Michael J. Pavol

Foot displacement but not velocity predicts the outcome of a slip induced in young subjects while walking

The purpose of the present study was to induce slips in healthy subjects as a means to determine if recovery from an induced slip is possible under conditions in which the displacements and velocities of the slipping foot exceed the generally accepted limits of 10cm and 50cm/s, respectively, and to determine if there are gait-related variables that predispose an individual to falling after a slip. Thirty-three young and barefoot adults, protected by an instrumented safety harness, were subjected to a single slipping trial following a series of unperturbed walking trials. The slip was induced when the bare foot contacted a vinyl sheet coated with mineral oil. Lower extremity kinematics were acquired using a video-based motion capture system. Fourteen and 12 subjects could be unambiguously categorized as having fallen or recovered, respectively. Four variables demonstrated significant between-group differences and two were used to compute the probability of the slip outcome using logistic regression. The variables were the displacement of the foot during the slip and the angle of the shank relative to the ground at the instant of ground contact just prior to the slip. Separate univariate logistic regressions using each variable were significant and correctly classified about 70% of the slip outcomes. The results demonstrated that previously published values for the displacement and velocity of the slipping foot, 10cm and 50cm/s, respectively, may not accurately represent the upper limits beyond which recovery is not possible. The results also demonstrated that heel-strike angle, reflective of stride length, exerts a significant influence on the outcome of a slip.

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.

Mechanisms of failed recovery following postural perturbations on a motorized treadmill mimic those associated with an actual forward trip.

OBJECTIVE To examine the recovery strategies employed during a treadmill acceleration task, to determine if mechanisms that contributed to failed recoveries on a motorized treadmill are the same general biomechanical mechanisms that contributed to falls from a trip, and to determine if failed recovery responses could be modified to allow for successful recoveries on subsequent trials. DESIGN A motorized treadmill was used to induce postural perturbations in healthy older adults. BACKGROUND Previously, we induced trips in older adults to identify the mechanisms of failed recovery. However, inducing trips is not a clinically practical test for identifying older adults who are predisposed to falling. METHODS Safety-harnessed older adults stood on a treadmill that was accelerated from 0 to 0.89 m/s to impose a postural perturbation. Recoveries were classified as successful (n=42) or failed (n=23). Selected biomechanical variables were calculated using motion analysis methods. RESULTS Initial failed recoveries had slower reaction times, shorter step lengths, and greater trunk flexion angles and velocities. Subjects who failed on the initial attempt modified their recovery strategy to successfully recover. The biomechanics of these recoveries resembled those used by subjects who successfully recovered on their initial attempt. CONCLUSIONS The biomechanical mechanisms involved with a failed treadmill recovery mimic those responsible for failed recoveries from an induced trip. Subjects who failed on their initial recovery response made modifications allowing successful recoveries on subsequent attempts. RELEVANCE This protocol may be useful as a testing and rehabilitation tool for fall recovery.

Measures of Postural Stability Are Not Predictors of Recovery from Large Postural Disturbances in Healthy Older Adults

OBJECTIVES: To determine, in healthy older adults, the relationship between postural steadiness, stability limits, and the ability to recover balance from three postural disturbances requiring anteriorly directed stepping responses.

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.

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.

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.

The Acute Effects of a Warm-up Including Static or Dynamic Stretching on Countermovement Jump Height, Reaction Time, and Flexibility

Perrier, ET, Pavol, MJ, and Hoffman, MA. The acute effects of a warm-up including static or dynamic stretching on countermovement jump height, reaction time, and flexibility. J Strength Cond Res 25(7): 1925-1931, 2011—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 ones 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.

Three-Dimensional Knee Joint Kinetics During a Golf Swing Influences of Skill Level and Footwear

This study characterized knee joint kinetics during a golf swing and determined the influence of shoe type and golfer skill on the peak knee joint loads. Thirteen golfers each hit a golf ball using a five iron under two footware conditions: spiked and spikeless golf shoes. Data from a video-based motion capture system and force plates were used to compute the knee joint kinetics. Mean peak forces and moments differed significantly between the lead and trail knees, but these peak loads were not significantly affected by shoe type. Only the lead knee flexion and internal rotation moments were significantly correlated to skill level. The magnitude of some of the peak loads at the knee during the golf swing approached those reached during activities prohibited until late-stage knee rehabilitation. We concluded the following: The type of shoe worn and the skill level of the golfer need not be considered in deciding time to return to golfing; however, the leg that is recovering from surgery or injury should be considered. The most stressful phase of the golf swing, relative to the knee, is the downswing. There is probably no “normal” swing; each golfer seems to possess consistent, characteristic, patterns of knee loading.