Ankle Strategy Research Articles

Postural steadiness during quiet stance does not associate with ability to recover balance in older women

Fall risk depends on ability to maintain balance during daily activities, and on ability to recover balance following a perturbation such as a slip or trip. We examined whether similar neuromuscular variables govern these two domains of postural stability. We conducted experiments with 25 older women (mean age=78 yrs, SD=7 yrs). We acquired measures of postural steadiness during quiet stance (mean amplitude, velocity, and frequency of centre-of-pressure movement when standing with eyes open or closed, on a rigid or compliant surface). We also measured ability to recover balance using the ankle strategy after release from a forward leaning position (based on the maximum release angle where recovery was possible, and corresponding values of reaction time, rate of ankle torque generation, and peak ankle torque). We found that balance recovery variables were not strongly or consistently correlated with postural steadiness variables. The maximum release angle associated with only three of the sixteen postural steadiness variables (mean frequency in rigid, eyes open condition (r=0.36, P=.041), and mean amplitude (r=0.41, P=.038) and velocity (r=0.49, P=.015) in compliant, eyes closed condition). Reaction time and peak torque did not correlate with any steadiness variables, and rate of torque generation correlated moderately with the mean amplitude and velocity of the centre-of-pressure in the compliant, eyes closed condition (r=0.48-0.60). Our results indicate that postural steadiness during quiet stance is not predictive of ability to recover balance with the ankle strategy. Accordingly, balance assessment and fall prevention programs should individually target these two components of postural stability.

Human standing posture control system depending on adopted strategies

Control of the standing posture of humans involves at least two distinct modes of operation to restore the body balance in the sagittal plane: the ankle strategy and the hip strategy. The objective of the study was to estimate the contribution of vestibular, visual and somatosensory feedbacks to these distinct strategies. The body dynamics was described as the motion of two linked rigid segments that represented the legs and the rest of the body. The posture controller received the inclination angles of the two body segments as inputs and regulated the moments around the ankle and hip joints. The controller had four feedback paths that were characterised by transfer functions connecting the two inputs and the two outputs. To evoke the distinct strategies, the floor conditions were varied by narrowing the support surface under the feet. A continuous pseudo-random external disturbing force was applied to the waist and the thigh independently. The inclination angles of the body segments and the ground reaction force were measured, and the transfer functions of the controller were estimated with the maximum-likelihood system identification procedure. Six healthy male adult subjects participated in the experiment. When the hip strategy became evident under the narrow support surface conditions, the transfer function relating the leg inclination angle and the ankle joint moment decreased its DC gain (16%), whereas the other three transfer functions increased the gains (20-140%) (ANOVA, p < 0.05). Based on a criterion for simplicity in the modification of the posture controller, these changes suggest a new hypothesis that, when posture control becomes difficult, the central nervous system selectively activates the somatosensory feedback paths from the hip joint angle to the moments around the ankle and hip joints.

Mechanisms underlying age-related differences in ability to recover balance with the ankle strategy

Falls cause substantial death and morbidity in the elderly. An important prerequisite to the development of fall prevention interventions is improved understanding of the biomechanical and neuromuscular variables that govern ability to recover balance. In the present study, we examined the relative importance of strength versus speed of response variables in explaining differences in balance recovery performance with the ankle strategy between young and elderly women. Twenty-five young (19-36 years) and 25 community-dwelling elderly women (66-90 years) participated in balance recovery experiments. Subjects were supported in an inclined standing position by a horizontal tether and instructed to recover an upright vertical standing position by contracting their ankle muscles. The maximum initial lean angle from which they could recover balance without release of the tether (which depends primarily on strength) was 19.6% smaller for elderly than young. The maximum initial lean angle from which they could recover balance after the tether was suddenly released (which depends on both strength and speed of response) was 36.1% smaller for elderly than young. Moreover, between-group differences in performance were related to both strength and speed of response. Peak ankle torque was 7.7% smaller in elderly than young during dynamic recovery trials; reaction time was 27% slower in elderly, due to a lengthened muscle response latency, and rate of ankle torque generation was 15.6% slower in elderly. These results suggest that differences in ability to recover balance between young and elderly women associate with variables related to strength and speed of response, and that exercise-based fall prevention programs should include balance and agility training, in addition to strength training.

Posturographic description of the regaining of postural stability following stroke

Failing balance and increased liability to falling are common complaints among hemiplegic subjects. During rehabilitation much effort is put into regaining postural stability. To describe the regaining of postural stability during rehabilitation for the first year following stroke in hemiplegic patients Twenty-six patients were included within 5 days of suffering a stroke resulting from a CT-verified clearly defined thromboembolic lesion, localized in the vascular bed of middle cerebral artery on either side. Posturographic evaluation of sway was performed on a commercially available force plate system (Balance Master Pro). Measured parameters included per cent maximum stability (PMS), per cent ankle strategy (AST), and average angular velocity (AVE). An additional parameter was derived by calculating the slope (SLP) of the linear relationship between stability and ankle strategy. SLP reflects the predisposition of the stroke patient to keep using ankle strategy when faced with increasingly difficult balance tasks. The presence of familiarization to the measurement method was examined by including a preliminary measurement not otherwise included in the analysis of changes during the first year of follow-up. The temporal evolvement of sway was described by measuring sway parameters 1, 2, 4, 8 and 52 weeks following stroke. The proportions of the variation in sway caused by measurement error and between patients were compared. All parameters improved over time, as demonstrated by significant improvements from each of week 1, 2, 4 and 8 to week 52. There was evidence of familiarisation for PMS, AST and AVE, but not for SLP. Estimation of variance components showed that between patient variation accounts for between two-thirds and four-fifths of the total variation. A considerable part of this variation was caused by individual differences in the temporal improvement of sway over time. Patients considered the evaluation of postural sway demanding and time consuming resulting in a relatively large loss to follow-up: 15 patients completed the 8-week visit while seven completed the 1 year visit. Postural stability increases for the first year following stroke. There is evidence of familiarization towards the measurements process for the traditional posturographic parameters PMS, AST and AVE, but not for the derived parameter SLP, which might make the latter a candidate for a sway parameter reflecting basic mechanisms of upholding upright stance in hemiplegic patients. The estimated variance components stress the considerable between-patient variation and question the ability of the force plate method to monitor individual sway performance of such patients during the rehabilitation process.

Falls, fractures, and hip pads

Improvement of balance along with bone-enhancing pharmacotherapy can improve the level of an individual's physical activity and mobility. Balance can be improved with enhancement of postural proprioception and muscular strength. Postural deformities have been shown to impair quality of life of osteoporotic individuals. Kyphotic posture has been demonstrated to contribute to propensity to fall in osteoporotic individuals. Kyphotic posturing and gait disorders can be managed through proprioceptive training, use of a weighted kypho-orthosis, muscle re-education, and safe resistance exercises. Proprioceptive balance training can reduce falls and fracture. Sarcopenia and osteoporotic fractures create musculoskeletal challenges that cannot be met with pharmacotherapy alone. Bone loss, imbalance, and gait disorder along with cognitive concerns can increase with aging. Even in healthy persons, predisposition to falls increases with age-related neuromuscular changes. Muscle strength decreases approximately 50% from age 30 to 80. Furthermore, the amount of body sway increases with reduction of proprioception. Therefore, measures that can decrease imbalance can reduce the risk for falls and fracture. In normal balance, ankle strategies are recruited rather than hip strategies. Strengthening of the lower extremity muscles reduces the risk for falls. Gait aids can also decrease the risk for falls. During a fall, the risk for hip fracture increases 30-fold if there is direct impact to the hip. The use of hip protectors can decrease the risk for hip fracture during a sideways fall. Training in effective safe-landing strategies should be included in fall prevention programs.

Role of Sensory Input and Muscle Strength in Maintenance of Balance, Gait, and Posture in Parkinson???s Disease

The purpose of this study was to simultaneously evaluate multiple components of disequilibrium in patients with idiopathic Parkinson's disease (PD) in ON and OFF states and healthy age- and sex-matched controls on tests of balance, gait, and dynamometry. Thirty subjects with Parkinson's disease and 30 controls were matched for age and sex. Isokinetic and balance laboratories of a clinical research center were used for assessment. Performance results for static and dynamic balance by dynamic posturography for sensory organization tests (SOT), limits of stability, clinical gait assessment, and dynamometric assessment for the trunk, hip, and ankle at different speeds for concentric muscle strength were obtained. Tests were done both in ON and OFF state in Parkinson's disease patients and results compared. Between OFF state and controls, a significant difference was observed for SOT-2 (proprioception, P < 0.005), SOT-6 (conflicting vision, P < 0.001), and SOT-4 (eyes open with sway support, P < 0.038), and there was less use of ankle strategy in SOT-3 (sway vision, P < 0.04). No significant difference was observed for vestibular function (SOT-5). Significant difference was also observed (P < 0.001) for all variables in limits of stability except for reaction time and for muscle strength of trunk, hip, and ankle (P < 0.001) between OFF state and controls. After antiparkinsonian medications, significant improvement was observed for gait velocity (P < 0.002), muscle strength (P < 0.001), and strategy score in SOT-3 between OFF and ON states. A positive correlation was observed between muscle strength (ankle, hip, and trunk) and gait velocity (ON state r = 0.37, OFF state r = 0.56) and movement velocity (ON state r = 0.39). A positive correlation was also seen between ankle strength and gait velocity in both ON (r = 0.393) and OFF states (r = 0.397) and between ankle strength and ankle strategy in all SOTs except SOT-3 in the OFF state. The quantitative reduction of muscle strength in the spine, hip, and ankle, along with impaired proprioception, visual sense, and smaller base of support, were the main causes for postural instability in Parkinson's disease patients. A correlation was seen between muscle strength, static and dynamic balance, and gait in both ON and OFF states. In contrast to the previous studies, the present study showed that medications improved the muscle strength, gait speed, and use of ankle strategy but did not worsen proprioceptive sense.

歩行時の外乱刺激適応課題に対する高齢者と若年者の姿勢制御反応の比較

The purpose of this study was to examine postural control against perturbation during treadmill walking. In addition, we hypothesized a strategy of postural control in such a situation and suggested the effect of aging on postural control.Ten young and twenty-nine elderly subjects participated in this study. We used a separated-belt treadmill, and perturbations were produced by rapidly decelerating one side of the walking-belt for 500 ms while walking. To young subjects, two types of the perturbation were given five times each in three minutes of walking: 50% deceleration of the initial speed (moderate perturbation) and 100% deceleration of the initial speed (strong perturbation). To elderly subjects, moderate perturbations were given five times in three minutes of walking. The electromyogram responses of leg, thigh, and trunk muscles on both sides and acceleration at the pelvis were measured. We classified subjects from reaction patterns of muscles, and compared them.Four reaction patterns of muscles were observed: “ankle strategy,” “mixed ankle and hip strategy,” and others. Comparing the ankle strategy seen in young and elderly subjects, the response of tibialis anterior on the perturbed side after the perturbation was significantly delayed in elderly subjects.The “mixed ankle and hip strategy” observed in this study did not change with advancing trials, although the mixed strategy in standing is defined as the transitory pattern to pure ankle strategy or hip strategy when the stimulus exceeds the control limit. We therefore concluded that the mixed strategy observed in this study was different from the mixed strategy seen in standing. The pattern of muscle recruitment was immediate antero-distal muscle activities followed by postero-proximal muscle activities after perturbation. This pattern resembled whiplash. We suggest this mixed strategy be established as a new strategy. It is thought that the posture of a subject moves like striking a whip since this new strategy showed the ankle strategy followed by the hip strategy, and we considered this new strategy to be a “whiplash strategy”. Also, this new strategy was seen when the body shake was large, so it is suggested that it is an important strategy for subjects who have low ability of postural control.From these results, we concluded that there is a specific postural control strategy in walking, and there are differences in postural control ability between elderly and young subjects.

Frequency spectral characteristics of standing balance in children and young adults

Past research has shown that spectral frequency characteristics of a balance control system may be useful for early detection of minor changes in the system. Since there is a lack of information regarding the frequency spectrum of children’s balance control system, this study was undertaken to investigate and compare the spectral frequency characteristics of standing balance control between children and young adults under altered sensory environments. Seventeen children (9 females, 8 males, mean age 7.8±0.9 years) and the same number of female and male young adults (mean age 21.1±1.3 years) were tested for standing balance under six sensory conditions. These conditions were created by crossing the three levels of the visual factor (open eye, closed eye, sway-referenced vision) with the two levels of the somatosensory factor (fixed foot support, compliant foot support). The median spectral frequencies of the shear forces in the anterior–posterior (A/P) and the medial–lateral (M/L) directions were used as dependent variables. The results showed that children had higher median spectral frequency of the A/P shear force than young adults and this difference was not affected by the somatosensory factor. The median spectral frequency in the M/L direction was not different between the groups. The higher rate of body mass vibration of children in the A/P direction implies that children may not have fully developed the ankle strategy for maintaining standing balance, but nonetheless have developed the same efficiency of using vision for their reference of standing balance.

Stratégies posturales et chutes chez la personne âgée et le sujet parkinsonien

Objective: To use a posture analysis to show the evolution of postural pattern connected with falls. Material and Method: It is a prospective study on two groups of 16 persons of more than 60 years. A group concerns 16 small disability off drug parkinsonian patients, a group concerns 16 healthy witnesses. All the persons benefited from a posture recording by means of a force platform and were followed during 1 year. Results: Data analysis underlines three groups of persons corresponding to three postural patterns, independently of the presence of Parkinson disease. A group ( n = 18) did not contain fallers, the second ( n = 10 ) contained 20% of fallers, the third ( n = 4) contained 100% of fallers. Differences between the groups were identified on 16 posturographic parameters. Discussion: A group has a good functional value and one does not record any fall. Its characteristics, which correspond to a category of persons who compensate well for the phenomena of ageing, are found in the literature. A group has an intermediate functional value and regrets 20% of fallers. Kinetic profile reveals a tendency to the stiffness of the posture. This group is going to operate rather ankle strategies. A group has an inferior functional value and regrets 100% of fallers. Kinetic profile seems disrupted and not to be able to adapt itself in a satisfactory way to the situation otherwise than by stereotypical reactions. This group is going to operate systematically much less stabilizing hip strategies. Conclusion: A close determinism between physiological neuromotor ageing and Parkinson disease does exist. We showed with a prospective follow-up, the arisen of fall and showed the evolution of postural patterns related to fall. It appears as well that evolution mainly follows three stages leading from a small risk of fall gait pattern to a major risk of fall gait pattern.

Effect of strength and speed of torque development on balance recovery with the ankle strategy.

In the event of an unexpected disturbance to balance, the ability to recover a stable upright stance should depend not only on the magnitude of torque that can be generated by contraction of muscles spanning the lower extremity joints but also on how quickly these torques can be developed. In the present study, we used a combination of experimental and mathematical models of balance recovery by sway (feet in place responses) to test this hypothesis. Twenty-three young subjects participated in experiments in which they were supported in an inclined standing position by a horizontal tether and instructed to recover balance by contracting only their ankle muscles. The maximum lean angle where they could recover balance without release of the tether (static recovery limit) averaged 14.9 +/- 1.4 degrees (mean +/- SD). The maximum initial lean angle where they could recover balance after the tether was unexpectedly released and the ankles were initially relaxed (dynamic recovery limit) averaged 5.9 +/- 1.1 degrees, or 60 +/- 11% smaller than the static recovery limit. Peak ankle torque did not differ significantly between the two conditions (and averaged 116 +/- 32 Nm), indicating the strong effect on recovery ability of latencies in the onset and subsequent rates of torque generation (which averaged 99 +/- 13 ms and 372 +/- 267 N. m/s, respectively). Additional experiments indicated that dynamic recovery limits increased 11 +/- 14% with increases in the baseline ankle torques prior to release (from an average value of 31 +/- 18 to 54 +/- 24 N. m). These trends are in agreement with predictions from a computer simulation based on an inverted pendulum model, which illustrate the specific combinations of baseline ankle torque, rate of torque generation, and peak ankle torque that are required to attain target recovery limits.

Characteristics of somatosensory feedback in postural control during standing.

In the present study, the function of the somatosensory feedback system in postural control was investigated. For the sake of simplicity, the present study considered only balancing in the anteroposterior direction using the ankle strategy, in which the ankle moment is mainly used to maintain balance. To suppress the vestibular and visual feedback paths, a subject stood on a force-measuring platform with a fixed back support. Because the subject's body was immovable under these conditions, the subject controlled a computer model that simulated the subject's load at the ankles. Information about the sway angle of the model was fed through the somatosensory feedback path. Frequency response functions of the ankle moment in response to the sway angle were calculated. The experimental results suggest that the human somatosensory feedback system has derivative characteristics and, consequently, can maintain an upright posture by itself. The results were compared with those of previous studies on vestibular and visual feedback systems. The comparison reveals that subject-to-subject variance in the somatosensory system is significantly smaller than that in the other systems. This may indicate that the somatosensory feedback is the most automatic of the systems and plays a dominant role when a subject maintains an upright posture using the ankle strategy.

Biomechanical analysis of movement strategies in human forward trunk bending. II. Experimental study

The large mass of the human upper trunk, its elevated position during erect stance, and the small area limited by the size of the feet, stress the importance of equilibrium control during trunk movements. The objective of the present study was to perform a biomechanical analysis of fast forward trunk movements in order to understand the coordination between movement and posture. The analysis is based on a comparison between experimentally observed bending and hypothetical "optimal bending" performed on an infinitely narrow support, as presented in a companion paper. The experimental data were obtained from 16 subjects who performed fast forward bending while standing on a wide platform or on a narrow beam. The analysis is performed by decomposition of the movement into three dynamically independent components, each representing a movement along one of the three eigenvectors of the motion equation. The eigenmovements are termed "hip", "ankle", and "knee" eigenmovements, according to the dominant joint. The experimentally observed movement is characterized mainly by the hip and ankle eigenmovements, whereas the knee eigenmovement is negligible. Similarly to the "optimal bending" the ankle eigenmovement starts earlier and lasts longer than the hip eigenmovement. An early forward acceleration of the center of gravity in the ankle eigenmovement is caused by anticipatory changes in the ankle joint torque. This clarifies the role of the early tibialis anterior burst and/or soleus inhibition usually observed in electromyographic recordings during forward bending. The results suggest that the hip and the ankle eigenmovements can be treated as independently controlled motion units aimed at functionally different behavioral goals: the bending per se and postural adjustment. It is proposed that the central nervous system has to control these motion units sequentially in order to perform the movement and maintain equilibrium. It is also suggested that the hip and ankle eigenmovements can be regarded as a biomechanical background for the hip and ankle strategies introduced by Horak and Nashner (1986) on the basis of electromyographic recordings and kinematic patterns in response to postural perturbations.

The Effect of Selected Residential Carpet and Pad on the Balance of Healthy Older Adults

The purpose of this research was to determine whether selected residential carpet and pad contributed to balance problems among healthy, community-dwelling older adults. Twenty-five older adults (mean age 73.25; SD± 7.48) were recruited, and balance was measured using the NeuroCom Computerized Posturography balance machine. Participants were exposed to the Sensory Organization Test while standing on the selected residential carpet (i.e., a 36-ounce, 1/2-inch pile height, 1/8-inch gauge, cut pile carpet) and pad (i.e., a rebonded polyurethane, 6-pound density, 7/16-inch thick padding). Postural sway significantly increased on the carpet only when participants had their eyes closed and the forceplate on the balance machine moved. Balance strategy was not affected by the carpet. Participants were able to adjust to the sensory limitations by adopting an ankle strategy regardless of the floor surface. The results of this study indicate limited participant difficulty maintaining static balance when standing on the selected carpet.

Correlation of Functional Reach Distance, Sagittal Displacement and Envelope Area of the Center of Gravity in Functional Reach Test by Hip, Ankle, and Heels-up Strategy in Normal Subjects.

本稿の目的はFunctional Reach Test(FRT)遂行における運動戦略(ストラテジ)の相違がFunctional Reach距離(FR距離)及び重心移動の要因にどのように影響するか確認することである。対象は健常者19名(平均年齢21.6±4.3歳)とした。FRTは股 · 足 · 踵上げストラテジ(股 · 足 · 踵上げFRT)の3条件で遂行させ,FR距離とともに重心の前後移動(前後移動),重心動揺外周面積(動揺面積)を測定した。その結果,FR距離は踵上げFRT,股FRT,足FRTの順にそれぞれ有意(p≤0.05)に大きく,前後移動,動揺面積はともに踵上げFRTが股FRT,足FRTより有意に(p≤0.01)大きかった。FR距離,前後移動、動揺面積の間の相関は有意でなかった。主成分分析の結果では主に前後移動を反映する成分(寄与率30.7%)と,FR距離と前後移動を分ける成分(寄与率25.4%)が挙げられた。踵上げFRTは前後移動や重心動揺も大きく,より高度な姿勢制御能力が要される特徴があった。また,同一のストラテジでFRTが行われる場合はFR距離と重心移動に有意な相関はあるといえないことがわかった。

Predicted region of stability for balance recovery:: motion at the knee joint can improve termination of forward movement

Earlier experimental studies on balance recovery following perturbation have identified two discrete strategies commonly employed by humans, i.e. hip and ankle strategies. It has hence been implied that the knee joint plays a relatively minor role in balance recovery. The purpose of this study was to determine whether the size of the feasible stability region (FSR) would be affected by allowing knee motion in sagittal plane movement termination. The FSR was defined as the feasible range of anterior velocities of the center of mass (COM) of a human subject that could be reduced to zero with the final COM position within the base of support (BOS) limits. The FSR was computed using a four-segment biomechanical model and optimization routine based on Simulated Annealing algorithm for three scenarios: unrestricted knee motion (UK), restricted knee motion (RK), and unrestricted knee motion with an initial posture that matches RK (UKM). We found that movement termination could benefit little from UK condition when the COM ( x COM) was initially located in the forefoot region [0.00 (toe) > x COM⩾−0.50 (mid-foot)] with no more than a 17% increase in FSR compared to RK. The effect of knee motion increased in the rear foot region with a 25% increase in FSR at x COM=−1 (heel). Close to half of this difference (12%) was attributable to the knee-related restriction on initial posture and the rest to movement termination per se. These findings illustrated a theoretical role of knee motion in standing humans’ repertoire of effective posture responses, which include hip and ankle strategies and their variants for balance recovery with stationary BOS.

Postural activity of constrained subject in response to disturbance in sagittal plane.

This study examines the postural activity of partially constrained subjects for three different initial standing postures in response to disturbances in the sagittal plane. When the ankle strategy suffices for disturbance rejection in response to anterior disturbances, a mostly linear relationship between the ankle torque and ankle angle was observed, resulting in a constant stiffness at the ankles. However, when the ankle torque saturation was reached, a combined ankle-trunk postural strategy was utilized mainly in response to posterior perturbations due to the properties of the base-of-support. This caused a nonlinear scaling of ankle responses, thereby increasing variability of ankle stiffness. Distinctions in the ankle responses were also observed for different initial standing postures. The anterior initial stance generally increases the overall postural stability and renders the utilization of ankle strategy feasible, even for the rejection of posterior disturbances. Therefore, a linear torque-angle relationship at the ankles was observed for the anterior initial stance, regardless of the perturbation parameters.

Stance balance control with orthoses in a group of children with spastic cerebral palsy

Although ankle‐foot orthoses (AFOs) are frequently prescribed to correct skeletal malalignment in children with spastic diplegia, their effect on standing balance abilities has not been documented. This study investigated balance differences related to the presence of pathology and orthotic conditions during conditions of unexpected stance perturbation by comparing four children aged between 3 and 15 years with spastic cerebral palsy and four control children matched for years of independent walking experience. Electromyographic and kinematic data were collected and compared between groups and in three orthotic conditions (no AFOs, solid AFOs, dynamic AFOs). Results revealed that balance responses of children with spasticity were characterized by: increased coactivation of muscles as opposed to distal to proximal recruitment, decreased presence of upright posture in stance, increased use of‘on‐toes’strategies, and different sway characteristics compared with the typically developing children. In both groups of children, the use of solid AFOs during perturbed stance resulted in: decreased activation of gastrocnemius muscles, disorganized muscle‐response patterns, decreased use of ankle strategies, and increased joint angular velocities at the knee compared with conditions without AFOs or with dynamic AFOs. These preliminary results support the use of dynamic AFOs to correct skeletal malalignment in children with spastic diplegia.

Ankle and hip postural strategies defined by joint torques.

Previous studies have identified two discrete strategies for the control of posture in the sagittal plane based on EMG activations, body kinematics, and ground reaction forces. The ankle strategy was characterized by body sway resembling a single-segment-inverted pendulum and was elicited on flat support surfaces. In contrast, the hip strategy was characterized by body sway resembling a double-segment inverted pendulum divided at the hip and was elicited on short or compliant support surfaces. However, biomechanical optimization models have suggested that hip strategy should be observed in response to fast translations on a flat surface also, provided the feet are constrained to remain in contact with the floor and the knee is constrained to remain straight. The purpose of this study was to examine the experimental evidence for hip strategy in postural responses to backward translations of a flat support surface and to determine whether analyses of joint torques would provide evidence for two separate postural strategies. Normal subjects standing on a flat support surface were translated backward with a range of velocities from fast (55 cm/s) to slow (5 cm/s). EMG activations and joint kinematics showed pattern changes consistent with previous experimental descriptions of mixed hip and ankle strategy with increasing platform velocity. Joint torque analyses revealed the addition of a hip flexor torque to the ankle plantarflexor torque during fast translations. This finding indicates the addition of hip strategy to ankle strategy to produce a continuum of postural responses. Hip torque without accompanying ankle torque (pure hip strategy) was not observed. Although postural control strategies have previously been defined by how the body moves, we conclude that joint torques, which indicate how body movements are produced, are useful in defining postural control strategies. These results also illustrate how the biomechanics of the body can transform discrete control patterns into a continuum of postural corrections.

Turning strategies during human walking.

The mechanisms involved in rapidly turning during human walking were studied. Subjects were asked to walk at a comfortable speed and to turn toward the instructed direction as soon as they felt an electrical stimulus to the superficial peroneal nerve. Stimuli were presented repeatedly at random over 10- to 15-min periods of walking for turning in both directions. Electromyograms (EMGs), joint angular movements of the right leg, and forces under both feet were recorded. The step cycle was divided into 16 parts, and the responses to stimuli in each part were analyzed separately. Two turning strategies were used, depending on which leg was placed in front for braking. For example, to turn to the right when the right foot was placed in front, subjects generally altered direction by spinning the body around the right foot (spin turn). To turn left when the right foot was in front, subjects shifted weight to the right leg, externally rotated the left hip, stepped onto the left leg, and continued turning until the right leg stepped in the new direction (step turn). The step turn is easy and stable because the base of support during the turn is much wider than in the spin turn, so some subjects used it in all parts of the cycle. Initially, the deceleration of walking is similar to a rapid stopping task, which has been previously examined. The deceleration mechanism involves a sequence of distal-to-proximal activation of muscles on one side of the body (soleus, biceps femoris, and erector spinae). This pattern is similar to the "ankle strategy" used in postural control during forward sway. The control of foot placement in the swing leg and muscle activities for rotating the trunk in the stance leg occurred within a step after the cue. The action of ankle inverters and elevation of the pelvis by activity of gluteus medius may contribute to the control of trunk rotation. This activity was closely related to the timing of the opposite foot strike, independent of the part of the step cycle when the stimulus was applied. In most subjects, the turn was completed without resetting the underlying walking rhythm. This first EMG analysis of rapid turning shows how common strategies for postural sway and stopping can be combined with one of two turning strategies. This simplifies the complex task of turning at a random time in the step cycle.

P058 Biomechanical analysis of hip and ankle strategies during fast forward upper trunk bending

P058 Biomechanical analysis of hip and ankle strategies during fast forward upper trunk bending