Age-related reduction in sagittal plane center of mass motion during obstacle crossing

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Abstract

Accidental falls are a leading cause of injury and death in the growing elderly population. Traumatic falls are frequent, costly, and debilitating. Control of balance during locomotion is critical for safe ambulation, but relatively little is known about the natural effect of aging on dynamic balance control. Samples of healthy young (n=13) and elderly (n=13) subjects were compared in the interactive measures of center of mass (COM) and center of pressure (COP) during level walking and obstacle crossing conditions. Obstacle heights were normalized to individual body height (2.5%, 5%, 10%, and 15%). Temporal-distance (T-D) variables of gait were also compared. Statistical analyses were conducted using a two-way ANOVA for subject group and obstacle height. T-D parameters were not significantly different between groups; nor were frontal plane COM and COP parameters. Significant age differences did exist for antero-posterior (A/P) motion of the COM (decreased motion in the elderly), and its relationship with the COP (reduced separation between the two variables in the elderly). Anterior COM velocities were also significantly lower in the elderly group. The results confirm the ability of healthy elderly adults to maintain dynamic balance control in the frontal plane during locomotion. Reduced A/P distances between the COM and COP indicate a conservative reduction of the mechanical load on joints of the supporting limb. This conservative strategy may be related to a reduction in muscle strength as it occurs in the natural aging process.

Introduction

Accidental falls are the leading cause of injury for persons over the age of 65 and the leading cause of death in persons over the age of 85 (Pocinki, 1990). Hip fractures resulting from falls can be costly, with an overall cost of each fracture estimated at $35,000. Of these elderly adults who suffer from a hip fracture, 25% make a full recovery, 40% require nursing home admission, and 50% of all hip fracture patients require a cane or walker for assisted ambulation (AAOS, 1998). Not as easily measured is the psychological impact of these falls and subsequent fractures. Tinetti and Speechley (1989) proposed that the psychological trauma of an injurious fall may induce a reduction in activity levels, followed by strength reduction, leading to ever-increasing risk of future falls. It is therefore evident that falls are frequent, costly, and potentially debilitating (both physically and psychologically) in the aging population.

Imbalance and tripping over obstacles during gait were reported as two of the most common causes of falls in the elderly (Overstall et al., 1977; Blake et al., 1988; Tinetti and Speechley, 1989; Campbell et al., 1990). Several gait studies were performed to study the effect of obstacle height on the joint kinematics and kinetics (McFadyen and Winter, 1991; Patla and Rietdyk, 1993; Chou and Draganich (1997), Chou and Draganich (1998)) of either the trailing (i.e., limb crossing the obstacle last) or leading (i.e., limb crossing the obstacle first) limb. It was demonstrated that greater joint motion of both lower limbs during swing and greater joint kinetic demands (forces and torques) of the trailing limb during stance are required in young adults when stepping over an obstacle. Healthy elderly adults were reported to adopt a more conservative strategy when crossing obstacles, with slower crossing speed, shorter step length, and smaller step width, than young adults and demonstrated an increased risk for obstacle contact (Chen et al (1991), Chen et al. (1994)). Recent studies further revealed that, compared to young adults, limited frontal plane pelvic motion, shorter stride length, kinetic accommodation in the sagittal plane, and non-optimal foot placement might be contributing factors to a greater risk of tripping in elderly adults (Begg and Sparrow, 2000; McFadyen and Prince, 2002). However, no information about age-related differences in balance maintenance while negotiating obstacles was reported. To better understand mechanisms underlying the increased incidence of tripping and falling in the elderly, it is necessary to monitor the whole body's response during obstacle crossing.

It may be that the natural processes of aging do not directly affect dynamic stability, but that age-associated pathologies (i.e., vestibular hypofunction, peripheral neuropathy, osteoarthritis, etc.) are more responsible for reductions in dynamic stability. Control of dynamic balance is challenged by the extrinsic risks encountered during locomotion. When balance is perturbed, the control system applies a series of reactive and feed- forward corrections via the musculoskeletal system to continuously maintain whole body center of mass (COM) trajectory within close range of the center of pressure (COP). A few studies reported the adequacy of the COM–COP interaction in demonstrating dynamic stability (balance control during locomotion), with a consistent COM trajectory passing between the alternating COP of each supporting foot (Jian et al., 1993; MacKinnon and Winter, 1993; Prince et al., 1994; Winter, 1995). Recent results showed that COM medio- lateral (M/L) displacement and peak M/L velocity during obstacle crossing could be used to better detect dynamic instability in elderly adults (Chou et al (2001), Chou et al (2003)).

The purpose of this study was to determine the age-related differences in dynamic stability between healthy young and elderly adults during level walking and obstacle crossing tasks. It was hypothesized that there would be age-related differences in the measures of whole body dynamic stability. Specifically, it was expected that the COM trajectory would be conservatively adjusted to remain closer to the COP in elderly adults, as compared to young adults.

Section snippets

Methods

Thirteen young adults (7 male/6 female; 25.7±3.6 years, 171.8±9.2 cm, 74.2±13.1 kg) and thirteen elderly adults (8 male/5 female; 72.8±6.0 years, 168.9±11.4 cm, 72.2±14.8 kg) were recruited for this study from the University of Oregon campus and surrounding community, within the guidelines of the Institutional Review Board. Elderly subjects were noted to be active community members, with many of them currently involved in recreational sporting activities. Informed consent was obtained from each

Results

Initial screening indicated that 19 individual data points were outside reasonable variability (±3 inter-quartile ranges). Of the 6630 total data points in the analysis (17 variables×26 subjects×5 conditions×3 trials), the outlying data accounted for only 0.3% of the total data collected. Removal of these data from the analysis was therefore justified.

All subjects were able to complete the condition trials with no incidents of tripping. No significant age group differences were found for any of

Discussion

As age-related declines occur in the balance control system, it is reasonable to expect that a reduced ability to maintain dynamic stability would be evident in altered patterns of COM motion and its coordination with the COP. Results from this study revealed that there are age-dependent decreases in A/P ROM of the COM, maximum anterior COM–COP distance allowed, and the instantaneous anterior COM velocity at the timing of maximum anterior or posterior COM–COP separation. These significant

Acknowledgments

This work was supported by the Oregon Medical Research Foundation, National Institutes of Health (HD042039-01A1 and AG022204-01) and the International Society of Biomechanics Dissertation Matching Grant. The authors gratefully acknowledge the assistance of Heng-Ju Lee and Marisa Hastie in data collection and analysis.

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