Gait characteristics of adults with Down syndrome explain their greater metabolic rate during walking.

The altered gait patterns of adults with Down syndrome (DS) may contribute to their higher net metabolic rate (net-MR) during walking than adults without DS, leading to mobility limitations. This study examined the extent to which gait characteristics explain differences in net-MR during walking between adults with and without DS. Fifteen adults with DS (27 ± 8 years) and 15 adults without DS (28 ± 6 years) completed two testing sessions in which expiratory gases and kinematic data were collected, respectively, during treadmill walking. Participants walked at six, randomly-presented dimensionless speeds, ranging from slow to fast. Hierarchical and stepwise regressions were used to determine the proportion of the variance in net-MR explained by gait variables that differed between groups, after controlling for variance due to walking speed. Positive work rate, the range of the body center of mass (COM) mediolateral position and its square, variability in the time-course of COM anteroposterior velocity, and the variability of step length, step width, and step time significantly predicted net-MR (p < .05). These variables collectively explained 73.9% of the variance in net-MR that was explained by DS but not by walking speed. After accounting for shared variance among predictors, step length variability made the greatest unique contribution (10.6%) to the higher net-MR in adults with DS, followed by the range of COM mediolateral motion (6.3%), step width variability (2.8%), and variability in COM anteroposterior velocity (0.7%). Therefore, the gait characteristics of adults with DS appear to largely explain their higher net-MR during walking.

Meeting physical activity guidelines through community-based group exercise: "better bones and balance".

OBJECTIVE: Community-based exercise programs are popular for achieving physical activity among older adults, but the amount of physical activity obtained through such programs is unknown. This study quantified the bone-loading forces and levels of cardiovascular activity associated with participation in "Better Bones and Balance" (BBB), a community-based fall- and fracture-prevention program for older adults. METHODS: Thirty-six postmenopausal women age 73.2 ± 7.6 yr engages in BBB participated in this study. Ground-reaction forces (GRFs) associated with BBB exercises were evaluated using a force platform. Session and weekly totals of minutes of moderate to vigorous physical activity (MVPA) and total time spent above 55% maximum heart rate (HR) were measured using accelerometers and HR monitors, respectively. RESULTS: BBB exercises produced mean 1-leg GRFs of 1.4-2.2 units body weight. Weekly BBB participation was associated with 126 ± 31 min of MVPA. CONCLUSION: Activity obtained by BBB participation meets recommended guidelines for skeletal and cardiovascular health.

Effects of decision making on landing mechanics as a function of task and sex.

BACKGROUND: Factors that contribute to sex-differences in the incidence of anterior cruciate ligament injuries among athletes are not well understood. Of interest is whether decision making during landing influences biomechanical factors associated with anterior cruciate ligament injury. This study examined the effects of decision making on the mechanics of two-footed landing tasks in women and men. METHODS: Twenty-nine healthy young adults (13 women, 16 men) completed drop landings and drop-jumps under preplanned and decision-making conditions. Biomechanical data were collected and effects of decision making on lower extremity kinematics and kinetics were examined as a function of task and sex. FINDINGS: Landing mechanics were influenced by decision-making condition, task, and sex. During drop-jumps, participants exhibited lesser hip flexion (-3.3°), lesser knee flexion (-5.1°), and greater knee abduction (+1.0°) at initial contact under decision-making conditions. Under decision-making conditions, no differences were observed in these variables between tasks or with respect to preplanned drop landings. Across tasks and sexes, participants exhibited greater ankle plantarflexion at initial contact (+1.6°), greater peak knee external rotation (+1.5°), lesser peak knee internal rotation (-1.0°), and smaller hip adduction moments (-0.2% body weight×height) under decision-making conditions. Women but not men exhibited smaller ankle inversion moments (-0.1% body weight×height) under decision-making conditions. INTERPRETATION: Modifications in landing mechanics suggest a default towards the preplanned drop landing strategy under decision-making conditions. Across sexes, drop landings and drop-jumps may be no more dangerous under decision-making conditions, with respect to anterior cruciate ligament loading, than preplanned drop landings.

Effects of aging-related losses in strength on the ability to recover from a backward balance loss.

Although muscle weakness is a risk factor for falls, its direct influence on the ability to prevent a fall is largely unknown. This study therefore investigated the effect of aging-related losses in strength on the ability to restore static balance following a recovery step from a backward balance loss. A six-link, sagittal-plane musculoskeletal model with 10 Hill-type musculotendon actuators was developed to simulate the strength characteristics and balance recovery motions of young and older adults. Using this model, feasible regions for balance recovery were mapped for each age group for "slow" and "fast" initial conditions of backward and downward velocity. For both conditions, there was considerable overlap between the feasible regions of young and older adults, with both age groups able to restore static balance from similar initial hip heights. However, the ranges of initial center of mass positions, relative to the rear heel, for which balance could be restored did not extend as far anteriorly or posteriorly for older adults. The feasible region did not extend as far upward, downward, or posteriorly for the "fast" condition, with these differences between conditions being similar in each age group. The results suggest that aging-related losses in strength impair the ability to recover from a backward balance loss only if older adults take a very short or very long recovery step behind the center of mass, even for high velocities at step touchdown. Training of the stepping response might therefore be more effective than strength training in preventing backward falls.

The acute effects of a warm-up including static or dynamic stretching on countermovement jump height, reaction time, and flexibility.

The purpose of this research was to compare the effects of a warm-up with static vs. dynamic stretching on countermovement jump (CMJ) height, reaction time, and low-back and hamstring flexibility and to determine whether any observed performance deficits would persist throughout a series of CMJs. Twenty-one recreationally active men (24.4 ± 4.5 years) completed 3 data collection sessions. Each session included a 5-minute treadmill jog followed by 1 of the stretch treatments: no stretching (NS), static stretching (SS), or dynamic stretching (DS). After the jog and stretch treatment, the participant performed a sit-and-reach test. Next, the participant completed a series of 10 maximal-effort CMJs, during which he was asked to jump as quickly as possible after seeing a visual stimulus (light). The CMJ height and reaction time were determined from measured ground reaction forces. A treatment × jump repeated-measures analysis of variance for CMJ height revealed a significant main effect of treatment (p = 0.004). The CMJ height was greater for DS (43.0 cm) than for NS (41.4 cm) and SS (41.9 cm) and was not less for SS than for NS. Analysis also revealed a significant main effect of jump (p = 0.005) on CMJ height: Jump height decreased from the early to the late jumps. The analysis of reaction time showed no significant effect of treatment. Treatment had a main effect (p < 0.001) on flexibility, however. Flexibility was greater after both SS and DS compared to after NS, with no difference in flexibility between SS and DS. Athletes in sports requiring lower-extremity power should use DS techniques in warm-up to enhance flexibility while improving performance.

Inoculation against falls: rapid adaptation by young and older adults to slips during daily activities.

OBJECTIVE: To determine whether aging diminishes one's ability to rapidly learn to resist falls on repeated-slip exposure across different activities of daily living. DESIGN: Quasi-experimental controlled trial. SETTING: Two university-based research laboratories. PARTICIPANTS: Young (n=35) and older (n=38) adults underwent slips during walking. Young (n=60) and older (n=41) adults underwent slips during a sit-to-stand task. All (N=174) were healthy and community dwelling. INTERVENTION: Low-friction platforms induced unannounced blocks of 2 to 8 repeated slips interspersed with blocks of 3 to 5 nonslip trials during the designated task. MAIN OUTCOME MEASURES: The incidence of falls and balance loss. Dynamic stability (based on center of mass position and velocity) and limb support (based on hip height) 300 ms after slip onset. RESULTS: Under strictly controlled, identical low-friction conditions, all participants experienced balance loss, but older adults were over twice as likely as young to fall on the first, unannounced, novel slip in both tasks. Independent of age or task, participants adapted to avoid falls and balance loss, with most adaptation occurring in early trials. By the fifth slip, the incidence of falls and balance loss was less than 5% and 15%, respectively, regardless of age or task. Reductions in falls and balance loss for each task were accomplished through improved control of stability and limb support in both age groups. A rapidly reversible age- and task-dependent waning of motor learning occurred after a block of nonslip trials. Adaptation to walk slips reached a steady state in the second slip block regardless of age. CONCLUSIONS: The ability to rapidly acquire fall-resisting skills on repeated-slip exposure remains largely intact at older ages and across functional activities. Thus, repeated-slip exposure might be broadly effective in inoculating older adults against falls.

Knee joint kinematics and kinetics during a lateral false-step maneuver.

CONTEXT: Cutting maneuvers have been implicated as a mechanism of noncontact anterior cruciate ligament (ACL) injuries in collegiate female basketball players. OBJECTIVE: To investigate knee kinematics and kinetics during running when the width of a single step, relative to the path of travel, was manipulated, a lateral false-step maneuver. DESIGN: Crossover design. SETTING: University biomechanics laboratory. PATIENTS OR OTHER PARTICIPANTS: Thirteen female collegiate basketball athletes (age = 19.7 +/- 1.1 years, height = 172.3 +/- 8.3 cm, mass = 71.8 +/- 8.7 kg). INTERVENTION(S): Three conditions: normal straight-ahead running, lateral false step of width 20% of body height, and lateral false step of width 35% of body height. MAIN OUTCOME MEASURE(S): Peak angles and internal moments for knee flexion, extension, abduction, adduction, internal rotation, and external rotation. RESULTS: Differences were noted among conditions in peak knee angles (flexion [P < .01], extension [P = .02], abduction [P < .01], and internal rotation [P < .01]) and peak internal knee moments (abduction [P < .01], adduction [P < .01], and internal rotation [P = .03]). The lateral false step of width 35% of body height was associated with larger peak flexion, abduction, and internal rotation angles and larger peak abduction, adduction, and internal rotation moments than normal running. Peak flexion and internal rotation angles were also larger for the lateral false step of width 20% of body height than for normal running, whereas peak extension angle was smaller. Peak internal rotation angle increased progressively with increasing step width. CONCLUSIONS: Performing a lateral false-step maneuver resulted in changes in knee kinematics and kinetics compared with normal running. The differences observed for lateral false steps were consistent with proposed mechanisms of ACL loading, suggesting that lateral false steps represent a hitherto neglected mechanism of noncontact ACL injury.

Effects of Down syndrome on three-dimensional motion during walking at different speeds.

The inherent joint laxity and muscle hypotonia of adults with Down syndrome (DS) may result in reduced gait stability and increased energetic cost. These factors vary as a function of walking speed and may be reflected in gait patterns. The present study therefore examined whether the three-dimensional motion of the body center of mass (COM) and stepping characteristics differ between adults with and without DS as a function of speed. Fifteen adults with DS and 15 adults without DS underwent a series of treadmill walking trials. Walking speeds were determined as Froude numbers, based on leg length. Participants walked at Froude numbers of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and, for adults without DS, 0.7. Whole-body kinematic data were collected for 30-35 steps at each speed. Across speeds, adults with DS showed greater and more variable mediolateral COM motion than adults without DS. COM anteroposterior velocity and vertical motion did not differ in range between groups, but were more variable in adults with DS. Adults with DS also showed smaller-duration steps and varied their step widths and step lengths more than adults without DS. The results suggest a gait pattern with lesser stability and greater energetic cost among adults with DS.

Economy and preferred speed of walking in adults with and without Down syndrome.

This study examined whether the net rate of oxygen uptake (VO2net) and the net oxygen uptake per kilometer (VO2net/km) are affected during walking in adults with Down syndrome (DS) and whether their preferred walking speed (PWS) minimizes the VO2net/km. Respiratory gases were collected as 14 adults with DS and 15 adults without DS completed a series of treadmill walking trials. PWS was measured over 15 meters in a hallway. The VO2net and the VO2net/km were higher in adults with DS than adults without DS. The overground PWS normalized for leg length was the same for both groups and did not appear to minimize the VO2net/km. Thus, adults with DS are less economical during walking than adults without DS. The overground PWS does not minimize the metabolic cost during treadmill walking.

Gait transitions of persons with and without intellectual disability.

This study examined whether the walk-to-run transition speed (W-RTS) and the run-to-walk transition speed (R-WTS) were different or more variable between participants with and without intellectual disability (ID). Nine adults with ID and 10 adults without ID completed in a series of walk-to-run and run-to-walk trials on a treadmill. W-RTS and R-WTS were identified using force-sensitive resistors. When transition speeds were expressed as Froude numbers to account for differences in leg length, W-RTS was slower and intraindividual variability of W-RTS and R-WTS was greater in participants with ID. These findings support the idea that the unique constraints of individuals with ID result in altered attractor dynamics for walking and a weaker coupling between locomotor speed and order parameters governing gait transitions.

Bone contact forces on the distal tibia during the stance phase of running.

Although the tibia is a common site of stress fractures in runners, the loading of the tibia during running is not well understood. An integrated experimental and modeling approach was therefore used to estimate the bone contact forces acting on the distal end of the tibia during the stance phase of running, and the contributions of external and internal sources to these forces. Motion capture and force plate data were recorded for 10 male runners as they ran at 3.5-4 m/s. From these data, the joint reaction force (JRF), muscle forces, and bone contact force on the tibia were computed at the ankle using inverse dynamics and optimization methods. The distal end of the tibia was compressed and sheared posteriorly throughout most of stance, with respective peak forces of 9.00+/-1.13 and 0.57+/-0.18 body weights occurring during mid stance. Internal muscle forces were the primary source of tibial compression, whereas the JRF was the primary source of tibial shear due to the forward inclination of the leg relative to the external ground reaction force. The muscle forces and JRF both acted to compress the tibia, but induced tibial shear forces in opposing directions during stance, magnifying tibial compression and reducing tibial shear. The superposition of the peak compressive and posterior shear forces at mid stance may contribute to stress fractures in the posterior face of the tibia. The implications are that changes in running technique could potentially reduce stress fracture risk.

MRI-derived body segment parameters of children differ from age-based estimates derived using photogrammetry.

Body segment parameters are required when researching joint kinetics using inverse dynamics models. However, the only regression equations for estimating pediatric body segment parameters across a wide age range were developed, using photogrammetry, based on 12 boys and have not been validated to date (Jensen, R.K., 1986. Body segment mass, radius and radius of gyration proportions of children. Journal of Biomechanics 19, 359-368). To assess whether these equations could validly be applied to girls, we asked whether body segment parameters estimated by the equations differ from parameters measured using a validated magnetic resonance imaging (MRI) method. If so, do the differences cause significant differences in joint kinetics during normal gait? Body segment parameters were estimated from axial MRIs of the left thigh and shank of 10 healthy girls (9.6 +/- 0.9 years) and compared to those from Jensen's equations. Kinematics and kinetics were collected for 10 walking trials. Extrema in hip and knee moments and powers were compared between the two sets of body segment parameters. With the exception of the shank mass center and radius of gyration, body segment parameters measured using MRI were significantly different from those estimated using regression equations. These systematic differences in body segment parameters resulted in significant differences in sagittal-plane joint moments and powers during gait. Nevertheless, it is doubtful that even the greatest differences in kinetics are practically meaningful (0.3% BW x HT and 0.7% BW x HT/s for moments and power at the hip, respectively). Therefore, body segment parameters estimated using Jensen's regression equations are a suitable substitute for more detailed anatomical imaging of 8-10-year-old girls when quantifying joint kinetics during gait.

Deficient limb support is a major contributor to age differences in falling.

Older adults are more likely than young to fall upon a loss of balance, yet the factors responsible for this difference are not well understood. This study investigated whether age-related differences in movement stability, limb support, and protective stepping contribute to the greater likelihood of falling among older adults. Sixty young and 41 older, safety-harnessed, healthy adults were exposed to a novel and unexpected forward slip during a sit-to-stand task. More older than young adults fell (76% vs. 30%). Falls in both age groups were related to lesser stability and lower hip height at first step touchdown, with 97.1% of slip outcomes correctly classified based on these variables. Decreases in hip height at touchdown had over 20 times greater effect on the odds of falling than equivalent decreases in stability. Three age differences placed older adults at greater risk of falling: older adults had lower and more slowly rising hips at slip onset, they were less likely to respond to slipping with ample limb support, and they placed their stepping foot less posterior to their center of mass. The first two differences, each associated with deficient limb support, reduced hip ascent and increased hip descent. The third difference resulted in lesser stability at step touchdown. These results suggest that deficient limb support in normal movement patterns and in the reactive response to a perturbation is a major contributor to the high incidence of falls in older adults. Improving proactive and reactive limb support should be a focus of fall prevention efforts.

Mechanisms of limb collapse following a slip among young and older adults.

Recovery from a large perturbation, such as a slip, can be successful when stability of movement can be reestablished with protective stepping. Nevertheless, one dilemma for executing a protective step is that its liftoff can weaken support against limb collapse. This study investigated whether failures in limb support leading to falls after a protective step result from insufficient joint moment generation, and whether such insufficiency is greater among older fallers. A novel, unexpected slip was induced immediately following seat-off during a sit-to-stand. Joint work and mechanical energy were calculated for 43 young (9 falls, 34 recoveries) and 22 older (13 falls, 9 recoveries) adults who responded with a protective step. Comparisons of the work produced at three joints of the bilateral lower limbs revealed that insufficient concentric knee and hip extensor work prior to step liftoff was a primary differentiating factor between falling and recovery, regardless of age. Also, during stepping, fallers regardless of age failed to limit the eccentric knee extensor work at their stance limb sufficiently to retard rapid knee flexion and the consequent potential energy loss. We concluded that young and older fallers had comparable weak limb support. The greater fall incidence among the older adults likely resulted from a greater proportion of subjects who responded to the slip with insufficient knee extensor support, possibly attributable to age-differences in chair-rising. One strategy to address this dilemma may rely on task-specific training to enhance feedforward control that improves movement stability, and thus lessens the reliance on protective stepping.

Lower extremity strength plays only a small role in determining the maximum recoverable lean angle in older adults.

BACKGROUND: The purpose of this study was to determine the extent to which measures of lower extremity strength and power contribute to the ability of older men and women to restore postural equilibrium using a single-step recovery following a large postural disturbance. METHODS: The postural disturbance, which has been used as a surrogate for forward-directed falls, involved a sudden release from a forward-leaning angle. The ability to recover using a single step was evaluated as the maximum recoverable lean angle for 56 healthy older women and men. Maximum voluntary isometric and isokinetic strength was measured for ankle plantarflexion and dorsiflexion, knee flexion and extension, and hip flexion and extension. Discriminant analysis was used to determine the strength measures that best classified participants as members of the highest (n = 14) or lowest (n = 14) quartiles of maximum recoverable lean angle. Those variables were subsequently entered into a regression analysis to characterize the relationship between strength and maximum recoverable lean angle for the entire participant sample. RESULTS: Maximum isokinetic dorsiflexion strength at 90 degrees /s satisfied the criteria of the stepwise discriminant analysis, and correctly classified 82.1% of the participants in the highest or lowest quartiles of maximum recoverable lean angle. The multiple regression procedure, performed on all participants (n = 56) revealed a significant quadratic relationship between maximum isokinetic dorsiflexion strength at 90 degrees /s and maximum recoverable lean angle (R2 = 0.295; p <.001). CONCLUSIONS: Lower extremity strength makes a small, but significant contribution to maximum recoverable lean angle. However, because 70% of the shared variability remained unaccounted for, it is suggested that other performance factors, such as coordination, may be of greater importance to performance of this time-critical motor task.

Diminished stepping responses lead to a fall following a novel slip induced during a sit-to-stand.

While neuromotor responses associated with successfully restoring balance have been widely characterized, little is known of how responses employed to regain balance fail. We identified biomechanical factors that characterized the unsuccessful recovery responses to a novel perturbation. An unexpected forward slip was induced, using a low-friction platform, just after seat-off of a sit-to-stand in 41 young subjects. Kinematic variables describing the recovery responses were compared between fallers and those who recovered; "falls" into the fall-arrest system were identified based on center of mass (COM) descent. Twelve "single-step" falls, four "multi-step" falls, and 11 recoveries resulted. Single-step fallers exhibited a more posterior COM at slip onset, support (nonstepping) limb collapse resulting in hip vertical descent, and a shortened protective step backward. A factor analysis indicated that six common factors explained 88% of the variance in 22 kinematic variables describing the slip and recovery response. Single-step falls were associated with two factors, one related to initial or imposed instability and one to the reactive response. The former factor indicated that step length and COM position at step touchdown were related to COM position at slip onset. Support limb collapse comprised the latter factor. Multi-step fallers, by appearance, exhibited a shortened initial backward step that was ineffective at enabling balance recovery. As a whole, the results indicated that unsuccessful recovery was associated with a diminished stepping response. Fallers may have inadequately scaled an appropriate reflexive stepping response or might, inappropriately, have reflexively attempted to recover without stepping.

Young and older adults exhibit proactive and reactive adaptations to repeated slip exposure.

BACKGROUND: A previous study found that, with repeated exposure to slipping during a sit-to-stand task, fall incidence decreased at a similar exponential rate in young and older adults. This study investigated the adaptations responsible for this decrease. METHODS: Slips were induced, using bilateral low-friction platforms, during a sit-to-stand in 60 young and 41 older healthy safety-harnessed adults. Participants underwent 5 slips, then a 6th slip (reslip) after 3-4 nonslipping trials. Between-trial adjustments in body center of mass state at seat-off were examined and correlated to the likelihoods of falling and stepping. Changes in reactive response between the first slip and reslip were investigated. RESULTS: With repeated slipping, both young and older adults adjusted to increase their center of mass anterior position and forward velocity at seat-off (p <.001), contributing to decreased fall incidence and changes in step incidence and direction (p <.001). These proactive adjustments predicted fall incidence well in later trials, but underpredicted fall incidence upon the first slip by 9%-21%, suggesting that reactive response deficiencies also initially contributed to falls by both age groups. Ten participants who initially fell without stepping adapted by stepping to recover upon the reslip. Thirty-six participants who stepped backward initially and upon the reslip altered their nonstepping limb reactive response to reduce hip vertical descent during the step (p <.001). CONCLUSIONS: Young and older adults rapidly learned to avoid falling through similar proactive and reactive adaptations that persisted in the short term. Both proactive and reactive adaptations should be targeted in interventions to reduce older adult fall incidence.

Feedforward adaptations are used to compensate for a potential loss of balance.

The central nervous system (CNS) must routinely compensate for unpredictable perturbations that occur during postural tasks. Such compensations could take the form of feedforward or feedback control. This study investigated whether the CNS, when faced with a potential postural perturbation, employs feedforward adjustments to reduce the near-term and overall likelihood of balance loss. Slips were induced, using bilateral low-friction platforms, during a sit-to-stand task in 60 safety-harnessed young adults. Subjects underwent a block of slipping trials, a block of nonslipping trials, then a mixed block of trials. After the first novel and unexpected slip, subjects were aware that a slip "may or may not occur." The state (horizontal position and velocity) of the body center of mass (COM) at seat-off and the direction of balance loss (forward, no loss, backward) were determined for each trial. Feedforward adjustments were identified as between-trial changes in COM state at seat-off. Effects of these adjustments on the likelihood of balance loss were quantified using logistic regression. Results indicated that the likelihood of balance loss in each direction (forward, backward) under each condition (slipping, nonslipping) was significantly related to the COM state at seat-off. When faced with the potential perturbation, the CNS made near-term feedforward adjustments to reduce the likelihood of balance loss under the conditions last experienced; exposure to slipping and nonslipping conditions resulted in adjustments that reduced the likelihood of backward and forward balance losses, respectively. Subjects adapted their performance over the longer-term in a manner that significantly decreased their overall likelihood of balance loss in either direction under either condition. The CNS thus adapted to acquire an "optimal" movement strategy that reduced the reliance on reactive responses to maintain balance in an uncertain environment.

Age influences the outcome of a slipping perturbation during initial but not repeated exposures.

BACKGROUND: Fall incidence in older adults might be reduced through learning to better recover from or adjust to perturbations. Extents of age-related declines and limitations in the ability to recover are not well established, however. METHODS: Slips were induced, using bilateral low-friction platforms, during a sit-to-stand task in 60 young and 41 older, healthy, safety-harnessed adults. Subjects underwent five slips, a block of nonslipping trials, then two reexposures to the slip. The first slip was novel and unexpected. Age-group and trial effects on fall incidence (evidenced by excessive hip descent) and on the direction of the initial protective step were examined. RESULTS: More older than young adults fell upon the first slip (73% vs 28%; p <.001). With repeated exposure, fall incidence decreased at similar exponential rates in both age groups. All but one subject eventually learned to slip without falling, and two-thirds of fallers fell only once. Repeat fallers fell without stepping in 63% of falls. Upon later slip reexposure, more older than young adults fell (20% vs 2%), but fewer falls occurred than did originally (p =.001). Likelihoods of forward and backward stepping during successful recovery changed with repeated slip exposure and upon reexposure, but did not differ between age groups. CONCLUSIONS: Older adults are more likely to fall upon initial, unexpected perturbation exposure, but, upon repeated exposure, healthy young and older adults rapidly learn to avoid falling at a similar rate. Healthy older adults appear fully capable of learning to better recover from or adjust to a perturbation through repeated exposure.