Minimum energy trajectories of the swing ankle when stepping over obstacles of different heights

Best-Compromise Control Strategy Between Mechanical Energy Expenditure and Foot Clearance for Obstacle-Crossing in Older Adults: Effects of Tai-Chi Chuan Practice

Background: Obstacle-crossing increases the risk of falls in older people. This study aimed to identify the effects of long-term Tai-Chi Chuan (TCC) practice on the control strategies for obstacle-crossing in older people.

Methods: A multi-objective optimal control technique with measured gait data was used to identify the control strategies adopted by 15 long-term TCC practitioners and 15 healthy controls when crossing obstacles of different heights, in terms of the best-compromise weighting sets for the conflicting objectives of minimizing energy expenditure and maximizing the toe-obstacle and heel-obstacle clearances.

Results and Conclusions: The long-term TCC older practitioners adopted a best-compromise control strategy similar to those adopted by young adults, with greater weightings on the minimization of the mechanical energy expenditure and smaller weightings on foot-clearance as compared to non-TCC controls (TCC: 0.72, 0.14, 0.14; Control: 0.55, 0.225, 0.225). This strategy enabled the long-term TCC older practitioners to cross obstacles with significantly greater leading-toe clearances but with relatively less mechanical energy expenditure. With the current approach, further simulations of obstacle-crossing mechanics with a given weighting set will be useful for answering clinically relevant what-if questions, such as what abilities would be needed if the non-TCC older people were to cross obstacles using the crossing strategy of the TCC people.

Effects of grab bar on utilized friction and dynamic stability when elderly people enter the bathtub

BACKGROUND

The effect of the grab bar on dynamic stability when elderly people enter the bathtub remains unclear. The purpose of the present study is to examine the age-related effect of the grab bar on dynamic stability during lateral stepping over an obstacle when entering bathtub.

METHODS

Sixteen young, healthy adults and sixteen elderly adults participated. The subjects performed lateral stepping over an obstacle with and without vertical and horizontal bars. Displacement and velocity of the center of mass and utilized friction, which is the required coefficient of friction to avoid slipping, were simultaneously measured by a three-dimensional motion analysis system and two force plates.

FINDINGS

A post hoc test for two-way ANOVA revealed that velocity of the center of mass in the vertical direction (p<0.05) and peak-to-peak values of the center of mass in the lateral (p<0.05) and vertical directions (p<0.05) with each grab bar were significantly slower and smaller than those without the grab bar in young and elderly people. Moreover, the utilized friction at push off of the trailing leg with the vertical bar in elderly people was lower (p<0.05) than that in participants without the grab bar.

INTERPRETATION

The use of each grab bar while performing a lateral step over an obstacle may help maintaining balance in lateral and vertical directions. However, use of the vertical bar while lateral stepping over an object in elderly people may need low utilized friction to prevent slipping.

Obstacle Crossing During Gait in Children With Cerebral Palsy: Cross-Sectional Study With Kinematic Analysis of Dynamic Balance and Trunk Control

BACKGROUND

Balance problems are common in children who have cerebral palsy (CP) but are active and ambulant. Control of the whole-body center of mass is critical in maintaining dynamic stability during challenging mobility tasks, such as clearing an obstacle while walking.

OBJECTIVE

The objective of this study was to compare trunk and lower limb kinematics and center-of-mass control in children with CP and those in children with typical development during obstacle crossing.

DESIGN

This was a cross-sectional study. Thirty-four children who were 5 to 17 years of age (17 with CP and 17 with typical development) and matched in age and height completed 2 gait trials involving crossing a 10-cm obstacle.

METHODS

Three-dimensional kinematic and kinetic data were captured with a general-purpose 3-dimensional motion tracking system and forceplates. Trunk data were captured with a validated model.

RESULTS

All children cleared the obstacle with similar hip and knee kinematics, step length, and single-support duration. In children with CP, step width was increased by 4.81 cm, and center-of-mass velocity was significantly slower at lead limb toe-off (0.31 m/s) and during lead limb clearance (0.2 m/s). Children with CP showed altered trunk and pelvis movement, characterized by significantly greater pelvic obliquity, pelvic tilt, and trunk rotation throughout the task, increased lateral trunk lean during lead limb crossing (3.7°), and greater sagittal trunk movement as the trail limb crossed (5.1°).

LIMITATIONS

The study was not powered to analyze differences between children with diplegia and those with hemiplegia.

CONCLUSIONS

Children with CP required greater adjustments at the trunk and pelvis to achieve successful obstacle crossing. The increase in trunk movement could have been compensatory for reduced stability distally or for a primary problem reflecting poor proximal control. The findings suggest that rehabilitation should focus on both proximal trunk control and distal stability to improve balance.

Damage Models for Soft Tissues: A Survey

Damage to soft tissues in the human body has been investigated for applications in healthcare, sports, and biomedical engineering. This paper reviews and classifies damage models for soft tissues to summarize achievements, identify new directions, and facilitate finite element analysis. The main ideas of damage modeling methods are illustrated and interpreted. A few key issues related to damage models, such as experimental data curve-fitting, computational effort, connection between damage and fractures/cracks, damage model applications, and fracture/crack extension simulation, are discussed. Several new challenges in the field are identified and outlined. This review can be useful for developing more advanced damage models and extending damage modeling methods to a variety of soft tissues.

Restricting ankle motion via orthotic bracing reduces toe clearance when walking over obstacles

BACKGROUND

When trans-tibial amputees cross obstacles leading with their prosthesis, foot clearance is achieved using compensatory swing-phase kinematics. Such compensation would suggest able-bodied individuals normally use swing-phase ankle dorsiflexion to attain adequate obstacle clearance; however, direct evidence of such contribution is equivocal. This study determined the contribution of sagittal plane ankle motion in achieving lead-limb clearance during obstacle negotiation.

METHODS

Twelve male able-bodied individuals (ages 18-30) completed obstacle crossing trials while walking on a flat surface. Lead-limb (right) ankle motion was manipulated using a knee-ankle-foot orthosis. Trials were completed with the ankle restricted at a neutral angle or unrestricted (allowing ∼±15° plantar/dorsiflexion).

FINDINGS

Restricted ankle motion caused significant increase in trail-limb foot placement distance before the obstacle (p=0.005); significant decrease in vertical toe clearance (p<0.003), vertical heel clearance (p=0.045), and lead-limb foot placement distance after the obstacle (p=0.045); but no significant changes in knee angle at instant of crossing or in average walking speed.

INTERPRETATION

The shifts in foot placements altered the part of swing that the lead-limb was in when the foot crossed the obstacle, which led to a decrease in clearance. These adaptations may have been due to being unable to dorsiflex the ankle to 'lift' the toes in mid-swing or to plantarflex the ankle during initial contact following crossing, which changed how the lead-limb was to be loaded. These findings suggest individuals using ankle bracing or those with ankle arthrodesis, will have reduced gait safety when negotiating obstacles.

Mechanical energy assessment of adult with Down syndrome during walking with obstacle avoidance

The aim of this study is analyzing the differences between plane walking and stepping over an obstacle for two groups of healthy people and people with Down syndrome and then, evaluating the movement efficiency between the groups by comprising of their mechanical energy exchanges. 39 adults including two groups of 21 people with Down syndrome (age: 21.6 ± 7 years) and 18 healthy people (age: 25.1 ± 2.4 years) participated in this research. The test has been done in two conditions, first in plane walking and second in walking with an obstacle (10% of the subject's height). The gait data were acquired using quantitative movement analysis, composed of an optoelectronic system (Elite2002, BTS) with eight infrared cameras. Mechanical energy exchanges are computed by dedicated software and finally the data including spatiotemporal parameters, mechanical energy parameters and energy recovery of gait cycle are analyzed by statistical software to find significant differences. Regards to spatiotemporal parameters velocity and step length are lower in people with Down syndrome. Mechanical energy parameters particularly energy recovery does not change from healthy people to people with Down syndrome. However, there are some differences in inter-group through plane walking to obstacle avoidance and it means people with Down syndrome probably use their residual abilities in the most efficient way to achieve the main goal of an efficient energy recovery.

Safety margins in older adults increase with improved control of a dynamic object

Older adults face decreasing motor capabilities due to pervasive neuromuscular degradations. As a consequence, errors in movement control increase. Thus, older individuals should maintain larger safety margins than younger adults. While this has been shown for object manipulation tasks, several reports on whole-body activities, such as posture and locomotion, demonstrate age-related reductions in safety margins. This is despite increased costs for control errors, such as a fall. We posit that this paradox could be explained by the dynamic challenge presented by the body or also an external object, and that age-related reductions in safety margins are in part due to a decreased ability to control dynamics. To test this conjecture we used a virtual ball-in-cup task that had challenging dynamics, yet afforded an explicit rendering of the physics and safety margin. The hypotheses were: (1) When manipulating an object with challenging dynamics, older adults have smaller safety margins than younger adults. (2) Older adults increase their safety margins with practice. Nine young and 10 healthy older adults practiced moving the virtual ball-in-cup to a target location in exactly 2 s. The accuracy and precision of the timing error quantified skill, and the ball energy relative to an escape threshold quantified the safety margin. Compared to the young adults, older adults had increased timing errors, greater variability, and decreased safety margins. With practice, both young and older adults improved their ability to control the object with decreased timing errors and variability, and increased their safety margins. These results suggest that safety margins are related to the ability to control dynamics, and may explain why in tasks with simple dynamics older adults use adequate safety margins, but in more complex tasks, safety margins may be inadequate. Further, the results indicate that task-specific training may improve safety margins in older adults.

Best-compromise between mechanical energy expenditure and foot clearance predicts leading limb motion during obstacle-crossing

This study aimed to identify the control strategy of obstacle-crossing of different heights with a multi-objective optimal control technique. Twelve young healthy adults walked and crossed obstacles of three different heights while their kinematic and ground reaction force data were measured simultaneously. Obstacle-crossing was formulated as an optimal control problem with two conflicting objectives: minimization of mechanical energy expenditure and maximization of foot-obstacle clearance. The results supported the hypothesis that experimentally measured ankle trajectories and joint angles of the swing limb and the joint moments of the stance limb could be predicted by the best compromise between these objectives, which was also independent of obstacle height. This control strategy was fundamentally different from that for unobstructed gait, and appeared to be pre-programmed into the nervous system. The results will serve as baseline data and the current technique be used for identifying changes in obstacle-crossing control strategies in people at higher risk of falling.

Performance and three-dimensional kinematics of bipedal lizards during obstacle negotiation

Bipedal running is common among lizard species, but although the kinematics and performance of this gait have been well characterized, the advantages in biologically relevant situations are still unclear. Obstacle negotiation is a task that is ecologically relevant to many animals while moving at high speeds, such as during bipedal running, yet little is known about how obstacles impact locomotion and performance. We examined the effects of obstacle negotiation on the kinematics and performance of lizards during bipedal locomotion. We quantified three-dimensional kinematics from high-speed video (500 Hz) of six-lined racerunners (Aspidoscelis sexlineata) running on a 3 m racetrack both with and without an obstacle spanning the width of the track. The lizards did not alter their kinematics prior to contacting the obstacle. Although contact with the obstacle caused changes to the hindlimb kinematics, mean forward speed did not differ between treatments. The deviation of the vertical position of the body center of mass did not differ between treatments, suggesting that in the absence of a cost to overall performance, lizards forgo maintaining normal kinematics while negotiating obstacles in favor of a steady body center of mass height to avoid destabilizing locomotion.

Model-based estimation of ventricular deformation in the cat brain

The estimation of ventricular deformation has important clinical implications related to neuro-structural disorders such as hydrocephalus. In this paper, a poroelastic model was used to represent deformation effects resulting from the ventricular system and was studied in 5 feline experiments. Chronic or acute hydrocephalus was induced by injection of kaolin into the cisterna magna or saline into the ventricles; a catheter was then inserted in the lateral ventricle to drain the fluid out of the brain. The measured displacement data which was extracted from pre-drainage and post-drainage MR images were incorporated into the model through the Adjoint Equations Method. The results indicate that the computational model of the brain and ventricular system captured 33% of the ventricle deformation on average and the model-predicted intraventricular pressure was accurate to 90% of the recorded value during the chronic hydrocephalus experiments.

Characteristics of single and double obstacle avoidance strategies: a comparison between adults and children

Activities of daily living often require us to negotiate several obstacles in the travel path. To date, there is little work investigating how adults accomplish such tasks, and there is even less known about multiple obstacle avoidance strategies used by children. The current work will expand our knowledge about the role of vision in adults and children when avoiding two obstacles placed in their travel path under altered ambient lighting. Healthy 7-year old children (n=10; aged 7.51+/-0.2 years) and adults (n=10; aged 22.76+/-1.7 years) were instrumented with infrared markers (Optotrak, NDI) placed on anatomical landmarks and asked to walk along a ten meter path under three conditions: unobstructed, single obstacle, or double obstacle. These trials were performed under two lighting conditions: Full (simulating standard office lighting) and Low (simulating a dark hallway lit by nightlights). Data analyses included lead and trail clearance values, step length, step width and step velocity, take-off distance and Horizontal toe Displacement at Apex (HDA) which was defined as the distance between the horizontal position of the toe to the leading edge of the obstacle when the toe reaches its peak height. Adults were able to maintain consistent behaviour regardless of the number of obstacles in the travel path. Children, however, adjusted their foot placement for the second obstacle. This indicates that having multiple obstacles in the travel path is a more challenging task for 7-year old, and suggests that children at this age may not have fully developed anticipatory locomotor strategies. Children had larger clearance values than adults for the lead foot crossing the obstacle under all obstacle and lighting conditions, and consistently used larger HDA values than adults. Together, these findings suggest that children adopt more cautious strategies than adults in complex environments. Additionally, children decreased walking velocity, increased step width and decreased their step length in a Low light environment. These changes are all indicators of a more careful avoidance strategy, which implies that children at this age rely heavily on visual information to guide foot placements in a complex environment.

Redirection of gaze and switching of attention during rapid stepping reactions evoked by unpredictable postural perturbation

In many situations successful execution of a balance-recovery reaction requires visual information about the environment. In particular, reactions that involve rapid limb movements, such as stepping, must be controlled to avoid obstacles and accommodate other constraints on limb trajectory. However, it is unknown whether the central nervous system can acquire the necessary visuospatial information prior to perturbation onset or must, instead, redirect gaze at the floor during the execution of the stepping reaction. To study this we examined gaze behaviour, during rapid forward-directed stepping reactions triggered by unpredictable platform perturbation, in 12 healthy young adults. We also monitored switching of attention, as inferred from onset of significant error in performing a concurrent visuomotor tracking task. Obstacles and/or step targets were used as constraints, to increase demands for accurate foot movement. Downward gaze shifts towards the floor almost never occurred during stepping reactions when foot motion was unconstrained but did occur more frequently as the demands for accurate foot movement increased. Nonetheless, even in the most challenging condition (target plus obstacle), downward redirection of gaze occurred in less than 40% of the trials, and subjects were commonly well able to clear the obstacle and land the foot on the target without redirecting their gaze towards the floor. An apparent switching of attention, subsequent to perturbation onset, occurred frequently (>80% of trials) in all task conditions, independent of the gaze shifts. The findings indicate that visual fixation of the foot or floor was not essential for accurate control of the foot movement, nor was the apparent switching of attention that followed perturbation onset linked, in any consistent way, to overt changes in visual fixation. Spatial features of the support surface were apparently "remembered" prior to perturbation onset, thereby allowing both vision and attention to be directed to other demands during the execution of the balance reaction.

A review of the adaptability and recovery of locomotion after spinal cord injury

Spinal cord injury (SCI) is associated with multiple motor problems leading to the alteration and limited adaptation in the walking and postural behavior. This review addresses recent findings on locomotor and postural adaptations after spinal cord injury. The adaptation of the locomotor behavior to behavioral goals and external constraints constitute important functional prerequisites in the recovery of locomotion after spinal cord injury. Functional prerequisites in locomotion include coping with changes in speed, slope obstacle, weight support, interaction with walking aids, energy consumption and attentional demands. Various treatment approaches such as locomotor training using body weight support (BWS) and functional electrical stimulation (FES) will be discussed, in the context of functional prerequisites necessary in the recovery of locomotion. Understanding locomotor and postural adaptations will lead to a better appreciation of the normal and dysfunctional mechanisms, and culminate eventually in the development of appropriate rehabilitation assessment and treatment strategies.

A dynamic model for simulating a trip and fall during gait

The purpose of this study was to develop an analytical model to simulate a trip and fall during gait. The human body was modeled as a 12 degree-of-freedom linkage system. The kinematics of the lower extremity for one cycle of gait were obtained for a healthy subject using an optoelectronic three-dimensional data acquisition system. Inverse dynamics was used to compute the moments about the hip, knee and ankle joints of the lower extremity. These moments were then used as input actuators to the joints in to a forward dynamics model to simulate the swing phase of gait from toe-off to heel-strike. An optimization procedure to minimize errors associated with the computed experimental torque was applied to correct for mathematical instability. An experiment was performed to measure the three-dimensional foot--obstacle contact force for a healthy subject tripping on an obstacle during gait. The contact force was applied to the swing limb of the forward dynamics model for 0.09 s beginning at 0.04 s after toe-off. Tripping on an obstacle followed by a muscle-relaxed fall was simulated. The simulation results were visualized with animation software.

Effect of stroke on step characteristics of obstacle crossing

OBJECTIVE

To compare spatial and temporal measures during lead limb obstacle crossing between subjects with stroke and healthy subjects.

DESIGN

Experimental, observational, with matched controls.

SETTING

Geriatric rehabilitation unit in a tertiary referral hospital.

PARTICIPANTS

Distance data were available for 19 subjects with stroke and 19 able-bodied subjects. Temporal data were available for 16 subjects with stroke and 16 able-bodied subjects. Subjects with stroke were inpatients and had to be able to walk 10 meters without assistance or gait aid.

INTERVENTION

Subjects were required to step over high and wide obstacles, ranging from 1 to 8cm, and trials were videotaped.

MAIN OUTCOME MEASURES

Toe clearance, preobstacle distance, postobstacle distance, step length, proportion of step length preobstacle, step time, preobstacle step time, postobstacle step time, and proportion of step time preobstacle were measured.

RESULTS

Mann-Whitney U tests were performed to determine differences between the 2 groups. Subjects with stroke had significantly higher toe clearance, smaller postobstacle distances, and greater step times than healthy subjects. Subjects with stroke did not demonstrate a significant reduction in preobstacle distance.

CONCLUSION

By modifying their lead limb trajectory during obstacle crossing, persons with stroke reduce the risk of a trip due to toe contact, but the modification may expose them to other safety risks.

Motion of the whole body's center of mass when stepping over obstacles of different heights

Tripping over obstacles and imbalance during gait were reported as two of the most common causes of falls in the elderly. Imbalance of the whole body during obstacle crossing may cause inappropriate movement of the lower extremities and result in foot-obstacle contact. Thus, this study was performed to investigate the effect of obstacle height on the motion of the whole body's center of mass (COM) and its interaction with the center of pressure (COP) of the stance foot while negotiating obstacles. Six healthy young adults were instructed to perform unobstructed level walking and to step over obstacles of heights corresponding to 2.5, 5, 10, and 15% of the subject's height, all at a comfortable self-selected speed while walking barefoot. A 13-link biomechanical model of the human body was used to compute the kinematics of the whole body's COM. Stepping over the higher obstacles resulted in significantly greater ranges of motion of the COM in the anterior-posterior and vertical directions, a greater velocity of the COM in the vertical direction, and a greater anterior-posterior distance between the COM and COP. In contrast, the motion of the COM in the medial-lateral direction was less likely to be affected when negotiating obstacles of different heights.

Kinematic analysis of obstacle clearance during locomotion

This study investigated the effect of obstacles of different heights on the locomotion of 15 healthy subjects. The following parameters were studied: (1) the distance of the toe and heel markers from the obstacle during toe-off and heel contact, respectively, (2) the minimum clearance distance of the toe and heel markers, and (3) the angular displacements and velocities of the hip, knee, and ankle. Results show significant differences in joint angular kinematics and clearance distances as obstacle height increased. The kinematic and distance differences exhibited both strong linear and non-linear trends. Toe-off distance and heel contact distance did not change significantly with changes in obstacle height.

Obstacle crossing in subjects with stroke

OBJECTIVE

To study the ability of subjects with stroke to successfully step over an obstacle during ambulation.

SETTING

A geriatric rehabilitation unit in a tertiary referral hospital.

SUBJECTS

Twenty-four inpatients with stroke (median time poststroke 27 days, interquartile range 21 to 44.5 days) able to walk 10 m unassisted without walking aids; also, 22 healthy subjects.

METHOD

Subjects were required to step over obstacles of various heights and widths, ranging from 1cm to 8cm. A fail was scored if the obstacle was contacted by either lower limb or if assistance or upper limb support was required. The choice of leading limb and the presence of visual deficits and neglect were also recorded in the stroke subjects. Subjects were tested on two occasions.

RESULTS

Significantly more fails were recorded for stroke subjects, with 13 subjects failing at least once. No preference was shown for leading either with the affected or with the unaffected leg. Stroke subjects showed inconsistent performance over the two testing sessions.

CONCLUSION

The ability to negotiate obstacles was compromised and inconsistent in stroke subjects undergoing inpatient rehabilitation. This suggests that gait safety in this population remains threatened.

The control of foot placement during compensatory stepping reactions: does speed of response take precedence over stability?

Rapid, reflex-like stepping movements are a prevalent and functional compensatory reaction to destabilization, however, little is known about the underlying control. In this paper, a model is developed to examine how speed and stability demands affect control of foot placement during forward and backward compensatory stepping reactions. The concept of the velocity stability margin (VSM) is introduced to characterize the degree to which the horizontal velocity of the falling body approaches biomechanical limits on the capacity to decelerate the center of mass; analogous limits on center-of-mass displacement are quantified in terms of the displacement stability margin (DSM). The model is used to predict, for any initial step characteristics, the variation in DSM and VSM that would occur as a function of changes in timing of foot placement. The VSM was found to prevail over the DSM in establishing limits of stability. Model simulations demonstrated that there typically exists a minimum swing duration that maximizes speed of response while meeting minimum requirements for stability (VSM > or = 0), as well as a slower speed of response (longer swing duration) at which stability (VSM) is maximized. Experimental data from platform-perturbation tests in 20 healthy young (22-28) and older (65-81) adults were used, in conjunction with the model, to investigate whether speed or stability takes precedence during natural behavior. Control of single-step reactions appeared to favor stability; although the model predicted that a minimally stable step (VSM = 0) could be attained by swing durations as short as 30 ms, the observed swing durations were, on average, 135 ms longer than this, and the average VSM was nearly as large (80%) as the optimally stable value predicted by the model. Control of the initial step of multiple-step reactions was distinctly different. The average swing duration was only 55 ms greater than the minimally stable value and the average VSM was 81% smaller than in the single-step reactions. This reduction in VSM is consistent with a need to execute additional steps and appears to support the validity of the model. This model may help to provide insight into the biomechanical factors that govern the neural control of compensatory stepping reactions.

Increasing obstacle height and decreasing toe-obstacle distance affect the joint moments of the stance limb differently when stepping over an obstacle

Foot placement during gait is important in regulating the dynamics of the joints of the supporting limb and in maintaining balance of the whole body. We hypothesized that increasing obstacle height and decreasing toe-obstacle distance (distance between the trailing foot and the obstacle during stance of the trailing foot just prior to stepping over the obstacle) would affect the joint moments of the stance limb differently when stepping over an obstacle. A total of 14 healthy young adults stepped over an obstacle 51, 102, 153, and 204 mm in height in a self-selected manner (i.e. toe-obstacle distance was not controlled) and for toe-obstacle distance targets of 10, 20, 30, and 40% of their step lengths measured during unobstructed gait. The adduction and internal rotation moments at the ankle joint increased as toe-obstacle distance decreased. The adduction and internal rotation moments at the hip joint during early stance, the internal rotation moment at the knee joint during late stance, and the dorsiflexion moment at the ankle joint during late stance increased with obstacle height. Reductions in toe-obstacle distance had greater effects on the moments of the ankle joint, and increases in obstacle height had greater effects on the moments of the hip joint. These greater demands on joint moments may affect the abilities of those elderly having decreased muscle strengths to safely step over obstacles. Copyright 1998 Elsevier Science B.V.

Placing the trailing foot closer to an obstacle reduces flexion of the hip, knee, and ankle to increase the risk of tripping

This study was performed to test the hypothesis that reducing the horizontal distance between the trailing foot (foot crossing the obstacle last) and obstacle, during stance just prior to stepping over the obstacle, would reduce flexion of the hip, knee, and ankle joints of the trailing limb when the toe is over the obstacle to reduce the vertical toe-obstacle clearance and increase the risk of tripping. Fourteen healthy young adults stepped over an obstacle of 51, 102, 153, and 204 mm height in a self-selected manner (i.e., toe-obstacle distance was not controlled) and for toe-obstacle distance targets of 10, 20, 30, and 40% of their step lengths measured during unobstructed gait. The reductions in toe-obstacle distance resulted in linear decreases in flexion of the hip, knee, and ankle when the toe was over the obstacle. Toe-obstacle clearance of the trailing limb decreased significantly as toe-obstacle distance decreased. The reductions in toe-obstacle distance led to contact of the trailing (but not the leading) foot with the obstacle, the closer the obstacle the greater the number of contacts. The reductions also resulted in linear decreases in swing time of the trailing limb from toe-off to when the toe was over the obstacle. The height of the hip was not affected by toe-obstacle distance. Angular velocity of knee flexion was found to increase linearly as toe-obstacle distance decreased and appears to be of primary importance in avoiding obstacle contact.

Stepping over obstacles during locomotion: insights from multiobjective optimization on set of input parameters

In this study we investigate possible objectives that the central nervous system (CNS) may consider in planning a strategy for stepping over an obstacle. A link segment simulation model has been developed based on Lagrangian dynamics, with which muscle force inputs can be optimized to best satisfy the postulated objectives for landing stability, obstacle clearance, and efficiency of the movement. A direct optimization approach with multiobjective criteria based on the kinematic and kinetic characteristics of the swing phase of locomotion is used in the simulation. The role of initial conditions at toe-off and biarticular muscle forces during the swing phase was also investigated. The optimization was performed for both leading limb and the trailing limb during the swing phase. The simulation results demonstrate that the use of biarticular muscles is sufficient to clear a range of obstacles with the trailing limb (obstacle encountered during early swing). Stride length or landing stability objectives need not be specified suggesting a simpler control of trailing limb trajectory by the CNS (one of stride length or landing stability objectives were not necessary). In contrast while the use of biarticular muscles can be sufficient to clear obstacles with the leading limb (obstacle encountered during mid to late swing), a stable landing and smooth toe and knee trajectories are compromised without suitable initial conditions at toe-off. The results suggest that the set of postulated objectives for the lead limb is adequate, although not complete.