Biomedical subjects

Yi-Chung Pai

Publications and source records attributed to Yi-Chung Pai.

11 recordsLinked to original sources

PubMed2006

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.

Accidental Falls↗

PubMed2006

Predicted threshold against backward balance loss in gait.

The purpose of this study was to determine the minimum forward center of mass (COM) velocity required to prevent backward loss of balance in gait as function of the initial COM position. We hypothesized that these threshold values would be different from those previously published for standing because of the postural differences between gait and standing. To investigate this issue, we constructed a seven-link, nine-degree-of-freedom biomechanical model and employed dynamic optimization to estimate these threshold values under two initial postural conditions: (1) the posture at the beginning of swing phase (i.e., at toe-off), and (2) symmetric bipedal standing. In particular, for a range of possible COM positions posterior to the base of support (BOS), simulated annealing was used to search for the minimum velocity that could carry the COM into the BOS and avoid backward loss of balance. We found that the stability boundary against backward balance loss in walking had a similar overall trend as that previously published for standing. In general, the minimal COM velocity necessary to prevent a backward loss of balance in walking was greater than that in symmetric bipedal standing, and the difference could approach 30% or more when the COM started 0.5 and 1.0 foot-lengths behind the BOS. These discrepancies suggest that simpler biomechanical models, while being more efficient and easier to employ, may not always be adequate for exploring stability limits of humans.

Accidental Falls↗

PubMed2006

Quantitative analysis of the ankle strategy under translational platform disturbance.

The ankle strategy is one of the postural adjustment maneuvers humans utilize when the support platform is disturbed. This paper presents a quantitative analysis of the ankle strategy. A three-link sagittal biped model is considered. The first link represents the two legs locked together. The second link represents the two thighs locked together. The third link represents the hip, the torso, the upper limbs, the neck, and the head. The dynamics, control, and stability of the three-link biped, under platform translation, are considered. The disturbance of the platform is represented as an input and the effect of the muscular system is reduced to a set of torques applied to the joints and across the joints. Two digital computer simulations are presented to demonstrate the behavior of the biped under backward or forward platform disturbance. The simulations are compared with experimental measurements of humans subjected to postural disturbances. It is shown that the effect of a horizontal disturbance at the ankle appears to be about 40 times that of the effect of the disturbance at the knees and at least a few hundred times larger than the effect of a disturbance at the hip. This means that, under translational platform disturbance, the ankle angle is subjected to the largest excursion. The knee and the hip angle excursions are relatively minor. Consequently, the biped, as a whole, appears to move as a single inverted pendulum. Major postural corrections are initiated by the ankle excursion. Further, when the available ankle torque is limited or nonexistent, the stability requires resorting to the knee or hip strategies.

Ankle Joint↗

PubMed2005

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.

Accidental Falls↗

PubMed2005

Does knee osteoarthritis alter the neuromuscular responses to a perturbation during single lower limb stance?

PURPOSE: Evidence substantiating an association between knee osteoarthritis (OA) and altered joint protection responses is relevant to the management of knee OA. The purpose of this study was to detect neuromuscular response times of the vastus lateralis and biceps femoris muscles following a perturbation during single lower limb stance. We hypothesized that muscle response times are: (1) delayed in older adults with knee OA when compared to young and older adults (without diagnosed knee OA) and (2) dependent on the magnitude of load released. SUBJECTS: Ten young adults, 10 older adults, and 7 older adults with symptomatic knee OA participated. METHODS: While in single lower limb stance, the subjects flexed the knee into a range of 33 to 37 degrees while a posterior load of either 6 or 9 kg was applied at the proximal tibia. The load was released after the subjects held the required position for 1 full second. Muscle response times were measured by electromyography. Separate 3 (group) by 2 (load) mixed factorial analysis of variance procedures were performed for electromyography data from the vastus lateralis and biceps femoris. RESULTS: There was no difference in vastus lateralis response times between older adults with knee OA and older adults or between older adults with knee OA and young adults. Older adults did have longer vastus lateralis response times than young adults. There was no difference in biceps femoris response times between older adults with knee OA and older adults; however, both groups had longer biceps femoris response times than young adults. Furthermore, there were no differences in either vastus lateralis or biceps femoris response times between 6 kg and 9 kg loads. CONCLUSION: Although knee OA did not alter muscle responses in our study, the type of functional weightbearing perturbation described could be safely used in the physical therapy clinic to help improve balance and stability while decreasing discomfort in older adults with symptomatic knee OA.

Adult↗

PubMed2004

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.

Accidental Falls↗

PubMed2004

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.

Accidental Falls↗

PubMed2003

Physical functioning over three years in knee osteoarthritis: role of psychosocial, local mechanical, and neuromuscular factors.

OBJECTIVE: To identify factors that predict a poor physical function outcome over 3 years in individuals with knee osteoarthritis (OA), in an effort to aid in the development of strategies to prevent such functional limitations and consequential disability. METHODS: Community-recruited individuals with knee OA underwent baseline, 18-month, and 3-year assessments of candidate risk factors and physical function. Risk factors were age, body mass index (BMI), knee pain intensity (on a visual analog scale [VAS]), local mechanical and neuromuscular factors (varus-valgus laxity, malalignment, proprioceptive inaccuracy, quadriceps strength, hamstring strength), activity level (Physical Activity Scale for the Elderly, amount of aerobic exercise), and psychosocial factors (Short-Form 36 [SF-36] mental health and role-functioning emotional subscales, self-efficacy using the Arthritis Self-Efficacy Scale physical function subscale, and social support using the Medical Outcomes Study Social Support Survey). Outcome was assessed using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) physical function scale and rate of chair-stand performance. Participants were grouped by quintile of baseline WOMAC score. The baseline to 3-year outcome was considered "good" when function improved by 1 or more quintiles or remained within the 2 highest function groups, and was considered "poor" when function declined by 1 or more quintiles or remained within the 3 lowest function groups. The same approach was taken for chair-stand outcome. Logistic regression was used to evaluate both the baseline level and the baseline to 18-month change in each factor as a predictor of physical function outcome over 3 years, adjusting for age, BMI, knee pain intensity, disease severity, and additional potential confounders. RESULTS: Factors that significantly increased the likelihood of a poor WOMAC outcome were baseline laxity (crude odds ratio [OR] 1.48/3 degrees, 95% confidence interval [95% CI] 1.02-2.14), BMI (OR 1.26/5 units, 95% CI 1.01-1.57), knee pain intensity (OR 1.21/20 mm on VAS, 95% CI 1.00-1.47), and baseline to 18-month increase in knee pain (OR 1.32/20 mm on VAS, 95% CI 1.06-1.65). Factors that significantly protected against a poor WOMAC outcome were better baseline mental health (OR 0.62/5 points, 95% CI 0.44-0.87), self-efficacy (OR 0.79/5 points, 95% CI 0.67-0.93), and social support (OR 0.86/10 points, 95% CI 0.75-0.98), and greater amount of aerobic exercise (OR 0.75/60 minutes each week, 95% CI 0.63-0.89). Factors that increased the likelihood for a poor function outcome by the chair-stand performance rate were age and proprioceptive inaccuracy, and factors that reduced the likelihood for poor chair-stand outcome were strength (attenuated after adjusting for pain intensity or self-efficacy), self-efficacy, and aerobic exercise. Individuals who sustained high function and those who sustained low function over the 3 years were described. CONCLUSION: Factors placing individuals with knee OA at greater risk of a poor function outcome by at least 1 of the 2 function measures included the local factors laxity and proprioceptive inaccuracy, as well as age, BMI, and knee pain intensity. Factors protecting against a poor function outcome included strength, the psychosocial factors mental health, self-efficacy, and social support, and the activity level measured by the amount of aerobic exercise per week. The identification of these factors provides possible targets for rehabilitative and self-management strategies to prevent disability.

Aged↗

PubMed2003

Movement termination and stability in standing.

This brief review develops a new conceptual framework for describing the influence of center-of-mass position and velocity on ability to terminate movement. Limitations of this dynamic conceptual framework and its applications in physical rehabilitation are explored.

Accidental Falls↗

PubMed2002

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.

Adaptation, Physiological↗

PubMed2002

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.

Accidental Falls↗