The effects of age and step length on joint kinematics and kinetics of large out-and-back steps
Introduction
Falls are a leading cause of accidental death and injury for those over the age of 65 years (Centers for Disease Control and Prevention, 2006) and older women, especially those with balance impairments, are at a particularly high risk for fall-related injuries (Baker et al., 1992). The ability to recover from an imminent fall has been shown to be dependent on the ability to take a rapid and adequately long step in the direction of the fall (Alexander, 1994, Luchies et al., 1994, Maki and McIlroy, 1999, Thelen et al., 1997).
Most tests of volitional stepping evaluate only step timing and do not take into account step length (Cronin et al., 2003, Lord and Fitzpatrick, 2001, Luchies et al., 2002). While rapid stepping is important, longer steps may be required to recover balance and greater step distances are likely to place greater physiological demands on the leg musculature. Given that strength (Frontera et al., 1991, Lexell, 1995), peak contraction velocity (Hortobagyi et al., 1995, Larsson et al., 1979), and particularly power (Hakkinen et al., 1997, Izquierdo et al., 1999, Macaluso and De Vito, 2003) decline with age, might these declines be related to step length?
In one measure of stepping ability, volitional stepping, Medell and colleagues (Medell and Alexander, 2000) instructed subjects to step out as far as possible and return to the original stance position in one step. This maximum step length (MSL) was found to decline with age and balance impairment and correlate strongly with measures of balance, fall risk, mobility performance, and self-reported disability in balance-impaired older adults (Cho et al., 2004, Lindemann et al., 2003, Medell and Alexander, 2000). MSL was recently shown to also improve with training (Nnodim et al., 2006). MSL test–retest reliability was satisfactory and, while MSL was originally conducted in three directions (forward, sideways, and backwards) with both right and left feet, a simplified version more appropriate for clinical settings (right foot forward only) and equally predictive of the above functional outcomes (Cho et al., 2004) was evaluated in the present study.
MSL thus appears to be a measure of fall risk as well as other mobility-related factors. Yet, as in many clinical mobility tests, little is known about the mechanisms underlying decreased MSL values associated with increased age and fall risk. While recent findings have indicated that MSL correlates with knee and hip extension strength, speed, and power generation even after controlling for age group (Schulz et al., 2007), the actual joint kinematics and kinetics used to perform the MSL are not yet known.
Biomechanical analyses of relevant clinical tests such as the MSL help provide a better understanding of the key contributors to clinical test performance (such as in leg power), and ultimately, better insight into how changes in these parameters might lead to declines or improvements in mobility performance. Thus, the goal of this study was to determine the relationships of age and step length with the MSL kinematics and kinetics. To accomplish this we recorded body segment kinematics during the original out-and-back MSL version and calculated the stepping leg joint kinetics using an inverse dynamics model. We restricted our analysis to the MSL phase with the greatest joint moments and power. Thus, we hypothesized that maximal values for joint kinetics would occur during the “pushback” phase of the MSL. Furthermore, based on prior data (Schulz et al., 2007) showing little independent effect of age on the correlations between MSL and dynamometer-measured knee and hip extension strength, speed, and power, we similarly hypothesized that any correlations of step length with inverse-dynamics-calculated peak knee and hip extension velocity, moment, and power generated would be independent of age.
Section snippets
Subjects
Eleven younger (mean [SD] age 24 [3.4] years) and ten older (mean age 73 [5.3] years) unimpaired women participated in this study. All younger subjects completed a medical questionnaire and all older subjects were physically screened by a geriatric nurse-clinician supervised by the physician-geriatrician prior to testing. The older women had no significant abnormal neurological or musculoskeletal findings on directed history and physical. The two groups were of similar stature, but the younger
Subject characteristics and step lengths
The young weighed less than the older women but were similar in height (Table 1). The young stepped farther than the older women (mean [SD] 77 [5] and 55 [8]% height or 125 [9] and 88 [13] cm, respectively, P < 0.0001) and there was little or no overlap in the ranges of step lengths attained. Only one older subject ever stepped farther than the poorest-performing young subject, but when all acceptable step lengths were averaged into a single ensemble mean there was no overlap. The younger women
Discussion
The dual-contact with feet apart, or “pushback” phase, of the MSL was the most kinetically-demanding phase, thus supporting our first hypothesis. Within this phase, hip kinematic and kinetic peaks were greater for both larger steps and younger subjects, while knee flexion and power absorbed were only greater for younger subjects. Regarding the second hypothesis, the findings of order-dependent multistage regressions indicated that, in general, torso and hip kinematics and kinetics were
Conflict of interest statement
None of the authors have any financial or personal relationships with any people or organizations that could benefit from or bias this work in any way.
Acknowledgements
We thank Eric Battjes, Adriana Figueroa, Steve Luther, and Janet Kemp for their assistance with this study. The authors wish to acknowledge the support of National Institute on Aging (NIA) Claude Pepper Older Adults Independence Center Grant P60 AG08808, Institute of Gerontology Training Grant T32 AG000114, GEM predoctoral fellowship, NIA Grant P01 AG10542, and the Department of Veterans Affairs Research and Development and the VA Ann Arbor Health Care System GRECC. Dr. Alexander is also a
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