How do non-muscular torques contribute to the kinetics of postural recovery following a support surface translation?

The Feasibility of Using the Virtual Time-to-Contact Measure of Postural Stability to Examine Postural Recovery in People With Diabetes Mellitus

This study aimed to examine the feasibility of using time-to-contact measures during the perturbation protocol in people with diabetes mellitus. Three-dimension motion capture and force data were collected during 0.5-s perturbations in four directions (forward, backward, right, and left) and at two accelerations (20 and 40 cm/s2) to compute the time-to-contact. Time-to-contact analysis was divided into three phases: perturbation, initial recovery, and final recovery. The statistical analysis showed the main effects of Direction and Phase (p < .01) as well as a Direction by Phase interaction (p < .01). Backward perturbation with lower acceleration and backward/forward perturbation with higher acceleration had deleterious effects on postural stability in people with diabetes mellitus.

Postural Control and Adaptation Strategy of Young Adults on Unstable Surface

Balance control is essential for postural adjustment in physical activities. This study investigates the behavior of human postural control and the coordination and adaptation strategy of hip, knee, and ankle when standing on an unstable surface. Twenty participants were recruited. Four different conditions were investigated: a quiet bipedal stance with eyes open and eyes closed, and standing on an unstable surface with eyes open and eyes closed. Other than the joint angle, the standard body sway measures, such as sway area and sway velocity, were computed. A nonlinear time series measure, that is, sample entropy, was used to determine the regularity of the time series and body adaptability to change and perturbation. The results show that the body sway increases as the difficulty increases. This study also confirms the coordination of the hip, knee, and ankle to maintain body balance on the unstable surface by decreasing the joint angle and adopting a lower posture. Even though the individual joint has lower sample entropy value and is deemed to be rigid and less adaptive to perturbation, the postural control exhibits higher sample entropy value, particularly in the anterior–posterior direction, and has the ability to stabilize the body by manipulating the joints simultaneously. These outcomes suggest that an unstable surface not only challenges the human postural control, but also reduces the hip, knee, and ankle adaptability to perturbation, thus making it a great tool to train body balance.

The contribution of counter-rotation movements during fall recovery: A validation study

Three mechanisms of maintaining standing stability include M₁ - moving the COP within the base of support, M₂ - segment counter-rotation, and M₃ - applying an external force. To date, the contributions of these mechanisms have not been quantified for the response to an external postural disturbance. The purpose of this study was to evaluate the construct validity of measures that quantify the M₂ contribution to anteroposterior fall recovery. We evaluated the whole-body rotation contribution, as well as a measure specific to arm motion (M_ARMS). With segment counter-rotation as the main focus of this study, we examined standing feet-in-place responses to treadmill-induced falls. The treatment validity of our measures was assessed by comparing unconstrained responses to those with constrained arm motion. The convergent validity of our measures was assessed by correlating peak shoulder flexion and extension velocities with counter-rotation contributions. Eleven unimpaired participants responded to anteroposterior belt accelerations from a treadmill, and the M₂ and M_ARMS contributions were quantified from three-dimensional segment motion. The treatment validity of these measures was partially supported. Constraining the arms reduced M₂ for anterior, but not posterior falls. Conversely, M_ARMS was reduced for posterior, but not anterior falls. Convergent validity was supported for M_ARMS (r = 0.64-0.78), but not M₂ (r = -0.40 to -0.15). These results support the use of M_ARMS over M₂ when interested in the role of arm motion. Given that arm constraints did not change the contribution of M_ARMS during a forward fall, unimpaired participants may not necessarily rely on arm motion as part of their recovery strategy in this context.

Coordination of Upper and Lower Body during Balance Recovery following a Support Translation

Strategies for recovery of posture were studied after lateral mechanical perturbations. 11 participants standing in tandem stance were unexpectedly submitted to lateral support translations with the eyes open or closed at two translation amplitudes. The trajectories of the center of mass of the upper and lower body and muscle activities allowed identification of three strategies, involving either the ankle or the hip only, or both. Hip use increased with vision and with amplitude of perturbation. Short-to-medium latency electromyographic activities were observed in leg and trunk muscles, and long-latency responses in the back leg muscles. Vision increased the activity of both leg and trunk muscles but did not influence the onset of the muscular responses. These data suggest a hierarchy in the selection of these different strategies: the hip is mobilized when the perturbation is more destabilizing but this strategy has a cost and needs specific sensory information supplied by vision.

Postural and trunk responses to unexpected perturbations depend on the velocity and direction of platform motion

This study compares postural and trunk responses to translating platform perturbations of varied velocities and directions. A group of 18 young and physically active subjects were exposed to a set of postural perturbations at varied velocities (5, 10, 15, and 20 cm/s) and directions of platform movement (forward, backward, left-lateral, and right-lateral). The center of pressure (CoP) displacement measurement, in addition to the trunk motion (representing the center of mass (CoM) displacement), were both monitored. Results identified that the CoP displacement increased from slow to faster velocities of platform motion more widely in both anterior and posterior directions (50.4 % and 48.4 %) as compared to the CoM displacement (17.8 % and 14.9 %). However a greater increase in the peak CoM velocity (70.3 % and 69.6 %) and the peak CoM acceleration (60.5 % and 53.1 %) was observed. The values in the anterior and posterior direction only differed significantly at the highest velocity of platform motion (i.e. 20 cm/s). A similar tendency was observed in the medio-lateral direction, but there were no significant differences in any parameter in the left-lateral and right-lateral direction. The velocity of the platform motion highly correlated with peak velocity (r=0.92-0.97, P<0.01) and moderately with amplitude of trunk displacement (r=0.56-0.63, P<0.05). These findings indicate that the velocity of perturbation alters peak CoM velocity rather than the magnitude of CoM displacement. The effect of the direction of perturbations on the trunk response emerges only at a high velocity of platform motion, such that the peak CoM velocity and peak CoM acceleration are significantly greater in anterior than posterior direction.

Comparison between investigations of induced stepping postural responses and voluntary steps to better detect community-dwelling elderly fallers

In this paper, we review a physiological task that is predominant in preventing humans from falling, but that simultaneously also challenges balance: taking a step. In particular, two variants of this task are presented and compared: the voluntary step versus a step induced by an external and unpredictable perturbation. We show that, while these contribute different information, it is interesting to compare these. Indeed, they both are relevant in a global balance assessment and should be included within this, at the same level as tests usually dispensed in the clinical environment such as posturography. We choose to focus on the community-dwelling elderly population, to discuss means of early detection of risk of falls, in order to prescribe an appropriate prevention. An overview of posture-movement coordination and balance recovery strategies is also provided. Finally, a working hypothesis is suggested on how "compensatory protective" steps are controlled and how their evaluation could bring additional information to the global balance assessment of risk of fall.

The influence of age on the thresholds of compensatory stepping and dynamic stability maintenance

The purpose of this study was to investigate the effects of age on compensatory-stepping thresholds and dynamic stability maintenance in response to postural disturbances. It was hypothesized that, with older age, anterior but not posterior stepping thresholds would be reduced. Thirteen young adults (31.1 ± 0.8 years), 11 middle-aged adults (57.6 ± 2.5 years), and 11 older adults (73.8 ± 5.3 years) participated in this study. Surface translations were delivered as subjects stood on a microprocessor-controlled treadmill. Subjects were instructed to "try not to step". Stepping thresholds were defined as the largest displacement at a given peak treadmill-belt velocity for which a subject could prevent stepping. The margin of stability was calculated to estimate the minimum dynamic stability at the stepping thresholds. Age-related declines in the ability to prevent forward steps were apparent. Anterior stepping thresholds were reduced with age. The minimum margin of stability associated with anterior stepping thresholds was not influenced by age. Therefore, smaller disturbance displacements caused middle-aged and older subjects to become dynamically unstable to the point of stepping. Posterior stepping thresholds were not influenced by age. It is concluded that an age-related decline in anterior, but not posterior, stepping thresholds was due to an impaired ability to maintain dynamic stability after a disturbance.

Age-Related Changes in Postural Responses to Backward Platform Translation

The aim of the study was to investigate age-related changes in postural responses to platform translation with 3 various velocities. We focused on the influence of linear velocity using the smoothed profile of platform acceleration (till 100 cm.s−2). Eleven healthy young (20-31 years) and eleven healthy elderly (65-76 years) subjects were examined. The subjects stood on the force platform with their eyes closed. Each trial (lasting for 8 sec) with different velocity (10, 15, 20 cm.s−1) of 20 cm backward platform translation was repeated 4 times. We have recorded displacements of the centre of pressure (CoP) and the EMG activity of gastrocnemius muscle (GS) and tibialis anterior muscle (TA). The results showed increased maximal values of CoP responses to the platform translation. There was also observed a scaling delay of CoP responses to platform translation with different velocities in elderly. The EMG activity of GS muscle during backward platform translation was of about similar shape in both groups during the slowest platform velocity, but it increased depending on rising velocity. EMG activity of TA was not related to the platform velocity. Early parts of postural responses showed significant co-activation of TA and GS muscles of elderly. It is likely that elderly increased body stiffening in order to help their further balance control.

A feedback model explains the differential scaling of human postural responses to perturbation acceleration and velocity

Although the neural basis of balance control remains unknown, recent studies suggest that a feedback law on center-of-mass (CoM) kinematics determines the temporal patterning of muscle activity during human postural responses. We hypothesized that the same feedback law would also explain variations in muscle activity to support-surface translation as perturbation characteristics vary. Subject CoM motion was experimentally modulated using 34 different anterior-posterior support-surface translations of varying peak acceleration and velocity but the same total displacement. Electromyographic (EMG) recordings from several muscles of the lower limbs and trunk were compared to predicted EMG patterns from an inverted pendulum model under delayed feedback control. In both recorded and predicted EMG patterns, the initial burst of muscle activity scaled linearly with peak acceleration, whereas the tonic "plateau" region scaled with peak velocity. The relatively invariant duration of the initial burst was modeled by incorporating a transient, time-limited encoding of CoM acceleration inspired by muscle spindle primary afferent dynamic responses. The entire time course of recorded and predicted muscle activity compared favorably across all conditions, suggesting that the initial burst of muscle activity is not generated by feedforward neural mechanisms. Perturbation conditions were presented randomly and subjects maintained relatively constant feedback gains across all conditions. In contrast, an optimal feedback solution based on a trade-off between CoM stabilization and energy expenditure predicted that feedback gains should change with perturbation characteristics. These results suggest that an invariant feedback law was used to generate the entire time course of muscle activity across a variety of postural disturbances.

Musculature and biomechanics of the trunk in the maintenance of upright posture

Surface perturbation has been used for decades to study balance and postural control; however the behavior of the trunk in these postural responses has been largely overlooked. Thirteen healthy males (18-23 yrs) were exposed to horizontal support surface translations delivered randomly in one of eight different horizontal directions in both sitting and standing. A 4-segment model of the trunk was used to estimate the kinematics and kinetics associated with the postural response, while surface EMG was acquired, bilaterally, from seven trunk muscles and one hip muscle. Multi-segmental movement was observed in the trunk in both test postures. Both the biomechanical and neuromuscular aspects of the trunk response were significantly affected by translation direction and test posture, with an interaction effect between these variables. The response in sitting was closely tied to the movement of the support surface, while the response in standing occurred in two phases: the first related to the dynamic response in the lower limbs, and the second tied to the movement of the support surface. As such, the observed postural responses could be largely explained by the biomechanical constraints of the system, such that the neural control of trunk equilibrium is simplified.

Responses to multi-directional surface translations involve redistribution of proximal versus distal strategies to maintain upright posture

Evaluation of postural control in multiple planes is necessary to determine the movement strategies used to respond to unexpected perturbations. The present study quantified net joint torques of the lower limbs and trunk in the sagittal and frontal planes following multi-directional surface translations. Twenty-one healthy subjects stood with feet on separate force plates mounted on a moveable platform, translated unexpectedly in one of 12 directions. Peak net torque magnitudes and latencies following perturbation onset were determined as were the relative contributions of each joint to total torque production. Magnitude of net torque generated by each leg varied by perturbation direction, with the largest individual joint magnitude elicited in directions of limb loading. Relative contributions of individual joint torques to the total response were dependent upon perturbation direction. Results suggest that a redistribution of the relative contributions of hip/trunk versus ankle strategies occurs dependent on perturbation direction, with a significant contribution by the knee joint in response to forward perturbations. Direction-specific redistribution of proximal versus distal strategies appears to depend upon the biomechanical constraints imposed by a given perturbation direction. Thus, it appears that sagittal and frontal plane posture-righting responses may not be uniquely controlled, and may instead be governed similarly, with modulation of relative torque contributions among joints when necessary, given direction-specific anatomical constraints.

A feedback model reproduces muscle activity during human postural responses to support-surface translations

Although feedback models have been used to simulate body motions in human postural control, it is not known whether muscle activation patterns generated by the nervous system during postural responses can also be explained by a feedback control process. We investigated whether a simple feedback law could explain temporal patterns of muscle activation in response to support-surface translations in human subjects. Previously, we used a single-link inverted-pendulum model with a delayed feedback controller to reproduce temporal patterns of muscle activity during postural responses in cats. We scaled this model to human dimensions and determined whether it could reproduce human muscle activity during forward and backward support-surface perturbations. Through optimization, we found three feedback gains (on pendulum acceleration, velocity, and displacement) and a common time delay that allowed the model to best match measured electromyographic (EMG) signals. For each muscle and each subject, the entire time courses of EMG signals during postural responses were well reconstructed in muscles throughout the lower body and resembled the solution derived from an optimal control model. In ankle muscles, >75% of the EMG variability was accounted for by model reconstructions. Surprisingly, >67% of the EMG variability was also accounted for in knee, hip, and pelvis muscles, even though motion at these joints was minimal. Although not explicitly required by our optimization, pendulum kinematics were well matched to subject center-of-mass (CoM) kinematics. Together, these results suggest that a common set of feedback signals related to task-level control of CoM motion is used in the temporal formation of muscle activity during postural control.

Lower extremity kinetics for balance control in children with cerebral palsy

The authors examined and compared the effect of support-surface perturbations of various magnitudes on lower extremity kinetics of 7 children with cerebral palsy (CP) and 8 typically developing (TD) children. Results showed that the highest velocity tolerated without stepping was slower in children with CP than in either age-matched TD or younger TD children. Multimodal torque profiles were more frequent in children with CP than in TD controls. TD groups temporally and spatially organized torque activation, whereas children with CP activated all joints simultaneously and showed altered torque contribution patterns among joints. Those results suggest that impairments in reactive postural control in children with CP result not only from developmental delay but also from pathology. Evidence for pathology included increased numbers of torque bursts required to regain stability and less efficient temporal and spatial organization of torque activation patterns.

Kinematic response characteristics of the CAREN moving platform system for use in posture and balance research

The CAREN system is a new and unique device for use in postural and balance research in clinical settings due to its ability to independently perturb the support surface in each of six degrees of freedom. Users of this system need knowledge of its technical performance which is not available. The aim of this study was to determine the technical performance of the CAREN system by defining its kinematic response characteristics to two commonly used input functions (sine and ramp) for each of its six translational and rotational axes. The translational and rotational displacement, velocity and acceleration limits of the CAREN system suggest that it is a mid-range system with regard to single degree of freedom moving platform devices reported in the literature. The maximum average displacement cross-talk was 1.5% of the viable working range in any specified direction. The maximum average velocity cross-talk was 3.3% of its maximum velocity in any specified direction. The CAREN system was able to respond to ramp input functions within its displacement and velocity limits although, for short duration ramps, there was evidence that target velocity was not reached. It is concluded that the CAREN system is an appropriate device for postural and balance research with some unique features. This specification of its technical performance should help researchers to identify the tasks for which it is most suitable.

Parallels in control of voluntary and perturbation-evoked reach-to-grasp movements: EMG and kinematics

To determine the potential differences in control underlying compensatory and voluntary reach-to-grasp movements the current study compared the kinematic and electromyographic profiles associated with upper limb movement. Postural perturbations were delivered to evoke compensatory reach-to-grasp in ten healthy young adult volunteers while seated on a chair that tilted as an inverted pendulum in the frontal plane. Participants reached to grasp a laterally positioned stable handhold and pulled (or pushed) to return the chair to vertical. The distinguishing characteristic between the two behaviors was the onset latency and speed of movement. Consistent with compensatory balance reactions, the perturbation-evoked reach response was initiated very rapidly (137 vs. 239 ms for voluntary). As well the movement time was shorter, and peak velocity was greater for PERT movements. In spite of the profound differences in timing, the sequence of muscle activity onsets and the order of specific kinematic events were not different between maximum velocity voluntary (VOL) and perturbation-evoked (PERT) reach-to-grasp movements. Peak velocity and grasp aperture occurred prior to hand contact with the target for PERT and VOL movements, and wrist trajectory was influenced by the direction of perturbation relative to the target. To achieve such target specific control for responses initiated within 100 ms of the perturbation, and when characteristics of body movement were unpredictable, the perturbation-evoked movements would need to incorporate sensory cues associated with body movement relative to the target into the earliest aspects of the movement. This suggests reliance on an internal spatial map constructed prior to the onset of perturbation. Parallels in electromyographic and kinematic profiles between compensatory and voluntary reach-to-grasp movements, in spite of temporal differences, lead to the view they are controlled by common neural mechanisms.

A kinematic analysis of the rapid step test in balance-impaired and unimpaired older women

Little is known about the kinematic and kinetic determinants that might explain age and balance-impairment alterations in the results of volitional stepping performance tests. Maximal unipedal stance time (UST) was used to distinguish "balance-impaired" old (BI, UST<10s, N=15, mean age=76 years) from unimpaired old (O, UST>30s, N=12, mean age=71 years) before they and healthy young females (Y, UST>30s, N=13, mean age=23 years) performed the rapid step test (RST). The RST evaluates the time required to take volitional front, side, and back steps of at least 80% maximum step length in response to verbal commands. Kinematic and kinetic data were recorded during the RST. The results indicate that the initiation phase of the step was the major source of age- and balance impairment-related delays. The delays in BI were primarily caused by increased postural adjustments prior to step initiation, as measured by center-of-pressure (COP) path length (p<0.003). The Step landing phase showed similar, but non-significant, temporal trends. Step length and peak center-of-mass (COM) deceleration during the Step-Out landing decreased in O by 18% (p=0.0002) and 24% (p=0.001), respectively, and a further 12% (p=0.04) and 18% (p=0.08) in BI. We conclude that the delay in BI step initiation was due to the increase in their postural adjustments prior to step initiation.

Ankle force and rate of force production increase following high intensity strength training in frail older adults

BACKGROUND AND AIMS

The most common cause of accidental injury and death in people over age 65 results from impacts associated with falling. Balance impaired older adults have poorer balance control than healthy young adults or healthy older adults. Lack of sufficient lower extremity strength and inability to rapidly produce muscle force may contribute to diminished balance control in the elderly. This study evaluated the effect of a 10-week high intensity strength-training program targeting key lower extremity muscles for the purpose of improving postural control in frail older adults.

METHODS

Thirteen experimental and fourteen control subjects, all balance impaired older adults were evaluated in response to unexpected platform perturbations that simulated slips.

RESULTS

Following strength training the experimental group was significantly stronger than the control group. Mean ankle moments improved in the experimental group following strength training during forward sway (Right: p=0.067, Left: p=0.009) and backward sway (Right: p=0.031, Left: p=0.058). For the backward sway condition the ankle rate of torque production increased significantly in the experimental group (Right: p=0.016, Left: p=0.031).

CONCLUSIONS

Enhancement of lower extremity strength contributed to improvements in balance stability demonstrated by greater ankle force production, in response to balance threats.

The influence of natural body sway on neuromuscular responses to an unpredictable surface translation

Previous research has shown that the postural configuration adopted by a subject, such as active leaning, influences the postural response to an unpredictable support surface translation. While those studies have examined large differences in postural conditions, it is of additional interest to examine the effects of naturally occurring changes in standing posture. Thus, it was hypothesized that the normal postural sway observed during quiet standing would affect the responses to an unpredictable support surface translation. Seventeen young adults stood quietly on a moveable platform and were perturbed in either the forward or backward direction when the location of the center of pressure (COP) was either 1.5 standard deviations anterior or posterior to the mean baseline COP signal. Postural responses, in the form of electromyographic (EMG) latencies and amplitudes, were recorded from lower limb and trunk muscles. When the location of the COP at the time of the translation was in the opposite, as compared to the same, direction as the upcoming translation, there was a significantly earlier onset of the antagonists (10-23%, i.e. 15-45 ms) and a greater EMG amplitude (14-39%) in four of the six recorded muscles. Stepping responses were most frequently observed during trials where the position of the COP was opposite to the direction of the translation. The results support the hypothesis that postural responses to unpredictable support surface translations are influenced by the normal movements of postural sway. The results may help to explain the large variability of postural responses found between past studies.

Compensatory stepping in response to waist pulls in balance-impaired and unimpaired women

An effective stepping response is often critical in avoiding a fall. Our objective was to study the effects of age and balance impairment on anterior and posterior compensatory stepping strategies in response to waist pull perturbations of 1-5% body weight (BW). Based on maximal unipedal stance time (UST), we tested 15 balance-impaired old (BI, UST < 10s, mean age = 76 years), 12 healthy old (O, UST>30s, mean age = 71 years), and 13 healthy young women (Y, UST >30s, mean age=23 years). Randomized anterior and posterior pulls of 1-5% body weight (BW) were applied to the waist while kinematic and kinetic recovery responses were recorded. Results show that O and BI required 0.5 more steps than Y to recover balance for posterior pulls of 4-5% BW (P < 0.01). For anterior pulls of 4-5% BW, only BI had a greater probability of step initiation (P<or=0.05 or <0.02) and mean number of steps required to recover balance (P < 0.03). The Y used 93% greater torso extension and 24% greater torso flexion in responding to anterior and posterior pulls, respectively. In the posterior, but not anterior direction, O employed smaller (P < 0.007), but more laterally-directed (P < 0.03) steps than the Y. The BI were less able to attenuate their momentum during the step landing in both directions. We conclude that the additional steps required by the BI in both directions occurred because their initial step failed to properly arrest their momentum. Controlling torso inclination before step liftoff and linear momentum after step landing are critical components of successful compensatory stepping.

Deceleration affects anticipatory and reactive components of triggered postural responses

Understanding the physiological and psychological factors that contribute to healthy and pathological balance control in man has been made difficult by the confounding effects of the perturbations used to test balance reactions. The present study examined how postural responses were influenced by the acceleration-deceleration interval of an unexpected horizontal translation. Twelve adult males maintained balance during unexpected forward and backward surface translations with two different acceleration-deceleration intervals and presentation orders (serial or random). "SHORT" perturbations consisted of an initial acceleration (peak acceleration 1.3 m s(-2); duration 300 ms) followed 100 ms later by a deceleration. "LONG" perturbations had the same acceleration as SHORT perturbations, followed by a 2-s interval of constant velocity before deceleration. Surface and intra-muscular electromyography (EMG) from the leg, trunk, and shoulder muscles were recorded along with motion and force plate data. LONG perturbations induced larger trunk displacements compared to SHORT perturbations when presented randomly and larger EMG responses in proximal and distal muscles during later (500-800 ms) response intervals. During SHORT perturbations, activity in some antagonist muscles was found to be associated with deceleration and not the initial acceleration of the support surface. When predictable, SHORT perturbations facilitated the use of anticipatory mechanisms to attenuate early (100-400 ms) EMG response amplitudes, ankle torque change and trunk displacement. In contrast, LONG perturbations, without an early deceleration effect, did not facilitate anticipatory changes when presented in a predictable order. Therefore, perturbations with a short acceleration-deceleration interval can influence triggered postural responses through reactive effects and, when predictable with repeated exposure, through anticipatory mechanisms.

An Emerging Postural Response: Is Control of the Hip Possible in the Newly Walking Child?

Previous researchers have proposed that because of their slow muscle-response latencies, 1- to 2-year-old children are unable to control hip-dominant postural responses when responding to balance threats (G. McCollum & T. Leen, 1989). To test that proposition, the authors exposed 41 children to backward support-surface translations and recorded muscle activations and movement kinematics. Children between 10 months and 10 years of age stood on a platform that was unexpectedly moved. Passive hip-dominant responses were observed among the least experienced walkers. In contrast, older children produced an active response, signified by higher levels of abdominal and quadriceps muscle activity and accompanied by larger hip flexor torques. Greater success in withstanding large magnitudes of perturbations were associated with actively generated hip responses.

Reactive balance adjustments to unexpected perturbations during human walking

The purpose of this investigation was to determine the effect of unexpected forward perturbations (FP) during gait on lower extremity joint mechanics and muscle Electromyographic (EMG) patterns in healthy adults. The muscles surrounding the hip were found to be most important in maintaining control of the trunk and preventing collapse in response to the FP. Distinct lower extremity joint moment and power patterns were observed in response to the FP but an overall positive moment of support (M(s)) was maintained. Therefore, reactive balance control was a synchronized effort of the lower extremity joints to prevent collapse during the FP.

Age-related differences in lower extremity power after support surface perturbations

OBJECTIVES

The purpose of this study was to examine a comprehensive measure relating to the ability to generate, absorb, and transfer mechanical energy introduced by a perturbation. It was hypothesized that this measure would reveal age-related differences leading to different balance recovery responses (i.e., feet-in-place and compensatory step).

DESIGN

Cross-sectional, descriptive study.

SETTING

The Motor Control Laboratory at the University of Oregon.

PARTICIPANTS

Eighteen younger (aged 18-35) and 21 older (aged 65-85) women received forward and backward support surface translations of varying amplitudes and velocities.

MEASUREMENTS

Lower extremity peak positive joint power and peak negative muscle power were examined at the largest perturbation in which a feet-in-place response was used and the subsequent perturbation where a step occurred. Peak positive joint power depicts the maximum rate of passive transfer of energy into a segment and is indicative of the maximum destabilization of that segment. Peak negative muscle power is the maximum rate of energy absorption by muscle, which reduces the effect of the perturbation on the segment.

RESULTS

After a backward perturbation, there was a significant main effect of age for muscle power and a significant main effect of response type for joint power. For the forward condition, there was a significant main effect of response type for muscle power only.

CONCLUSIONS

The mechanical reason for the emergence of the compensatory step after support-surface perturbations is clear; the feet-in-place response is energetically more demanding than the compensatory step. The energy analysis suggests a mechanical basis as one reason for older adults' reliance on the compensatory step: older adult muscles absorb less energy. The results of this study also highlight the importance of the proximal knee and hip musculature versus the distal ankle musculature.

The translating platform paradigm: perturbation displacement waveform alters the postural response

The translating platform paradigm is widely used to investigate the regulation of upright standing and locomotion. This study investigated how the displacement waveform characteristics underlying the translating platform perturbation are revealed in the resulting postural response. Eight participants experienced a series of backward-directed perturbations using a hydraulically driven forceplate. Two ranges of platform displacement (5 and 15 cm) in combination with two peak velocities (40 and 60 cm/s) were achieved using three distinct waveforms for platform displacement: (a) RAMP: ramp onset and ramp offset, (b) Ramp-to-Parabola (R-P): ramp onset with parabolic offset and (c) SINE: sine-wave onset with sine wave offset. Our findings indicated that the unique and distinctive acceleration and deceleration characteristics that result from the three different platform displacement waveforms significantly altered the postural response to the perturbation.