too simple to make predictions at a level adequate to anticipate specific morphology except at the most general level.

Load carrying with flexible bamboo poles: optimization of a coupled oscillator system.

In Asia, flexible bamboo poles are routinely used to carry substantial loads on the shoulder. Various advantages have been attributed to this load-carrying strategy (e.g. reduced energy consumption), but experimental evidence remains inconsistent - possibly because carriers in previous studies were inexperienced. Theoretical models typically neglect the individual's capacity to optimize interactions with the oscillating load, leaving the complete dynamics underexplored. This study used a trajectory optimization model to predict gait adaptations that minimize work-based costs associated with carrying compliant loads and compared the outcomes with naturally selected gait adaptations of experienced pole carriers. Gait parameters and load interactions (e.g. relative amplitude and frequency, phase) were measured in rural farmworkers in Vietnam. Participants carried a range of loads with compliant and rigid poles and the energetic consequences of step frequency adjustments were evaluated using the model. When carrying large loads, the empirical step frequency changes associated with pole type (compliant versus rigid) were largely consistent with model predictions, in terms of direction (increase or decrease) and magnitude (by how much). Work-minimizing strategies explain changes in leg compliance, harmonic frequency oscillations and fluctuations in energetic cost associated with carrying loads on a compliant bamboo pole.

'Mini-interval gait' switching: understanding the positive implications of a novel training regime.

The neuromechanical reorganization required to change gaits imposes an energetic cost 75% greater than either a walking or running step at the same speed. By combining walking and running with the requisite gait switching transition steps, an exercise protocol can be generated with virtually any desired metabolic output even at relatively slow treadmill speed. Gait switching increases metabolic demand through discrete events, which can be tolerated more easily by individuals recovering from health problems, just as interval training allows greater work production for healthy individuals. In addition to cardio-respiratory benefits, 'mini-intervals' with frequent gait switching also provides positive effects and attributes such as distributing muscle group activation, re-training neural coordination, and avoiding repetitive joint overloading. It has the added benefit of developing stability during transitions while a safety hand rail is present which can lead to greater stability in more complex natural environments. Finally, increased mental focus may help avoid the monotony of usual treadmill workouts, aiding adherence to an exercise program. We review evidence for the cost increase of the gait transition step and explain the mechanisms involved. We also discuss literature supporting the range of benefits for mini-interval gait switching as a training and rehabilitation tool.

The Landscape of Movement Control in Locomotion: Cost, Strategy, and Solution.

Features of gait are determined at multiple levels, from the selection of the gait itself (e.g., walk or run) through the specific parameters utilized (stride length, frequency, etc.) to the pattern of muscular excitation. The ultimate choices are determined neurally, but what is involved with deciding on the appropriate strategy? Human locomotion appears stereotyped not so much because the pattern is predetermined, but because these movement patterns are good solutions for providing movement utilizing the machinery available to the individual (the legs and their requisite components). Under different circumstances the appropriate solution may differ broadly (different gait) or subtly (different parameters). Interpretation of the neural decision making process would benefit from understanding the influences that are utilized in the selection of the appropriate solution in any set of circumstances, including normal conditions. In this review we survey an array of studies that point to energetic cost as a key input to the gait coordination system, and not just an outcome of the gait pattern implemented. We then use that information to rigorously define the construct proposed by Sparrow and Newell (1998) where the effects of environment, organism, and task act as constraints determining the solution set available, and the coordination pattern is then implemented under pressure for energetic economy. The fit between the environment and the organism define affordances that can be actualized. We rely on a novel conceptualization of task that recognizes that the task goal needs to be separated from the mechanisms that achieve it so that the selection of a particular implementation strategy can be exposed and understood. This reformulation of the Sparrow and Newell construct is then linked to the proposed pressure for economy by considering it as an optimization problem, where the most readily selected gait strategy will be the one that achieves the task goal at (or near) the energetic minimum.

Determinants of optimal leg use strategy: horizontal to vertical transition in the parkour wall climb.

This study examined the mechanics of the horizontal to vertical transition used by parkour athletes in wall climbing. We used this task as an alternative to normal running - where the functional options differ substantially - exposing the movement control priorities required to successfully complete the task. Ground reaction forces were measured in several expert parkour athletes and centre of mass trajectory was calculated from force plates embedded in the ground and the wall. Empirical measures were compared with movements predicted by a work-based control optimization model. The model captured the fundamental dynamics of the transition and therefore allowed an exploration of parameter sensitivity for success at the manoeuvre (run-up speed, foot placement, etc.). The optimal transition of both the model and the parkour athletes used a common intermediate run-up speed and appears determined largely by a trade-off between positive and negative leg work that accomplishes the task with minimum overall work.

Properties of traditional bamboo carrying poles have implications for user interactions.

Compliant bamboo poles have long been used for load carriage in Asian cultures. Although this custom differs from Western conventions of rigid body attachments (e.g. backpack), potential benefits include reduced peak shoulder forces as well as metabolic transport cost savings. Evidence that carrying a flexible pole benefits locomotion remains mixed, perhaps in part because the properties of pole design (e.g. bamboo material, structural geometry, etc.) have largely been neglected. These properties influence vibrational forces and consequently, the energy required by the user to manage the oscillations. We collected authentic bamboo poles from northern Vietnam and characterized their design parameters. Four poles were extensively studied in the lab (load-deflection testing, resonance testing, and computed tomography scans of three-dimensional geometry), and 10 others were tested at a rural Vietnamese farm site (basic measures of form and resonance). A mass-spring-damper model was used to characterize a relationship between resonant frequency (which affects the energetics of the pole-carrier system) and pole properties concerning stiffness, damping, etc. Model predictions of resonant frequencies agreed well with empirical data. Although measured properties suggest the poles are not optimally designed to reduce peak oscillation forces, resonant frequencies are within range of a typical human walking cadence, and this is likely to have a consequence on locomotion energetics.

Watch Where You're Going? Interferer Velocity and Visual Behavior Predicts Avoidance Strategy During Pedestrian Encounters.

Pedestrians can avoid collisions with other pedestrians by modifying some combination of their velocity and their path. The authors investigated how path constraints (constrained or unconstrained), interferer velocity (slow or fast), and vision (looking or not looking; time spent looking at the interferer) influenced collision avoidance to an oblivious interferer walking on a perpendicular path. Ten participants walked 6 m to either a point or line target on either a constrained or unconstrained path while wearing an eye-tracking device and avoiding an oblivious interferer that walked at 2 speeds. Looking behavior and interferer velocity were reliable predictors of determining whether a pedestrian would pass in front of or behind the interferer, while path constraints were less reliable. These findings highlight the degeneracy in human movement systems and suggest that, in complex environments, behavior may not always be optimized for efficiency.

Children's perception of action boundaries and how it affects their climbing behavior.

There are some concerns that children today may be less calibrated to their action capabilities because of the "risk-free" culture that has proliferated during recent decades. This study investigated the extent to which judgments of reaching affordances presented in different directions (i.e., overhead, diagonal, and horizontal) are related to children's climbing behavior on a climbing wall. A sample of 30 schoolchildren from 6 to 11years old (20 boys and 10 girls) estimated maximum reach and grasp distances and subsequently attempted to climb across an indoor climbing wall. Children who perceived the extents of their reach more accurately completed the climb more often and more quickly. Judgments in the primary directions of climbing locomotion (horizontal and diagonal) were better predictors of success than vertical judgments. Judgments about whether objects are reachable and graspable are complex and influenced by various dynamic factors (including perceptual-motor calibration), and as such different levels of accuracy are likely in different reaching directions. It appears that young children are relatively sensitive to their action boundaries for climbing and, therefore, may be able to make informed decisions themselves about whether a surface is climbable.

Affordance Boundaries Are Defined by Dynamic Capabilities of Parkour Athletes in Dropping from Various Heights.

Available behaviors are determined by the fit between features of the individual and reciprocal features of the environment. Beyond some critical boundary certain behaviors become impossible causing sudden transitions from one movement pattern to another. Parkour athletes have developed multiple movement patterns to deal with their momentum during landing. We were interested in whether drop distance would cause a sudden transition between a two-footed (precision) landing and a load-distributing roll and whether the transition height could be predicted by dynamic and geometric characteristics of individual subjects. Kinematics and ground reaction forces were measured as Parkour athletes stepped off a box from heights that were incrementally increased or decreased from 0.6 to 2.3 m. Individuals were more likely to roll from higher drops; those with greater body mass and less explosive leg power, were more likely to transition to a roll landing at a lower height. At some height a two-footed landing is no longer feasible but for some athletes this height was well within the maximum drop height used in this study. During low drops the primary task constraint of managing momentum could be achieved with either a precision landing or a roll. This meant that participants were free to select their preferred landing strategy, which was only partially influenced by the physical demands of the task. However, athletes with greater leg power appeared capable of managing impulse absorption through a leg mediated strategy up to a greater drop height.

Role of route previewing strategies on climbing fluency and exploratory movements.

This study examined the role of route previewing strategies on climbing fluency and on exploratory movements of the limbs, in order to understand whether previewing helps people to perceive and to realize affordances. Eight inexperienced and ten experienced climbers previewed a 10 m high route of 5b difficulty on French scale, then climbed it with a top-rope as fluently as possible. Gaze behavior was collected from an eye tracking system during the preview and allowed us to determine the number of times they scanned the route, and which of four route previewing strategies (fragmentary, ascending, zigzagging, and sequence-of-blocks) they used. Five inertial measurement units (IMU) (3D accelerometer, 3D gyroscope, 3D magnetometer) were attached to the hip, both feet, and forearms to analyze the vertical acceleration and direction of each limb and hip during the ascent. We were able to detect movement and immobility phases of each IMU using segmentation and classification processes. Depending on whether the limbs and/or hip were moving, five states of behavior were detected: immobility, postural regulation, hold exploration, hold change, and hold traction. Using cluster analysis we identified four clusters of gaze behavior during route previewing depending on route preview duration, number of scan paths, fixations duration, ascending, zigzagging, and sequence-of-blocks strategies. The number of scan paths was positively correlated with relative duration of exploration and negatively correlated with relative duration of hold changes during the ascent. Additionally, a high relative duration of sequence-of-blocks strategy and zigzagging strategy were associated with a high relative duration of immobility during the ascent. Route previewing might help to pick up functional information about reachable, graspable, and usable holds, in order to chain movements together and to find the route. In other words, route previewing might contribute to perceiving and realizing nested affordances.

The goal of locomotion: Separating the fundamental task from the mechanisms that accomplish it.

Human locomotion has been well described but is still not well understood. This is largely true because the observable aspects of locomotion-neuromuscular activity that generates forces and motions-relate to both the task solution and the problem being solved. Identifying the fundamental task achieved in locomotion makes it possible to critically evaluate the motor control strategy used to accomplish the task goal. We contend that the readily observed movements and activities of locomotion should be considered mechanism(s). Our proposal is that the fundamental task of walking and running is analogous to flight, and should be defined in terms of the interaction of the individual's mass with the medium in which it moves: a low-density fluid for flight, or the supporting substrate for legged locomotion. A rigorous definition of the fundamental task can help identify the constraints and opportunities that influence its solution and guide the selection of appropriate mechanisms to accomplish the task effectively. The results from robotics-based modeling studies have demonstrated how the interaction of the mass and substrate can be optimized, making the goal of movement a defined trajectory of the individual's mass. We assessed these concepts by evaluating the ground reaction forces generated by an optimization model that satisfies the task but uses none of the mechanisms that are available to the human leg. Then we compared this model to normal human walking. Although it is obvious that the specific task of locomotion changes with a variety of movement challenges, clearly identifying the fundamental task of locomotion puts all other features in an interpretable context.

A New Qualitative Typology to Classify Treading Water Movement Patterns.

This study proposes a new qualitative typology that can be used to classify learners treading water into different skill-based categories. To establish the typology, 38 participants were videotaped while treading water and their movement patterns were qualitatively analyzed by two experienced biomechanists. 13 sport science students were then asked to classify eight of the original participants after watching a brief tutorial video about how to use the typology. To examine intra-rater consistency, each participant was presented in a random order three times. Generalizability (G) and Decision (D) studies were performed to estimate the importance variance due to rater, occasion, video and the interactions between them, and to determine the reliability of the raters' answers. A typology of five general classes of coordination was defined amongst the original 38 participants. The G-study showed an accurate and reliable assessment of different pattern type, with a percentage of correct classification of 80.1%, an overall Fleiss' Kappa coefficient K = 0.6, and an overall generalizability φ coefficient of 0.99. This study showed that the new typology proposed to characterize the behaviour of individuals treading water was both accurate and highly reliable. Movement pattern classification using the typology might help practitioners distinguish between different skill-based behaviours and potentially guide instruction of key aquatic survival skills. Key pointsTreading water behavioral adaptation can be classified along two dimensions: the type of force created (drag vs lift), and the frequency of the force impulsesBased on these concepts, 9 behavioral types can be identified, providing the basis for a typologyProvided with macroscopic descriptors (movements of the limb relative to the water, and synchronous vs asynchronous movements), analysts can characterize behavioral type accurately and reliably.

Interacting Factors Associated with Adult Male Drowning in New Zealand.

OBJECTIVES: i) to identify factors that contribute to the global trend of the higher incidence of male drowning relative to females, and; ii) to explore relationships between such factors from mortality data in New Zealand. METHODS: Drownings from 1983 to 2012 were examined for: Age, Ethnicity, Site, Activity, Buoyancy and Alcohol. Conditional frequency tables presented as mosaic plots were used to assess the interactions of these factors. RESULTS: Alcohol was involved in a high proportion of Accidental Immersion drownings (61%) and was highest for males aged 20-24 years. When alcohol was involved there were proportionally more incidences where a life jacket was Available But Not Worn and less incidences where a life jacket was Worn. Many 30-39 year old males drowned during underwater activities (e.g., snorkeling, diving). Older men (aged +55 years old) had a high incidence of drowning while boating. Different ethnicities were over-represented in different age groups (Asian men aged 25-29, and European men aged 65-74) and when involved in different activities. CONCLUSIONS: Numerous interacting factors are responsible for male drownings. In New Zealand, drowning locations and activities differ by age and ethnicity which require targeted intervention strategies.

Integrative physiological and behavioural responses to sudden cold-water immersion are similar in skilled and less-skilled swimmers.

We examined the initial physiological responses and subsequent capacity to swim following cold-water immersion. An ecologically-valid model was used whereby immersion was sudden (<2s) and participants had to actively remain afloat. Participants (15 skilled swimmers, 17 less-skilled swimmers) undertook four experimental test sessions: a physiological test and a swimming test in both cold (10°C) water and temperate (27°C) water in a swimming flume (temperature order counter-balanced). For physiological testing, measures of brain perfusion [flow velocity (MCAv, Doppler) and oxygenation (NIRS)] and cardiorespiratory function [ventilation parameters and end-tidal PCO2 (PETCO2)] were recorded whilst treading water for 150s. The swimming test involved treading water (150s) before swimming at 60% (up to 120s) and 90% (to intolerance) of pre-determined maximum velocity. Multifactorial analysis revealed that swimming duration was influenced most heavily by water temperature, followed by respiratory variables and MCAv in the first 30s of immersion. The time course and severity of cold shock were similar in both groups (p=0.99), in terms of initial physiological changes (MCAv down ~20 ± 11%, respiratory frequency increased to 58 ± 18 breaths·min(-1), PETCO2 dropped to 12 ± 9 mmHg). Treading water following cold-water immersion increased MCAv by 30% above resting values despite maintained cold-shock-induced hyperventilation. In comparison to temperate water, swimming capacity was also reduced similarly between groups in the cold (i.e., distance decreased by 34 ± 26% skilled; 41 ± 33% less-skilled, p=0.99). These integrative findings verify that sudden cold-water immersion followed by physical activity leads to similar physiological responses in humans when contrasting between skilled and less-skilled swimmers.

Responses to sudden cold-water immersion in inexperienced swimmers following training.

BACKGROUND: When suddenly immersed in cold water, humans typically exhibit the cold shock response, although training can attenuate hyperventilation. This study extends previous findings by considering the influence of physical activity to maintain buoyancy and subsequent swimming performance. METHODS: Six inexperienced swimmers (three men and three women; mean age 22.8) received 1 wk of cold-water head-out immersions (10 x 3 min at 15 degrees C) alongside mental skills training to improve their treading water technique and to control hyperventilation upon immersion. Six inexperienced control swimmers (four men and two women; mean age 21.8) received immersions in temperate water (27 degrees C). Ventilation, brain blood flow velocity, and blood oxygenation were measured during a physiological test in which participants trod water for 150 s. In a subsequent simulated survival test, performance (swimming duration and distance) and perception of effort were recorded. All the tests were in 10 degrees C water with the head out. RESULTS: There were significant improvements in the intervention group's ability to suppress rapid increases in respiratory frequency; 62 +/- 24 breaths x min(-1) to 33 +/- 12. The drop in brain blood flow was smaller and more transient than that previously reported due to the hypertensive response associated with treading water. DISCUSSION: Inexperienced swimmers could benefit from cold-water habituation combined with mental skills training in order to improve voluntary control over the respiratory portion of the cold shock response as part of learning to tread water. This may improve survival prospects in a real-life emergency scenario such as an overturned boat.

Effects of bicycle saddle height on knee injury risk and cycling performance.

Incorrect bicycle configuration may predispose athletes to injury and reduce their cycling performance. There is disagreement within scientific and coaching communities regarding optimal configuration of bicycles for athletes. This review summarizes literature on methods for determining bicycle saddle height and the effects of bicycle saddle height on measures of cycling performance and lower limb injury risk. Peer-reviewed journals, books, theses and conference proceedings published since 1960 were searched using MEDLINE, Scopus, ISI Web of Knowledge, EBSCO and Google Scholar databases, resulting in 62 references being reviewed. Keywords searched included 'body positioning', 'saddle', 'posture, 'cycling' and 'injury'. The review revealed that methods for determining optimal saddle height are varied and not well established, and have been based on relationships between saddle height and lower limb length (Hamley and Thomas, trochanteric length, length from ischial tuberosity to floor, LeMond, heel methods) or a reference range of knee joint flexion. There is limited information on the effects of saddle height on lower limb injury risk (lower limb kinematics, knee joint forces and moments and muscle mechanics), but more information on the effects of saddle height on cycling performance (performance time, energy expenditure/oxygen uptake, power output, pedal force application). Increasing saddle height can cause increased shortening of the vastii muscle group, but no change in hamstring length. Length and velocity of contraction in the soleus seems to be more affected by saddle height than that in the gastrocnemius. The majority of evidence suggested that a 5% change in saddle height affected knee joint kinematics by 35% and moments by 16%. Patellofemoral compressive force seems to be inversely related to saddle height but the effects on tibiofemoral forces are uncertain. Changes of less than 4% in trochanteric length do not seem to affect injury risk or performance. The main limitations from the reported studies are that different methods have been employed for determining saddle height, small sample sizes have been used, cyclists with low levels of expertise have mostly been evaluated and different outcome variables have been measured. Given that the occurrence of overuse knee joint pain is 50% in cyclists, future studies may focus on how saddle height can be optimized to improve cycling performance and reduce knee joint forces to reduce lower limb injury risk. On the basis of the conflicting evidence on the effects of saddle height changes on performance and lower limb injury risk in cycling, we suggest the saddle height may be set using the knee flexion angle method (25-30°) to reduce the risk of knee injuries and to minimize oxygen uptake.