The independent effects of speed and propulsive force on joint power generation in walking

Walking speed is modulated using propulsive forces (F_P) during push-off and both preferred speed and F_P decrease with aging. However, even prior to walking slower, reduced F_P may be accompanied by potentially unfavorable changes in joint power generation. For example, compared to young adults, older adults exhibit a redistribution of mechanical power generation from the propulsive plantarflexor muscles to more proximal muscles acting across the knee and hip. Here, we used visual biofeedback based on real-time F_P measurements to decouple and investigate the interaction between joint-level coordination, whole-body F_P, and walking speed. 12 healthy young subjects walked on a dual-belt instrumented treadmill at a range of speeds (0.9–1.3 m/s). We immediately calculated the average F_P from each speed. Subjects then walked at 1.3 m/s while completing a series of biofeedback trials with instructions to match their instantaneous F_P to their averaged F_P from slower speeds. Walking slower decreased F_P and total positive joint work with little effect on relative joint-level contributions. Conversely, subjects walked at a constant speed with reduced F_P, not by reducing total positive joint work, but by redistributing the mechanical demands of each step from the plantarflexor muscles during push-off to more proximal leg muscles during single support. Interestingly, these naturally emergent joint- and limb-level biomechanical changes, in the absence of neuromuscular constraints, resemble those due to aging. Our findings provide important reference data to understand the presumably complex interactions between joint power generation, whole-body F_P, and walking speed in our aging population.     

Older Adults Show Preserved Equilibrium but Impaired Step Length Control in Motor-Equivalent Stabilization of Gait

Stable walking depends on the coordination of multiple biomechanical degrees of freedom to ensure the dynamic maintenance of whole-body equilibrium as well as continuous forward progression. We investigated adult age-related differences in whole-body coordination underlying stabilization of center of mass (CoM) position and step pattern during locomotion. Sixteen younger (20-30 years) and 16 healthy older men (65–80 years) walked on a motorized treadmill at 80%, 100% and 120% of their self-selected preferred speed. Preferred speeds did not differ between the age groups. Motor-equivalent stabilization of step parameters (step length and width) and CoM position relative to the support (back and front foot) was examined using a generalized covariation analysis. Across age groups, covariation indices were highest for CoM position relative to the front foot, the measure most directly related to body equilibrium. Compared to younger adults, older adults showed lower covariation indices with respect to step length, extending previous findings of age-related differences in motor-equivalent coordination. In contrast, no reliable age differences were found regarding stabilization of step width or any of the CoM parameters. The observed pattern of results may reflect robust prioritization of balance over step pattern regularity, which may be adaptive in the face of age-associated sensorimotor losses and decline of coordinative capacities.     

Force–velocity,force–power relationships of bilateral and unilateral leg multi-joint movements in young and elderly women

The present study investigated force–velocity and force–power relationships of bilateral and unilateral knee-hip extension movement in young and elderly women. Twelve healthy young (age, 19–31 yr) and 12 healthy elderly (age, 60–82 yr) women performed bilateral and unilateral knee-hip extension movements on the dynamometer against loads controlled by the servo system. Under the isotonic force condition, force–velocity relationships were measured. The maximum isometric force (F_max), unloaded velocity (V_max) and power output (P_max) of the movements were calculated from extrapolating force–velocity and force–power relationships. F_max and P_max of bilateral and unilateral knee-hip extension movements were 20–30% lower in elderly than in young women. On the other hand, there were no significant differences in V_max between young and elderly women and between bilateral and unilateral movements. Bilateral deficit was larger as the generation of force was larger in both young and elderly women. Also, bilateral deficit of F_max and P_max were not different between young and elderly women. The results were that lower maximum power output of bilateral and unilateral leg multi-joint movements in elderly women did not depend on the intrinsic shortening velocity of muscle action, but largely on reduction in force generating capacity. This suggests the importance of preventing a loss of force generating capacity of muscles during leg multi-joint movements in elderly women.     

A bipedal compliant walking model generates periodic gait cycles with realistic swing dynamics

A simple spring mechanics model can capture the dynamics of the center of mass (CoM) during human walking, which is coordinated by multiple joints. This simple spring model, however, only describes the CoM during the stance phase, and the mechanics involved in the bipedality of the human gait are limited. In this study, a bipedal spring walking model was proposed to demonstrate the dynamics of bipedal walking, including swing dynamics followed by the step-to-step transition. The model consists of two springs with different stiffnesses and rest lengths representing the stance leg and swing leg. One end of each spring has a foot mass, and the other end is attached to the body mass. To induce a forward swing that matches the gait phase, a torsional hip joint spring was introduced at each leg. To reflect the active knee flexion for foot clearance, the rest length of the swing leg was set shorter than that of the stance leg, generating a discrete elastic restoring force. The number of model parameters was reduced by introducing dependencies among stiffness parameters. The proposed model generates periodic gaits with dynamics-driven step-to-step transitions and realistic swing dynamics. While preserving the mimicry of the CoM and ground reaction force (GRF) data at various gait speeds, the proposed model emulated the kinematics of the swing leg. This result implies that the dynamics of human walking generated by the actuations of multiple body segments is describable by a simple spring mechanics.     

Compliant bipedal model with the center of pressure excursion associated with oscillatory behavior of the center of mass reproduces the human gait dynamics

Although the compliant bipedal model could reproduce qualitative ground reaction force (GRF) of human walking, the model with a fixed pivot showed overestimations in stance leg rotation and the ratio of horizontal to vertical GRF. The human walking data showed a continuous forward progression of the center of pressure (CoP) during the stance phase and the suspension of the CoP near the forefoot before the onset of step transition. To better describe human gait dynamics with a minimal expense of model complexity, we proposed a compliant bipedal model with the accelerated pivot which associated the CoP excursion with the oscillatory behavior of the center of mass (CoM) with the existing simulation parameter and leg stiffness. Owing to the pivot acceleration defined to emulate human CoP profile, the arrival of the CoP at the limit of the stance foot over the single stance duration initiated the step-to-step transition. The proposed model showed an improved match of walking data. As the forward motion of CoM during single stance was partly accounted by forward pivot translation, the previously overestimated rotation of the stance leg was reduced and the corresponding horizontal GRF became closer to human data. The walking solutions of the model ranged over higher speed ranges (~1.7 m/s) than those of the fixed pivoted compliant bipedal model (~1.5 m/s) and exhibited other gait parameters, such as touchdown angle, step length and step frequency, comparable to the experimental observations. The good matches between the model and experimental GRF data imply that the continuous pivot acceleration associated with CoM oscillatory behavior could serve as a useful framework of bipedal model.     

Resonance-based oscillations could describe human gait mechanics under various loading conditions

The oscillatory behavior of the center of mass (CoM) and the corresponding ground reaction force (GRF) of human gait for various gait speeds can be accurately described in terms of resonance using a spring–mass bipedal model. Resonance is a mechanical phenomenon that reflects the maximum responsiveness and energetic efficiency of a system. To use resonance to describe human gait, we need to investigate whether resonant mechanics is a common property under multiple walking conditions. Body mass and leg stiffness are determinants of resonance; thus, in this study, we investigated the following questions: (1) whether the estimated leg stiffness increased with inertia, (2) whether a resonance-based CoM oscillation could be sustained during a change in the stiffness, and (3) whether these relationships were consistently observed for different walking speeds. Seven healthy young subjects participated in over-ground walking trials at three different gait speeds with and without a 25-kg backpack. We measured the GRFs and the joint kinematics using three force platforms and a motion capture system. The leg stiffness was incorporated using a stiffness parameter in a compliant bipedal model that best fitted the empirical GRF data. The results showed that the leg stiffness increased with the load such that the resonance-based oscillatory behavior of the CoM was maintained for a given gait speed. The results imply that the resonance-based oscillation of the CoM is a consistent gait property and that resonant mechanics may be useful for modeling human gait.     

Leg stiffness increases with speed to modulate gait frequency and propulsion energy

Bipedal walking models with compliant legs have been employed to represent the ground reaction forces (GRFs) observed in human subjects. Quantification of the leg stiffness at varying gait speeds, therefore, would improve our understanding of the contributions of spring-like leg behavior to gait dynamics. In this study, we tuned a model of bipedal walking with damped compliant legs to match human GRFs at different gait speeds. Eight subjects walked at four different gait speeds, ranging from their self-selected speed to their maximum speed, in a random order. To examine the correlation between leg stiffness and the oscillatory behavior of the center of mass (CoM) during the single support phase, the damped natural frequency of the single compliant leg was compared with the duration of the single support phase. We observed that leg stiffness increased with speed and that the damping ratio was low and increased slightly with speed. The duration of the single support phase correlated well with the oscillation period of the damped complaint walking model, suggesting that CoM oscillations during single support may take advantage of resonance characteristics of the spring-like leg. The theoretical leg stiffness that maximizes the elastic energy stored in the compliant leg at the end of the single support phase is approximated by the empirical leg stiffness used to match model GRFs to human GRFs. This result implies that the CoM momentum change during the double support phase requires maximum forward propulsion and that an increase in leg stiffness with speed would beneficially increase the propulsion energy. Our results suggest that humans emulate, and may benefit from, spring-like leg mechanics.     

Features of temporal dynamics of oscillatory brain activity during creative problem solving in young and elderly adults

Considering evidence from psychological research, successful aging is accompanied by long-term preservation of creative potential despite slowing of mental processes; however, the neurophysiological mechanisms that ensure the maintenance of those abilities are unclear. In this study, we compared temporal dynamics of changes induced by divergent task electrical activity (event-related spectral perturbations, ERSP) in a wide range of EEG frequencies in the younger (YA, N = 80, 22.6 ± 3 years) and older (OA, N = 80, 63.4 ± 6.7 years) age groups. The groups were sex-matched. EEG was recorded while participants performed the “alternate uses task”. The time ranges 200–400, 400–600 and 600–800 ms after stimulus presentation were analyzed. It was found that task performance was associated with distinct patterns of ERSP changes in the θ and α₃ rhythms in young and elderly subjects. The elderly subjects exhibited smaller θ-desyn-chronization of anterior brain areas at the initial stage of creative thinking as compared to young participants. The gradient of fronto-parietal activation was unchanged during the entire interval of analysis in the elderly subjects, whereas it was observed in young adults in the interval 200–400 ms only. Decrease in desynchronization of the parieto-occipital area in the α₃ rhythm in the interval 600–800 ms in elderly subjects was revealed, and it resulted in disappearance of differences between parietal and fronto-temporal areas, while they were preserved in the young group. Significant correlations between ERSP in the α₃ band and originality, in the β₁ band and solution rate were obtained in old adults exclusively. Identified age-related changes in oscillatory activity may be the basis of different strategies in solving creative task in young and elderly adults.     

Balance control during stair negotiation in older adults

Stair negotiation is among the most challenging and hazardous types of locomotion for older people. However, the effect of aging on balance control during stair negotiation has not been investigated. Instantaneous inclination angles between the center of mass (CoM) and center of pressure (CoP) have been reported to detect gait instability effectively in the elderly. The purpose of this study was to compare the CoM-CoP inclination angles between 12 healthy elderly and 13 healthy young adults when performing stair ascent (SA) and descent (SD) on a three-step staircase. Whole body motion data were collected with an eight-camera motion analysis system. Four force plates were mounted on the floor as well as the first two steps to measure ground reaction forces. No significant group differences were detected in any of the temporal-distance gait measures and CoM-CoP inclination angles during SA and SD. Compared to the floor-to-stair transition phase, both groups demonstrated a significantly greater CoM-CoP medial inclination angle while ascending the stairs. However, a significant reduction in medial inclination was only detected in young adults when transferring from SD to level ground walking. Elderly adults were found to demonstrate a significantly greater medial inclination angle during the stair-to-floor transition phase when compared to young adults. Age-related degenerations in the elderly could compromise their ability to regulate body sway during the stair-to-floor transition, which may subsequently increase the risk of falling.     

Maximum walking speed and lower limb length in hominids

In 1984, Helene (Am. J. Physics 52:656) and Alexander (Am. Scientist 72:348–354) presented equations which purported to explain how lower limb length limited maximum walking speed in humans. The equations were based on a simplified model of human walking in which the center of mass (CoM) “vaults” over the supporting leg. Increasing walking speed by increasing stride frequency or stride length would increase the upward acceleration of the CoM in the first half of stance phase, to the point that it would be greater than the downward pull of gravity, and the individual would become airborne. This constitutes running by most definitions. While these models ignored various mechanical factors, such as knee flexion during midstance, that reduce the vertical movement of the CoM, the general idea is plausible inasmuch as the CoM of the body does oscillate vertically with each step. One hypothesis tested here is whether it is indeed the interaction between the pull of gravity and the individual's own upward acceleration that determines at what speed (or cadence) he changes from walking to running. Another hypothesis considered is that increased lower limb length (L) was selected for in early hominids, because of the locomotor advantages of longer lower limbs. Results indicate, however, that while L was clearly related to maximum possible walking speed, it was not an important factor in determining maximum “comfortable” walking speed. These and other results from the recent literature suggest that increased lower limb length provided no selective advantage in locomotion, and other explanations should be sought. © 1996 Wiley-Liss, Inc.     

Age-related changes in the behavior of the muscle-tendon unit of the gastrocnemius medialis during upright stance

Mechanical properties of the muscle-tendon unit change with aging, but it is not known how these modifications influence the control of lower leg muscles during upright stance. In this study, young and elderly adults stood upright on a force platform with and without vision while muscle architecture and myotendinous junction movements (expressed relative to the change in the moment on the x-axis of the force platform) were recorded by ultrasonography and muscle activity by electromyography. The results show that the maximal amplitude of the sway in the antero-posterior direction was greater in elderly adults (age effect, P < 0.05) and was accompanied by an increase in lower leg muscle activity compared with young adults. Moreover, the data highlight that fascicles shorten during forward sway and lengthen during backward sways but more so for young (-4 ± 3 and -4 ± 3 mm/Nm, respectively) than elderly adults (-0.7 ± 3 and 0.8 ± 3 mm/Nm, respectively; age × sway, P < 0.001). Concurrently, the pennation angle increased and decreased during forward and backward sways, respectively, with greater changes in young than elderly adults (age × sway, P < 0.001). In contrast, no significant differences were observed between age groups for tendon lengthening and shortening during sways. The results indicate that, compared with young, elderly adults increase the stiffness of the muscular portion of the muscle-tendon unit during upright stance that may compensate for the age-related decrease in tendon stiffness. These observations suggest a shift in the control strategy used to maintain balance.     

Muscle activities used by young and old adults when stepping to regain balance during a forward fall

The current study was undertaken to determine if age-related differences in muscle activities might relate to older adults being significantly less able than young adults to recover balance during a forward fall. Fourteen young and twelve older healthy males were released from forward leans of various magnitudes and asked to regain standing balance by taking a single forward step. Myoelectric signals were recorded from 12 lower extremity muscles and processed to compare the muscle activation patterns of young and older adults. Young adults successfully recovered from significantly larger leans than older adults using a single step (32.2° vs. 23.5°). Muscular latency times, the time between release and activity onset, ranged from 73 to 114 ms with no significant age-related differences in the shortest muscular latency times. The overall response muscular activation patterns were similar for young and older adults. However older adults were slower to deactivate three stance leg muscles and also demonstrated delays in activating the step leg hip flexors and knee extensors prior to and during the swing phase. In the forward fall paradigm studied, age-differences in balance recovery performance do not seem due to slowness in response onset but may relate to differences in muscle activation timing during the stepping movement.     

The oscillatory behavior of the CoM facilitates mechanical energy balance between push-off and heel strike

Humans use equal push-off and heel strike work during the double support phase to minimize the mechanical work done on the center of mass (CoM) during the gait. Recently, a step-to-step transition was reported to occur over a period of time greater than that of the double support phase, which brings into question whether the energetic optimality is sensitive to the definition of the step-to-step transition. To answer this question, the ground reaction forces (GRFs) of seven normal human subjects walking at four different speeds (1.1-2.4 m/s) were measured, and the push-off and heel strike work for three differently defined step-to-step transitions were computed based on the force, work, and velocity. To examine the optimality of the work and the impulse data, a hybrid theoretical-empirical analysis is presented using a dynamic walking model that allows finite time for step-to-step transitions and incorporates the effects of gravity within this period. The changes in the work and impulse were examined parametrically across a range of speeds. The results showed that the push-off work on the CoM was well balanced by the heel strike work for all three definitions of the step-to-step transition. The impulse data were well matched by the optimal impulse predictions (R(2)>0.7) that minimized the mechanical work done on the CoM during the gait. The results suggest that the balance of push-off and heel strike energy is a consistent property arising from the overall gait dynamics, which implies an inherited oscillatory behavior of the CoM, possibly by spring-like leg mechanics.     

Ground reaction forces and center of mass mechanics of bipedal capuchin monkeys: Implications for the evolution of human bipedalism

Tufted capuchin monkeys are known to use both quadrupedalism and bipedalism in their natural environments. Although previous studies have investigated limb kinematics and metabolic costs, their ground reaction forces (GRFs) and center of mass (CoM) mechanics during two and four‐legged locomotion are unknown. Here, we determine the hind limb GRFs and CoM energy, work, and power during bipedalism and quadrupedalism over a range of speeds and gaits to investigate the effect of differential limb number on locomotor performance. Our results indicate that capuchin monkeys use a “grounded run” during bipedalism (0.83–1.43 ms^?1) and primarily ambling and galloping gaits during quadrupedalism (0.91–6.0 ms^?1). CoM energy recoveries are quite low during bipedalism (2–17%), and in general higher during quadrupedalism (4–72%). Consistent with this, hind limb vertical GRFs as well as CoM work, power, and collisional losses are higher in bipedalism than quadrupedalism. The positive CoM work is 2.04 ± 0.40 Jkg^?1 m^?1 (bipedalism) and 0.70 ± 0.29 Jkg^?1 m^?1 (quadrupedalism), which is within the range of published values for two and four‐legged terrestrial animals. The results of this study confirm that facultative bipedalism in capuchins and other nonhuman primates need not be restricted to a pendulum‐like walking gait, but rather can include running, albeit without an aerial phase. Based on these results and similar studies of other facultative bipeds, we suggest that important transitions in the evolution of hominin locomotor performance were the emergences of an obligate, pendulum‐like walking gait and a bouncy running gait that included a whole‐body aerial phase. Am J Phys Anthropol, 2013. © 2012 Wiley Periodicals, Inc.     

Individual limb work does not explain the greater metabolic cost of walking in elderly adults.

Elderly adults consume more metabolic energy during walking than young adults. Our study tested the hypothesis that elderly adults consume more metabolic energy during walking than young adults because they perform more individual limb work on the center of mass. Thus we compared how much individual limb work young and elderly adults performed on the center of mass during walking. We measured metabolic rate and ground reaction force while 10 elderly and 10 young subjects walked at 5 speeds between 0.7 and 1.8 m/s. Compared with young subjects, elderly subjects consumed an average of 20% more metabolic energy (P=0.010), whereas they performed an average of 10% less individual limb work during walking over the range of speeds (P=0.028). During the single-support phase, elderly and young subjects both conserved approximately 80% of the center of mass mechanical energy by inverted pendulum energy exchange and performed a similar amount of individual limb work (P=0.473). However, during double support, elderly subjects performed an average of 17% less individual limb work than young subjects (P=0.007) because their forward speed fluctuated less (P=0.006). We conclude that the greater metabolic cost of walking in elderly adults cannot be explained by a difference in individual limb work. Future studies should examine whether a greater metabolic cost of stabilization, reduced muscle efficiency, greater antagonist cocontraction, and/or a greater cost of generating muscle force cause the elevated metabolic cost of walking in elderly adults.     

Decreased lower limb muscle recruitment contributes to the inability of older adults to recover with a single step following a forward loss of balance

In response to a balance disturbance, older individuals often require multiple steps to prevent a fall. Reliance on multiple steps to recover balance is predictive of a future fall, so studies should determine the mechanisms underlying differences between older adults who can and cannot recover balance with a single step. This study compared neural activation parameters of the major leg muscles during balance recovery from a sudden forward loss of balance in older individuals capable of recovering with a single step and those who required multiple steps to regain balance. Eighty-one healthy, community dwelling adults aged 70 ± 3 participated. Loss of balance was induced by releasing participants from a static forward lean. Participants performed four trials at three initial lean magnitudes and were subsequently classified as single or multiple steppers. Although step length was shorter in multiple compared to single steppers (F = 9.64; p = 0.02), no significant differences were found between groups in EMG onset time in the step limb muscles (F = 0.033–0.769; p = 0.478–0.967). However, peak EMG normalised to values obtained during maximal voluntary contraction was significantly higher in single steppers in 6 of the 7 stepping limb muscles (F = 1.054–4.167; p = 0.045–0.024). These data suggest that compared to multiple steppers, single steppers recruit a larger proportion of the available motor unit pool during balance recovery. Thus, modulation of EMG amplitude plays a larger role in balance recovery than EMG timing in this context.     

Mechanical work performed by the individual legs during uphill and downhill walking

Previous studies of the mechanical work performed during uphill and downhill walking have neglected the simultaneous negative and positive work performed by the leading and trailing legs during double support. Our goal was to quantify the mechanical work performed by the individual legs across a range of uphill and downhill grades. We hypothesized that during double support, (1) with steeper uphill grade, the negative work performed by the leading leg would become negligible and the trailing leg would perform progressively greater positive work to raise the center of mass (CoM), and (2) with steeper downhill grade, the leading leg would perform progressively greater negative work to lower the CoM and the positive work performed by the trailing leg would become negligible. 11 healthy young adults (6 M/5 F, 71.0±12.3 kg) walked at 1.25 m/s on a dual-belt force-measuring treadmill at seven grades (0, ±3, ±6, ±9°). We collected three-dimensional ground reaction forces (GRFs) and used the individual limbs method to calculate the mechanical work performed by each leg. As hypothesized, the trailing leg performed progressively greater positive work with steeper uphill grade, and the leading leg performed progressively greater negative work with steeper downhill grade (p<0.005). To our surprise, unlike level-ground walking, during double support the leading leg performed considerable positive work when walking uphill and the trailing leg performed considerable negative work when walking downhill (p<0.005). To understand how humans walk uphill and downhill, it is important to consider these revealing biomechanical aspects of individual leg function and interaction during double support.     

Multi-day Longitudinal Assessment of Physical Activity and Sleep Behavior Among Healthy Young and Older Adults Using Wearable Sensors

ObjectivesThe number of elderly people is growing rapidly and aging is found to affect activities of daily living. Older adults are found to perform less physical activity when compared to younger ones. In the perspective of movement behavior, it is not well understood how are elderly different from younger ones. It is not known whether they produce only low frequency movement accelerations or the overall number of movements produced are reduced in elderly. It is also not known how elderly and younger ones perform movement transitions throughout the duration of a day and during night-time sleep.Material and methodsIn this study, 10 healthy young and 10 healthy old participants wore inertial measurement unit at their lower back for 3-days. The 24 hours of day were divided into four 6 hour time zones and transitions made by young and elderly were investigated. All participants performed their regular daily activities unhindered and longitudinal multi-day signals for acceleration and angular velocity were analyzed. Time-frequency analysis was performed using wavelet transform and frequency content of each movement performed was computed.ResultsWe found that both young and older adults performed significantly more low amplitude movements than medium and high amplitude movements. Healthy young adults produced significantly more movements at 1.1 Hz than older adults. Healthy young adults were also found to have produced significantly smaller number of transitions in the mid-phases of sleep. They were also found to produce significantly larger accelerations during night-time sleep transitions than their older counterparts.ConclusionThe advantages of collecting longitudinal data about human movement and sleep transition data can lead us to important clinical diagnosis. The information from longitudinal assessment can help develop lifestyle interventions for disease prevention, monitoring of chronic diseases to prevent or slow disease progression among elderly people.     

Obesity does not increase external mechanical work per kilogram body mass during walking

Walking is the most common type of physical activity prescribed for the treatment of obesity. The net metabolic rate during level walking (W/kg) is ~10% greater in obese vs. normal weight adults. External mechanical work (W_ext) is one of the primary determinants of the metabolic cost of walking, but the effects of obesity on W_ext have not been clearly established. The purpose of this study was to compare W_ext between obese and normal weight adults across a range of walking speeds. We hypothesized that W_ext (J/step) would be greater in obese adults but W_ext normalized to body mass would be similar in obese and normal weight adults. We collected right leg three-dimensional ground reaction forces (GRF) while twenty adults (10 obese, BMI=35.6 kg/m² and 10 normal weight, BMI=22.1 kg/m²) walked on a level, dual-belt force measuring treadmill at six speeds (0.50–1.75 m/s). We used the individual limb method (ILM) to calculate external work done on the center of mass. Absolute W_ext (J/step) was greater in obese vs. normal weight adults at each walking speed, but relative W_ext (J/step/kg) was similar between the groups. Step frequencies were not different. These results suggest that W_ext is not responsible for the greater metabolic cost of walking (W/kg) in moderately obese adults.     

Effects of hip torque during step-to-step transition on center-of-mass dynamics during human walking examined with numerical simulation

Besides the leg force actuator, humans also use a hip torque actuator during the step-to-step transition to redirect the velocity of CoM (Center of Mass). Although the leg force actuator has been widely studied, few researches analyze the hip torque actuator during the step-to-step transition. In this paper, we build a powered walking model which consists of a point mass linked with two compliant legs. Each leg has a spring and a damper in parallel. Two types of active actuators, the force actuator on the leg and the torque actuator at the hip, are added to simulate the leg force and hip torque actuator during the step-to-step transition. The cycle walk is solved by numerical simulations under different hip torque strength, and the energetics and stability are evaluated. The simulation results show that the hip torque actuator can reduce the energy cost and improve the stability of walking. Further analysis shows that the hip torque actuator can reduce mechanical works of both legs with small extra energy cost. To understand the principle of hip torque actuator, the CoM dynamics is analyzed. It is shown that the hip torque actuator is efficient on the redirection of CoM. Thus, it can improve the stability and reduce required forces of both legs, which decreases the energy cost. Our work provides a fundamental understanding of the hip torque during the step-to-step transition, and may help improve the design of bipedal robots and prosthesis.