Control of dynamic stability during gait termination on a slippery surface

Postural control during gait termination and prehension

BACKGROUND

Challenges to postural stability emerge in the transition from locomotion to a standing posture as during gait termination, often accompanied by another task (e.g., opening a door), which may complicate control. However, less is known about postural control during terminating gait while engaged in a secondary manual task.

RESEARCH QUESTION

What are the changes in postural control when terminating gait with and without a prehension task?

METHODS

In a cross-sectional design, 15 healthy young adults (M=8, F=7; 27±2 years; 69±13 kg; 171±8 cm) underwent both a single task gait termination (GTO) and dual task (gait termination plus reaching; GTR). Postural Time-to-Contact (TtC) was measured using Center of Pressure (CoP) and the sternum position in anterior-posterior (AP) and medial-lateral (ML) directions over two different phases: preparatory phase and stabilization phase. Five successful trials were recorded to obtain a mean TtC. For statistical analysis of TtC, a two-tailed paired t-test was used (p =.05) as normality was satisfied.

RESULTS

For the preparatory phase, there were no differences for the CoP, but TtC of the sternum position in AP was shorter in GTR than GTO (p =.001). Meanwhile, for the stabilization phase, TtCs of both the CoP and sternum position were longer in GTR in both AP and ML directions (p's <.001).

SIGNIFICANCE

We suggest that for the preparatory phase, the shorter TtC of the sternum position with intact TtC of the CoP in GTR indicates that healthy young individuals are flexible, in that they smoothly integrate CoP control with the upper body demands required to also perform the prehension task. Meanwhile, for the stabilization phase, the longer TtC in dual termination and prehension task indicates that the perturbation imposed by the prehension movement did not result in reduced stabilization when returning to an upright posture.

Situational risk factors for fall-related vertebral fractures in older men and women

Situational factors might help explain why most vertebral fractures occur in older people without a previous osteoporosis diagnosis. After adjusting for predisposing risk factors, the activity before the fall, type of fall, and falling direction remained as strong determinants of fall-related vertebral fractures in older men and women.

INTRODUCTION

A matched case-control study was conducted to investigate the effects of situational factors, in addition to predisposing factors, on clinical vertebral fractures in older men and women in Taiwan.

METHODS

Cases were community-dwelling individuals aged ≥ 65 years who visited emergency departments (EDs) of two university-affiliated hospitals due to a fall and had a primary diagnosis of a vertebral fracture during a 1-year period in 2017. Five control patients per case, matched by the time of falling, gender, and age, who sought care in the same ED due to a fall resulting in a soft tissue injury were selected. A total of 64 men (age range: 65 ~ 99 years) and 194 women (age range: 65 ~ 100 years), diagnosed with a vertebral fracture, participated in the study.

RESULTS

Multivariable logistic models were conducted separately for men and women. Results suggested that the following factors were significantly associated with an increased risk of vertebral fractures in men: a low educational level (adjusted odds ratio [OR] = 1.95; 95% confidence interval (CI), 1.02 ~ 3.71), asthma (OR = 2.96; 95% CI, 1.35 ~ 6.92), depression (OR = 4.31; 95% CI, 1.03 ~ 17.5), toileting (OR = 2.30; 95% CI, 1.04 ~ 4.94), slipping (OR = 5.27; 95% CI, 1.80 ~ 15.4), stepping down (OR = 3.99; 95% CI, 1.40 ~ 11.4), sudden leg weakness (OR = 3.73; 95% CI, 1.13 ~ 12.4), and falling backwards (OR = 3.78; 95% CI, 1.83 ~ 7.80); and in women: a fracture history (OR = 2.00; 95% CI, 1.07 ~ 3.76), osteoporosis (OR = 1.94; 95% CI, 1.15 ~ 3.49), getting in/out of the bed/chair (OR = 1.90; 95% CI, 1.07 ~ 3.39), stepping down (OR = 2.10; 95% CI, 1.17 ~ 3.77), and falling backwards (OR = 4.00; 95% CI, 2.39 ~ 6.68) and sideways (OR = 2.61; 95% CI, 1.38 ~ 4.96).

CONCLUSIONS

The combination of predisposing and situational risk factors may display a more comprehensive risk profile for the occurrence of VFs, and thus, interventions that add both types of risk factors may result in greater risk reduction of VFs, although those specifically targeted at situational risk factors during falls are limited and their effectiveness and efficiency remained to be explored.

Tails, Flails, and Sails: How Appendages Improve Terrestrial Maneuverability by Improving Stability

Trade-offs in maneuverability and stability are essential in ecologically relevant situations with respect to robustness of locomotion, with multiple strategies apparent in animal model systems depending on their habitat and ecology. Free appendages such as tails and ungrounded limbs may assist in navigating this trade-off by assisting with balance, thereby increasing the acceleration that can be achieved without destabilizing the body. This comparative analysis explores the inertial mechanisms and, in some cases, fluid dynamic mechanisms by which appendages contribute to the stabilization of gait and perturbation response behaviors in a wide variety of animals. Following a broad review of examples from nature and bio-inspired robotics that illustrate the importance of appendages to the control of body orientation, two specific cases are examined through preliminary experiments: the role of arm motion in bipedal gait termination is explored using trajectory optimization, and the role of the cheetah’s tail during a deceleration maneuver is analyzed based on motion capture data. In both these examples, forward rotation of the appendage in question is found to counteract the unwanted forward pitch caused by the braking forces. It is theorized that this stabilizing action may facilitate more rapid deceleration by allowing larger or longer-acting braking forces to be applied safely.

Investigating proactive balance control in individuals with incomplete spinal cord injury while walking on a known slippery surface

BACKGROUND

Up to 83 % of individuals with incomplete spinal cord injury (iSCI) experience ≥ 1 fall/year. Individuals with iSCI employ more cautious walking strategies than able-bodied (AB) individuals during normal walking. Whether individuals with iSCI can use proactive balance strategies to adapt to expected slip perturbations/reduce slip severity while walking has not been previously assessed.

METHODS

19 individuals with iSCI (AIS D; 14 males; 61 ± 18 years) and 17 AB individuals (13 males; 61 ± 18 years) completed 3 walking conditions: normal walking trials, an unexpected slip trial, and expected slip trials. Steel rollers induced a slip in the antero-posterior (AP) direction. Outcome variables included step length, center of mass velocity, foot-floor angle, AP margin of stability, and maximum post-slip velocity (PSV).

RESULTS

The iSCI group used a greater magnitude of cautious strategies (i.e. walking slower with shorter, flatter steps) than AB individuals in all conditions. However, the lack of significant interaction effects indicate that the proactive adaptations compared to normal walking (i.e. walking slower with shorter, flatter steps, and a more anterior xCOM-position) were similar between the two groups (AB & iSCI). Both groups showed a similar rate of adaptation (after just 1 slip) and these feedforward changes were maintained throughout the remaining slip trials which was effective at reducing maximum PSV.

CONCLUSIONS

Individuals with iSCI use proactive balance strategies to adapt to a known slippery surface in a similar manner to AB individuals both in terms of the proportion and timing of adaptation.

Mediolateral postural stability when carrying asymmetric loads during stair negotiation

The purpose of this study was to assess postural stability in the medial-lateral (ML) direction when carrying unilateral and bilateral loads during stair negotiation. Twenty-four healthy young adults were instructed to ascend and descend a three step staircase under three load conditions: no load, 20% body mass (BM) bilateral load, and 20% BM unilateral load. A modified time-to-contact (TTC) method was proposed to evaluate postural stability during stair negotiation. Carrying unilateral loads required more rapid postural adjustments as evidenced by lower minimum ML TTC and ML TTC percentage as compared bilateral loads and no load during stair descent. In addition, lower ML TTC and TTC percentage were found for loaded limb stance for stair descent. Taken together, unilateral loads and the loaded leg during stair descent are of concern when considering postural stability during load carriage. These results illustrate differing postural control challenges for stair ascent and descent during load carriage.

Compensatory spinopelvic motions with and without a handheld task following a perturbation in standing between subjects with and without chronic low back pain

BACKGROUND

A handheld task-related compensation strategy could be used to avoid injuries in subjects with chronic low back pain (LBP). Those who suffer with LBP demonstrate reduced spinopelvic motion; however, there is a lack of understanding on dynamic mobility with a handheld task.

RESEARCH QUESTION

Are there differences in three-dimensional spinopelvic motions following a treadmill-induced perturbation in subjects with LBP while considering a handheld task?

METHODS

Twenty-five subjects (11 female and 14 male) with LBP and 38 control subjects (15 female and 23 male) participated in the study. Each group was introduced to walking perturbations (0.86 m/sec, velocity in 0.1 sec) with and without a handheld tray in a standing position. The induced trip allowed subjects to recover by walking forward for a 5 second duration while the spinopelvic angles were measured during the trip and the subsequent recovery period.

RESULTS

There was no significant group difference in the three-dimensional (3D) spinopelvic motions while holding or not holding a tray. However, the groups demonstrated a significant interaction on tray usage with 3D motions in the spinopelvic regions (F = 13.46, p = 0.001). The sagittal plane motion was significantly minimized with a handheld task for both the lumbar spine and pelvis in the LBP group.

SIGNIFICANCE

The LBP group demonstrated minimized lumbar and pelvic motions in the sagittal plane, which underpins their concern to reduce motion while holding a tray. The significant interaction between groups on tray usage for spinopelvic compensation represents a strategy for avoiding injuries. Further studies are required to determine strategies to enhance lumbopelvic integration with handheld tasks in individuals with LBP.

Age-related changes in upper body contribution to braking forward locomotion in women

INTRODUCTION

Gait termination is a transitory task that requires the lower body to produce braking forces and inhibit forward propulsion. However, it is still unknown whether the upper body plays an active role in braking of gait and whether this mechanism is impaired with ageing.

RESEARCH QUESTION

Do older women exhibit an impaired control of upper body segments during gait termination with respect to young women?

METHODS

Ten young and 10 older women performed three gait termination trials at comfortable speed while fixing the gaze on a visual target. A 3D motion analysis system was used to measure head, trunk and pelvis angular displacement and velocity, and estimate neck, waist and hip moments through Plug-in Gait modeling. Cross-correlation analysis of kinematic waveforms between paired adjacent segments (head-trunk and trunk-pelvis) was performed to investigate upper body coordination. Surface EMG activity of erector spinae (L3), sternocleidomastoid and neck extensor muscles was recorded. Statistics was carried out by MANOVA.

RESULTS

Older participants exhibited delayed peak extensor torques of neck, waist and hip compared to young participants, along with lower progression speed. Both groups showed a slight flexion of the trunk counteracted by a backward tilt of head and pelvis during braking. In addition, older women displayed a peculiar upper body coordination pattern, with the head coupling with trunk motion, as shown by cross-correlation. Older women displayed shorter lumbar erector spinae onset latency relative to last heel contact than young (16 ± 68 ms vs 92 ± 37 ms).

SIGNIFICANCE

The upper body plays an active role in the braking of gait and this mechanism is impaired in older women. Moreover, the age-related coupling of head and trunk motion may produce an unbalancing effect on whole-body stability during the braking mechanism, thus leading to a higher risk of falls.

Visual-Vestibular Interaction for Postural Control During Sit-to-Stand: Effects of Aging

During sit-to-stand (STS), the vestibular system is highly stimulated in response to linear acceleration of the head and may play an important role, in addition to vision, for postural control. We examined the effects of aging on visual-vestibular interaction for postural control during STS in 15 young (22.5 ± 1.1 years) and 15 older (73.9 ± 5.3 years) participants. Vestibular information was manipulated using galvanic vestibular stimulation. Vision conditions involved normal (eyes open), suboptimal (blurring goggles), and no (eyes closed) vision. Older participants had significantly greater mediolateral peak-to-peak trunk roll (p = .025) and center of mass displacements (p < .001) than young participants. However, despite having greater mediolateral instability, older participants utilized similar strategies as young participants to overcome sensory perturbations during STS. Overall visual inputs were more dominantly used for mediolateral trunk control during STS than vestibular inputs.

Arm reactions in response to an unexpected slip-Impact of aging

Slips and falls represent a serious public safety concern in older adults, with the segment of the United States population over the age of 65 accounting for about three quarters of all fall related deaths. The majority of falls in older adults are due to trips and slips. The objective of this study was to investigate how age affects arm reactions generated in response to unexpected slips. Thirty-three participants divided into two age groups (16 young, 17 old) participated in this study. Participants were exposed to two conditions: known dry walking (baseline) and an unexpected slip initiated when stepping onto a glycerol-contaminated floor. The upper extremity parameters of interest included the timing and amplitude of the shoulder flexion moment generated in response to the slip as well as the resulting angular kinematics (trajectories). The analysis of the kinetic data revealed a delayed shoulder flexion reaction to slips in older adults compared to their young counterparts, as well as a greater flexion moment magnitude. Knowledge of such upper body reaction mechanisms to unexpected slips may help to improve balance recovery training in older adults, as well as aid in the implementation of environmental modifications, e.g. handrails, to reduce falls-related injuries.

Intersegmental coordination elicited by unexpected multidirectional slipping-like perturbations resembles that adopted during steady locomotion

This study aimed at testing the hypothesis that reactive biomechanical responses elicited by unexpected slipping-like perturbations delivered during steady walking are characterized by an intersegmental coordination strategy resembling that adopted during unperturbed walking. Fifteen healthy subjects were asked to manage multidirectional slipping-like perturbations delivered while they walked steadily. The planar covariation law of elevation angles related to lower limb segments was the main observed variable related to unperturbed and perturbed strides. Principal component analysis was used to verify whether elevation angles covaried, both before and after the onset of the perturbation, and, if so, the orientation of the related planes of covariation was compared. Results revealed that the planar covariation law of the unperturbed limb after onset of the perturbation was systematically similar to that seen during steady walking. This occurred despite differences in range of motion and intersubject variability of both elevation and joint angles. The analysis strongly corroborates the hypothesis that the planar covariation law emerges from the interaction between spinal neural networks and limb mechanical oscillators. In particular, fast and stereotyped reactive strategies may result from the interaction among activities of downstream neural networks encrypting well-trained motor schemes, such as those related to walking, limb dynamics, and sensory motor information gathered during the perturbation. In addition, our results allowed us to speculate that rehabilitative treatment based on unexpected perturbations and relying on the plasticity of the central nervous system may also be effective in eliciting unimpaired intralimb coordination in neurological patients.

Compensations during Unsteady Locomotion

Locomotion in a complex environment is often not steady, but the mechanisms used by animals to power and control unsteady locomotion (stability and maneuverability) are not well understood. We use behavioral, morphological, and impulsive perturbations to determine the compensations used during unsteady locomotion. At the level both of the whole-body and of joints, quasi-stiffness models are useful for describing adjustments to the functioning of legs and joints during maneuvers. However, alterations to the mechanics of legs and joints often are distinct for different phases of the step cycle or for specific joints. For example, negotiating steps involves independent changes of leg stiffness during compression and thrust phases of stance. Unsteady locomotion also involves parameters that are not part of the simplest reduced-parameter models of locomotion (e.g., the spring-loaded inverted pendulum) such as moments of the hip joint. Extensive coupling among translational and rotational parameters must be taken into account to stabilize locomotion or maneuver. For example, maneuvers with morphological perturbations (increased rotational inertial turns) involve changes to several aspects of movement, including the initial conditions of rotation and ground-reaction forces. Coupled changes to several parameters may be employed to control maneuvers on a trial-by-trial basis. Compensating for increased rotational inertia of the body during turns is facilitated by the opposing effects of several mechanical and behavioral parameters. However, the specific rules used by animals to control translation and rotation of the body to maintain stability or maneuver have not been fully characterized. We initiated direct-perturbation experiments to investigate the strategies used by humans to maintain stability following center-of-mass (COM) perturbations. When walking, humans showed more resistance to medio-lateral perturbations (lower COM displacement). However, when running, humans could recover from the point of maximum COM displacement faster than when walking. Consequently, the total time necessary for recovery was not significantly different between walking and running. Future experiments will determine the mechanisms used for compensations during unsteady locomotion at the behavioral, joint, and muscle levels. Using reduced-parameter models will allow common experimental and analytical frameworks for the study of both stability and maneuverability and the determination of general control strategies for unsteady locomotion.

Sudden stopping in patients with cerebellar ataxia

Stopping during walking, a dynamic motor task frequent in everyday life, is very challenging for ataxic patients, as it reduces their gait stability and increases the incidence of falls. This study was conducted to analyse the biomechanical characteristics of upper and lower body segments during abrupt stopping in ataxic patients in order to identify possible strategies used to counteract the instability in the sagittal and frontal plane. Twelve patients with primary degenerative cerebellar ataxia and 12 age- and sex-matched healthy subjects were studied. Time-distance parameters, dynamic stability of the centre of mass, upper body measures and lower joint kinematic and kinetic parameters were analysed. The results indicate that ataxic patients have a great difficulty in stopping abruptly during walking and adopt a multi-step stopping strategy, occasionally with feet parallel, to compensate for their inability to coordinate the upper body and to generate a well-coordinated lower limb joint flexor-extensor pattern and appropriate braking forces for progressively decelerating the progression of the body in the sagittal plane. A specific rehabilitation treatment designed to improve the ability of ataxic patients to transform unplanned stopping into planned stopping, to coordinate upper body and to execute an effective flexion-extension pattern of the hip and knee joints may be useful in these patients in order to improve their stopping performance and prevent falls.

Effects of Aqua Aerobic Therapy Exercise for Older Adults on Muscular Strength, Agility and Balance to Prevent Falling during Gait

[Purpose] The purpose of the present study was to examine the effects of an aqua aerobic therapy exercise for older adults on biomechanical and physiological factors affecting gait. [Subjects] A total of 15 subjects participated in this study and they were randomly divided into the experimental and the control group. [Methods] Physiological variables, leg strength, power and flexibility, and biomechanical variables, both kinematic and kinetic, were measured before and after the aqua aerobic therapy exercise. Each subject was instructed to walk along an elevated walkway and during the trials a trapdoor opened at random to create a 10 cm falling perturbation. Full body motion and kinetics was gathered during the gait. [Results] There were significant reductions in body weight, and body fat mass, and stride time after the perturbation. Significant increases in leg strength corresponded to the maximum joint moment of the landing leg showing that the subjects' ability for recovery of balance after the perturbation improved. [Conclusion] As the results showed significant improvements in gait pattern and recovery time after perturbed gait, we conclude that aqua aerobic therapy is an effective exercise method for training older adults to reduce their risk of falling.

Adaptation of gait termination on a slippery surface in Parkinson's disease

Parkinson's disease (PD) causes instability and difficulty adapting to changing environmental and task demands. We examined the effects of PD on the adaptation of gait termination (GT) on a slippery surface under unexpected and cued circumstances. An unexpected slip perturbation during GT was followed by a slip perturbation during GT under two conditions: planned over multiple steps and cued one step prior to GT. Feed forward and feedback-based responses to the perturbation were compared to determine (1) how PD affects the ability to integrate adaptive feed forward and feedback-based GT strategies on a slippery surface, (2) if adaptations can be implemented when GT is required within one step, and (3) if behaviour changes with repeated exposure. Similar to the control group (n=10), the PD group (n=8) adapted and integrated feed forward and feedback-based components of GT under both stop conditions. Feed forward adaptations included a shorter, wider step, and appropriate stability margin modifications. Feedback-based adaptations included a longer, wider subsequent step. When cued to stop quickly, both groups maintained most of these adaptations: foot angle at contact increased in the first cued stop but adapted with practice. The group with PD differed in their ability to adapt GT with slower, wider steps and less stability.

The contribution of light touch sensory cues to corrective reactions during treadmill locomotion

The arms play an important role in balance regulation during walking. In general, perturbations delivered during walking trigger whole-body corrective responses. For instance, holding to stable handles can largely attenuate and even suppress responses in the leg muscles to perturbations during walking. Particular attention has been given to the influence of light touch on postural control. During standing, lightly touching a stable contact greatly reduces body sway and enhances corrective responses to postural perturbations, whereas light touch during walking allows subjects to continue to walk on a treadmill with the eyes closed. We hypothesized that in the absence of mechanical support from the arms, sensory cues from the hands would modulate responses in the legs to balance disturbing perturbations delivered at the torso during walking. To test this, subjects walked on a treadmill while periodically being pulled backwards at the waist while walking. The amplitude of the responses evoked in tibialis anterior to these perturbations was compared across 4 test conditions, in a 2 × 2 design. Subjects either (a) lightly touched or (b) did not touch a stable contact, while the eyes were (c) open or (d) closed. Allowing the subjects to touch a stable contact resulted in a reduction in the amount of fore-aft oscillation of the body on the treadmill, which was accompanied by a reduction in the ongoing electromyographic activity in both tibialis anterior and soleus during undisturbed walking. In contrast, the provision of touch resulted in an increase in the amplitude of the evoked responses in tibialis anterior to the backward perturbations that was more evident when subjects walked with the eyes closed. These results indicate that light touch provides a sensory cue that can be used to assist in stabilizing the body while walking. In addition, the sensory information provided by light touch contributes to the regulation of corrective reactions initiated by balance disturbances encountered during walking.

BIOMECHANICS AND POTENTIAL INJURY MECHANISMS OF WRESTLING

Wrestling is one of the oldest and most popular competitive sports in the world, however, knowledge of the biomechanics of wrestling is not well established and the biomechanical risk factors of injuries unclear. The purpose of this study was to investigate the joint kinematics of the lower limbs and the center of pressure (COP) movements in Greco-Roman style (GR) and free style (FS) wrestlers during tackle defense. Eighteen male college wrestlers participated in the current study: 10 majored in GR (height: 171.1 ± 8.0 cm; weight: 73.9 ± 11.5, kg) and 8 in FS (height: 169.0 ± 5.2 cm; weight: 71.8 ± 11.4 kg). The wrestlers received tackle attacks from three different directions while their kinematic data measured by a 3D motion capture system and ground reaction forces from two AMTI forceplates. The wrestlers who majored in GR style tended to resist tackle attacks longer than the FS group. Compared to the GR group, the FS wrestlers tended to have greater A/P excursions of the COP with significant greater knee flexion. This flexed knee strategy may be related to the rule of the game and the training the FS wrestlers received. Significantly increased joint angles in the transverse and frontal planes at the knee and ankle found in the current study may be related to the risk of knee and ankle injuries commonly observed in wrestlers. Strengthening of the muscles of the lower extremity may be helpful for reducing these injuries during competitions.

Age-related changes of arm movements in dual task condition when walking on different surfaces

The purpose of this study was to investigate whether the dual task paradigm would influence arm movements during walking. Furthermore, we examined the effects of different walking surfaces on arm movements while performing dual tasks. The effects of age and gender were also investigated. Fifteen young adults and 15 older adults were included in this study. Subjects were asked to perform the walking task alone (single-task trial) and walking in combination with a cognitive task (dual-task trial). Four walking conditions (1 single task and 3 dual task trials)×two walking surfaces were encountered. Both age groups had greater elbow and trunk movement in the sagittal plane under the dual task trials as compared to the single task trial (p<.05). Subjects had greater upper extremity and upper body movement on the soft floor than on the hard floor (p<.05). Subjects had greater movement amplitude when confronting a challenging environment, especially in the contralateral side. Among gender, there was a group-gender interaction: the older females had smaller upper extremity movement than the older males (p<.05) but the opposite was true for the young adults. The results suggest that different age groups of males and females use different balance control strategy to deal with the challenging conditions.

Postural control while dressing on two surfaces in the elderly

Falling is a worldwide problem faced by the elderly, and it has serious consequences. The elderly falls in the bathroom very often, and this raises the concern of the connection between the high incidence of falls in the bathroom and postural control of putting on shorts. Because little is known about postural control while putting on shorts of any age group, this study investigated the effects of age and surface on postural control while putting on shorts. Healthy young adults (n =15; age range: 21-27 years) and the elderly participants (n = 15; age range: 63-78 years) were compared in the temporal-spatial parameters, movement of center of mass (COM), and its interactions with center of pressure (COP) and kinematic measures under two conditions: anti-slipping mat and tile. The results demonstrated that the elderly had lengthened cycle time, decreased single-limb-support ratio, decreased medial-lateral velocity of COM, decreased relative displacement between COM and COP, decreased hip abduction-adduction angle, knee flexion angle, and the height of foot elevation. The elderly demonstrated a conservative strategy that may reduce the mechanical load on the supporting limb but preserve medial-lateral balance during the task. The adopted strategy may be relevant to the reduction of muscle strength and proprioception loss during natural aging process. Although surface did not affect the motion patterns of participants while putting on shorts, the slippery surface should be avoided. Other environmental modification such as a grab bar and bright illumination may have advantages in reducing the risk of falls.

Motor patterns during walking on a slippery walkway

Friction and gravity represent two basic physical constraints of terrestrial locomotion that affect both motor patterns and the biomechanics of bipedal gait. To provide insights into the spatiotemporal organization of the motor output in connection with ground contact forces, we studied adaptation of human gait to steady low-friction conditions. Subjects walked along a slippery walkway (7 m long; friction coefficient approximately 0.06) or a normal, nonslippery floor at a natural speed. We recorded gait kinematics, ground reaction forces, and bilateral electromyographic (EMG) activity of 16 leg and trunk muscles and we mapped the recorded EMG patterns onto the spinal cord in approximate rostrocaudal locations of the motoneuron (MN) pools to characterize the spatiotemporal organization of the motor output. The results revealed several idiosyncratic features of walking on the slippery surface. The step length, cycle duration, and horizontal shear forces were significantly smaller, the head orientation tended to be stabilized in space, whereas arm movements, trunk rotations, and lateral trunk inclinations considerably increased and foot motion and gait kinematics resembled those of a nonplantigrade gait. Furthermore, walking on the slippery surface required stabilization of the hip and of the center-of-body mass in the frontal plane, which significantly improved with practice. Motor patterns were characterized by an enhanced (roughly twofold) level of MN activity, substantial decoupling of anatomical synergists, and the absence of systematic displacements of the center of MN activity in the lumbosacral enlargement. Overall, the results show that when subjects are confronted with unsteady surface conditions, like the slippery floor, they adopt a gait mode that tends to keep the COM centered over the supporting limbs and to increase limb stiffness. We suggest that this behavior may represent a distinct gait mode that is particularly suited to uncertain surface conditions in general.

Rapid gait termination: effects of age, walking surfaces and footwear characteristics

The aim of this study was to systematically investigate the influence of various walking surfaces and footwear characteristics on the ability to terminate gait rapidly in 10 young and 26 older people. Subjects walked at a self-selected speed in eight randomized shoe conditions (standard versus elevated heel, soft sole, hard sole, high-collar, flared sole, bevelled heel and tread sole) on three surfaces: control, irregular and wet. In response to an audible cue, subjects were required to stop as quickly as possible in three out of eight walking trials in each condition. Time to last foot contact, total stopping time, stopping distance, number of steps to stop, step length and step width post-cue and base of support length at total stop were calculated from kinematic data collected using two CODA scanner units. The older subjects took more time and a longer distance to last foot contact and were more frequently classified as using a three or more-steps stopping strategy compared to the young subjects. The wet surface impeded gait termination, as indicated by greater total stopping time and stopping distance. Subjects required more time to terminate gait in the soft sole shoes compared to the standard shoes. In contrast, the high-collar shoes reduced total stopping time on the wet surface. These findings suggest that older adults have more difficulty terminating gait rapidly than their younger counterparts and that footwear is likely to influence whole-body stability during challenging postural tasks on wet surfaces.

Maneuvers during legged locomotion

Maneuverability is essential for locomotion. For animals in the environment, maneuverability is directly related to survival. For humans, maneuvers such as turning are associated with increased risk for injury, either directly through tissue loading or indirectly through destabilization. Consequently, understanding the mechanics and motor control of maneuverability is a critical part of locomotion research. We briefly review the literature on maneuvering during locomotion with a focus on turning in bipeds. Walking turns can use one of several different strategies. Anticipation can be important to adjust kinematics and dynamics for smooth and stable maneuvers. During running, turns may be substantially constrained by the requirement for body orientation to match movement direction at the end of a turn. A simple mathematical model based on the requirement for rotation to match direction can describe leg forces used by bipeds (humans and ostriches). During running turns, both humans and ostriches control body rotation by generating fore-aft forces. However, whereas humans must generate large braking forces to prevent body over-rotation, ostriches do not. For ostriches, generating the lateral forces necessary to change movement direction results in appropriate body rotation. Although ostriches required smaller braking forces due in part to increased rotational inertia relative to body mass, other movement parameters also played a role. Turning performance resulted from the coordinated behavior of an integrated biomechanical system. Results from preliminary experiments on horizontal-plane stabilization support the hypothesis that controlling body rotation is an important aspect of stable maneuvers. In humans, body orientation relative to movement direction is rapidly stabilized during running turns within the minimum of two steps theoretically required to complete analogous maneuvers. During straight running and cutting turns, humans exhibit spring-mass behavior in the horizontal plane. Changes in the horizontal projection of leg length were linearly related to changes in horizontal-plane leg forces. Consequently, the passive dynamic stabilization associated with spring-mass behavior may contribute to stability during maneuvers in bipeds. Understanding the mechanics of maneuverability will be important for understanding the motor control of maneuvers and also potentially be useful for understanding stability.

Control of dynamic stability during gait termination on a slippery surface in Parkinson's disease

This study investigated how Parkinson's disease (PD) affects the ability to switch from locomotion to gait termination (GT) during planned and cued GT and examined the effect of PD on the integration of a reactive, balance maintenance strategy into voluntary GT. After a series of stops on a stable surface, eight participants with and 10 without PD stopped on a surface, which slid quickly and unexpectedly forward mimicking a slippery surface. PD caused instability during the completely voluntary nonslippery stops (P = 0.012) but not during the slippery stops, which required a reactive movement. The PD group walked slower [0.9-1.0 m/s vs. 1.3 m/s, respectively (P < 0.001)] with shorter steps during the first step of nonslippery GT (P = 0.016) and with wider steps during all steps of nonslippery GT (P <or= 0.05). Similar to controls, the PD group increased lateral stability during planned GT compared to cued GT (P = 0.007). The timing of gait termination was similar between groups in all conditions. During the unexpected perturbation, both groups used a generalized slip response to regain balance after the perturbation. PD did not affect the ability to stop walking or to integrate a balance-correcting response into GT but did affect movement speed, size, and stability of the voluntary movement.

Whole-Body Responses: Neural Control and Implications for Rehabilitation and Fall Prevention

Humans are one of the unique species that utilize bipedal gait to ambulate in our environment. Despite this fact, coordination of the arms with the legs and the rest of body is essential for many daily activities. As such, whole-body responses have emerged as the preferred strategy following perturbations to balance during both standing and walking. Complex neural circuitry may allow for this coordination through the use of propriospinal pathways linking lumbar and cervical pattern generators in the spinal cord, with supraspinal centers altering this control depending on the context of the situation. Based on these findings, we argue that whole-body reactions may be exploited for rehabilitation purposes. Preliminary results have indicated training programs designed to elicit whole-body responses are effective in reducing falls and improving functional mobility in older adults with and without neurological impairment.

Adaptations to normal human gait on potentially slippery surfaces: the effects of awareness and prior slip experience

Prior knowledge of potentially slippery conditions has been shown to alter normal human gait in slip and fall experiments. Here we quantify the effects of two aspects of prior knowledge - awareness of a possible slip and prior slip experience - on normal gait. Sixty-eight subjects (40F, 28M) each walked over 48 high-friction surfaces (control trials) and 12 low-friction surfaces. Within- and between-subject changes in lower limb muscle activation, gait kinematics and ground reaction forces were analyzed in three non-slip control trials: one before and one after the first unexpected slip exposure, and a third after repeated slip exposures. Subjects knew they might slip in the latter two trials but not the first trial. Twenty subjects slipped during their first low-friction exposure (early slip group), 32 in later low-friction exposures (late slip group), and 16 subjects did not slip at all. Simultaneous changes in awareness and experience between the first two analyzed trials of the early slip group altered the muscle activity in both limbs, reduced the foot and knee angles at heel strike in the slip limb and reduced the ground reaction forces, impulses and utilized friction after heel strike in the slip limb. A change in only awareness between the first two analyzed trials of the late slip group produced the same kinematic changes seen in the early slip group, but only small muscle activity change and no kinetic changes. Subsequent slip experience in the late slip group produced the muscle activation and kinetic changes observed in the early slip group, but no further kinematic changes. These results showed that awareness of a potential slip primarily alters how the slip-limb approaches the floor, whereas prior slip experience primarily alters the anticipatory muscle activation and how the foot interacts with the floor. These muscle, kinematic and kinetic changes were consistent with a more cautious "normal" gait, and can reduce the external validity of slip and fall experiments.

The effect of subject awareness and prior slip experience on tribometer-based predictions of slip probability

Prior knowledge of potentially slippery conditions has been shown to alter normal human gait in slip and fall experiments. We sought to quantify how the empirical relationship between slip probability and available floor friction was affected by subject awareness and prior slip experience. Sixty-eight subjects (40 females, 28 males) walked over three different low-friction surfaces inserted periodically between non-slip control trials. Three increasing levels of prior knowledge were used: deceived (unaware of the slippery surface), aware (20% chance of a slippery surface, but no prior slip experience) and experienced (aware with prior slip experience). Available friction was measured using a drag sled and a variable incidence tribometer. Of 620 low-friction trials, 124 generated slips greater than 27mm. The proportion of slips, the slip distance and the required friction (taken from the control trial immediately before a low-friction trial) generally decreased with increasing levels of prior knowledge. These adaptations were accommodated by logistically regressing slip outcome (yes/no) against the normalized friction (available friction minus required friction) rather than against available friction alone. The regressions showed that subject awareness biased the slip probability curve toward a lower slip risk for a given normalized friction, but that the subsequent addition of slip experience generated a slip risk curve that was not significantly different from that of deceived (and presumably unprepared) subjects. These findings suggest that data to validate a tribometer's ability to predict the risk of slipping (but not falling) can be acquired from subjects with prior slip experience.

Relationship between hip abductor rate of force development and mediolateral stability in older adults

OBJECTIVE

To examine the relationship between hip abductor rate of force development (RFD) and performance of reactive and voluntary balance tasks in older adults.

DESIGN

Descriptive study using correlation and regression analyses.

SETTING

University research laboratory.

PARTICIPANTS

A volunteer sample of 30 community-dwelling men and women over the age of 65 years.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Measures of hip abductor RFD, center of pressure (COP) displacement during compensatory stepping, and scores on 1-leg standing (OLS), and tandem gait tests. Hip abductor RFD was defined as the time required to go from 10% of maximum force to 60% of maximum force (10%-60% RFD) or to 90% of maximum force (10%-90% RFD).

RESULTS

Hip abductor RFD did not correlate with COP displacement during compensatory stepping, but did correlate significantly with OLS and tandem gait variables (P<.05). In predicting OLS scores, age (beta=-.485, P<.05) and 10% to 60% RFD (beta=-.354, P<.05) were significant predictors. In predicting tandem gait scores, 10% to 90% RFD (beta=.384, P<.05) was the only significant predictor in the final regression model.

CONCLUSIONS

Hip abductor RFD, tested under voluntary conditions, correlated with performance of clinical tests that challenge lateral stability. Our results support the idea that voluntary and reactive balance tasks involve different types of neuromuscular control.