The kinematics and kinetics of turning: limb asymmetries associated with walking a circular path

Abstract

The biomechanics of changing direction while walking has been largely neglected despite its obvious relevancy to functional mobility. The world is filled with turns that must be negotiated. These turns carry an increased risk of injury due to a decrease in stability. A VICON 612 system measured joint kinematics and kinetics on 10 normal subjects for straight line walking (ST); turning, inside foot strike (IN); and turning, outside foot strike (OUT). All trials were completed at a self-selected walking speed and across a range of speeds from 0.6 to 1.3 m/s; the turn radius was 1 m. Significant differences between the conditions were detected using a mixed effects repeated measures ANCOVA with walking speed as a covariate. The most pronounced differences were demonstrated in the mediolateral ground reaction force impulse: in straight walking the impulses tended to shift the body toward the contralateral limb. In turning, the IN and OUT impulses shifted the body toward the ipsilateral and contralateral limbs, respectively. Knee flexion during stance was increased on the IN limb, while ankle plantarflexion increased on the OUT limb consistent with body lean during turning; differences in joint kinetics during turning were negligible. Self-selected turning was significantly slower than walking straight ahead (0.96 ± 0.12 m/s versus 1.61 ± 0.22 m/s) and turning at very slow speeds showed a non-uniform center of mass trajectory. Understanding the mechanisms of turning will provide insights driving design, therapy and intervention to increase functional navigation in amputees, the elderly and individuals with neuromuscular pathologies.

Introduction

Nearly all research involving human gait is focused on walking in a straight line. Even though 20% of all steps in activities of daily living are turns [1], the joint motions and forces exerted to change direction have escaped formal study. In fact, more is known about cockroach turning [2] than human turning. Specifically, little research exists to explain how turning is accomplished, the alterations due to pathology, or the interventions to improve turning function and safety. Although turning is clearly a functional task of interest to researchers [3], [4], [5], [6] and clinicians [7], [8], [9], [10], the advances in motion capture systems allowing researchers to explicitly quantify turning behavior have only recently become available.

While the mechanics of turning has been largely ignored, other areas of study have focused research on turning. Previous research, mainly from a neuroscience perspective, investigated motor control and spatial orientation during walking. Hase and Stein [3] used EMG activity of the lower extremity muscles to speculate on which muscles were used to accomplish a 180° change in walking direction. Other investigators have examined the effect of turning on the relationship of the head, trunk and feet; significant differences associated with turning were documented in head spatio-temporal behavior, trunk shift and foot yaw [4]. While the past research reveals some fundamental aspects of turning and provides some of the first vocabulary, kinematic and kinetic measures are necessary to elucidate the mechanisms by which turning is accomplished.

Clinicians have developed experimental protocols to identify characteristics of successful mobility based on turning ability [7], [8], [9], [10]. Dite and Temple [9] developed and validated a clinical measure of turning for use with older adults and found that the number of steps and time taken to perform a turn was significantly higher for patients with a history of multiple falls, suggesting that patients who perceive a high risk of falling take additional steps while turning slowly in order to improve stability. Wall et al. [10] found similar results in a modified “get up and go” test [7], where subjects were asked to rise from a chair, walk forward 10 m, turn around and return. Elderly, at risk individuals took nearly twice as long to perform the turn as young, non-impaired subjects. The patient's fear of injury during a turning fall appears well founded; Cummings and Klineberg [11] found that individuals who fall during turning were nearly eight times more likely to fracture their hip than those who fell while walking straight. This evidence suggests that turning is a greater challenge for individuals with mobility problems than is walking straight ahead. Understanding the biomechanics of turning could lead to the development of focused interventions designed to improve mobility and reduce the risks of injury.

The goal of the present research was to explore how turning is accomplished by investigating the kinematic and kinetic changes in the joints of the lower extremities while walking around a 1 m circular path. Turning in this context is defined as a change in the center of mass (COM) trajectory in the horizontal (XY) plane with the concomitant rotation of the trunk to maintain its perpendicular orientation to the new path. Walking straight ahead requires equal forces imparted to the body from both limbs, while turning requires limb kinetic asymmetry: the inside limb must differ from the outside limb. Empirical observation has lead to the following four hypotheses:

Turning is accomplished by increasing mediolateral ground reaction forces throughout stance phase to propel the body in the desired direction of travel. The lateral impulse is hypothesized to be greater on the outside limb and less on the inside limb during turning than straight ahead walking.

Turning is accomplished by transverse plane rotational moments applied to the ground. The hypothesis is that external rotator moments of the hip, ankle or knee would be greater on the outside limb compared to straight walking, which would be greater than the inside limb. The internal rotator moments would be greater on the inside limb compared to straight walking, which would be greater than the outside limb.

Turning is accomplished by a reduction in stride and limb length on the inside limb compared to the outside limb. The outside limb is hypothesized to be functionally longer than the inside limb through changes in knee flexion-extension and ankle dorsi-plantarflexion.

Turning is accomplished by increasing ankle push-off force on the outside limb to push the COM in the direction of the turn and to rotate the trunk to the new heading. Therefore, ankle power generation is hypothesized to increase on the outside leg, especially in late stance. In addition, ankle power absorption should increase on both turning limbs in order to maintain the braking action necessary to complete the turn.