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Biomedical subjects

Michael J Pavol

Publications and source records attributed to Michael J Pavol.

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↗
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

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.

Accidental Falls↗
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↗
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

Body segment inertial parameter estimation for the general population of older adults.

The practical determination of accurate body segment inertial parameters for the general older adult population remains a problem, especially in estimating these parameters for women and accounting for variations in body type. A method is presented for determining the mass and center of mass location of the body segments of individuals within the general population of older adults. Effects of sex and body type on older adult mass distribution are accounted for using 32 easily obtainable body measurements. The method is based on existing results from different data sources and employs a combination of validated estimation approaches, including: body mass and segment length proportions, linear and nonlinear regression equations, and a mathematical model of the trunk. The method was applied to a validation sample of healthy, community-dwelling older adults (29 men, 50 women). Predicted body mass was 96.7+/-4.8% and 95.7+/-3.7% of measured body mass in the men and women, respectively. The estimates of body segment mass and trunk center of mass location for the sample population approximate those reported in the literature, supporting the validity of the described method. By producing practical, subject-specific estimates of body segment inertial parameters, the method should allow more accurate biomechanical analyses of the older adult population.

Age Factors↗
PubMed2002

Influence of lower extremity strength of healthy older adults on the outcome of an induced trip.

OBJECTIVES: To determine whether decreased lower extremity strength contributes to trip-related falls in older adults. DESIGN: A cross-sectional sample of older adults were safety-harnessed and tripped while walking using a concealed, mechanical obstacle. Lower extremity strength was compared between trip outcome groups. SETTING: A biomechanics research laboratory. PARTICIPANTS: Seventy-nine healthy, community-dwelling adults aged 65 and older (50 women). MEASUREMENTS: Ankle, knee, and hip flexion and extension strength were measured isometrically and isokinetically. Measured strengths were subjected to a factor analysis. Strength factor scores were compared between those who recovered from the trip and those who fell by three previously identified mechanisms: during-step, after-step, and elevating-response falls. RESULTS: Seven common factors, one associated with each direction of exertion at each joint and one with the time rate of moment increase, explained 88% of the variance in measured strength. The during-step (n=5) fallers were significantly stronger in the ankle extension (plantarflexion), knee flexion, overall extension, and total strength factors than those who successfully recovered using a similar, lowering strategy (n=26). The elevating-response faller (n=1) was stronger in the plantarflexion and overall extension factors than most of those who recovered using a similar, elevating strategy (n=11). Two of three after-step fallers were among the weakest subjects tested. CONCLUSION: Weak older adults and the strongest older adults may be at greater risk of falling from a trip, although by different mechanisms. High strength may increase the likelihood of a during-step or elevating-response fall; decreased strength may increase the likelihood of an after-step fall.

Accidental Falls↗
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↗
PubMed2002

Can fall-related hip fractures be prevented by characterizing the biomechanical mechanisms of failed recovery?

Unintentional injuries are the seventh leading cause of death in adults ages 65 and older, and the greatest number of these deaths results from fall-related injuries. In addition to the startling mortality, the morbidity associated with fall-related injuries, particularly hip fractures, has become a research imperative. This article reviews a series of studies that was undertaken to determine the biomechanical reasons that older adults are unable to recover from very large postural perturbations that are applied during locomotion that, if not corrected, can lead to a fall. Our protocol involves causing older adults to trip unexpectedly while walking normally in the laboratory. The results from this series of experiments were used to design an experiment that characterized the biomechanical similarities between recovery biomechanics after an induced trip and those following a large postural perturbation delivered by a motorized treadmill. Collectively, we have been able to document different recovery strategies and categories of falls by older adults following an induced trip; the biomechanical causes of these falls by older adults; and the very rapid motor adaptations that occur with repeated exposure to large perturbations that may be protective against falls from tripping and, therefore, reduce the substantial fall-related morbidity and mortality in older adults.

Accidental Falls↗