Effects of obesity on the biomechanics of walking at different speeds.

Effects of obesity on the biomechanics of walking at different speeds.
09/13/2007
Raymond C. Browning; Rodger Kram
Med Sci Sports Exerc.
Medscape

Raymond C. Browning, Rodger Kram, Locomotion Laboratory, Department of Integrative Physiology, University of Colorado, Boulder, CO

Abstract
Purpose: Walking is a recommended form of exercise for the treatment of obesity, but walking may be a critical source of biomechanical loads that link obesity and musculoskeletal pathology, particularly knee osteoarthritis. We hypothesized that compared with normal-weight adults 1) obese adults would have greater absolute ground-reaction forces (GRF) during walking, but their GRF would be reduced at slower walking speeds; and 2) obese adults would have greater sagittal-plane absolute leg-joint moments at a given walking speed, but these moments would be reduced at slower walking speeds.
Methods: We measured GRF and recorded sagittal-plane kinematics of 20 adults (10 obese and 10 normal weight) as they walked on a level, force-measuring treadmill at six speeds (0.5-1.75 m.s-1). We calculated sagittal-plane net muscle moments at the hip, knee, and ankle.
Results: Compared with their normal-weight peers, obese adults had much greater absolute GRF (N), stance-phase sagittal-plane net muscle moments (N.m) and step width (m).
Conclusions: Greater sagittal-plane knee moments in the obese subjects suggest that they walked with greater knee-joint loads than normal-weight adults. Walking slower reduced GRF and net muscle moments and may be a risk-lowering strategy for obese adults who wish to walk for exercise. When obese subjects walked at 1.0 versus 1.5 m.s-1, peak sagittal-plane knee moments were 45% less. Obese subjects walking at approximately 1.1 m.s-1 would have the same absolute peak sagittal-plane knee net muscle moment as normal-weight subjects when they walk at their typical preferred speed of 1.4 m.s-1.

Introduction
Obesity is the main preventable risk factor for large-joint (e.g. knee) osteoarthritis (OA).[30] Exercise combined with dietary intervention probably holds the best promise for combating the obesity epidemic.[17] Walking is a recommended and popular form of exercise[18] but weight-bearing exercise (i.e., walking) may be a critical source of the biomechanical loads that link obesity and OA.[15] Intuitively, it would seem likely that obesity greatly increases the biomechanical loads involved in walking and that these loads increase with walking speed. If so, the common prescription of brisk walking as exercise for treating obesity[28] may inadvertently increase the risk of musculoskeletal pathology.

Remarkably, few studies have been conducted on how obesity affects the biomechanical loads involved in walking.[30] Spyropoulos et al.[35] compared stride and joint angle differences between obese and normal-weight men. They reported that the obese males walked slower (1.09 m.s-1) with wider steps and similar knee flexion at midstance compared with their faster-walking (1.64 m.s-1) normal-weight counterparts, but they could not discern the obesity effects from the speed effects. However, according to Lelas et al.,[21] peak knee flexion during the loading response phase of stance increases with increased walking speed. Combining the two studies suggests that at matched speeds, obese men might adopt a more flexed knee during stance than their normal-weight peers.

Messier et al.[25] provide the only published reports of ground-reaction forces (GRF) for obese adults walking. Absolute peak vertical GRF increased in almost direct proportion with body weight. The peak vertical GRF were approximately 1.0 x body weight, which is consistent with the slow walking speeds selected by the subjects (1.0 m.s-1). Absolute peak anteroposterior and mediolateral GRF also increased proportionally with body weight. By design, Messier et al. compared arthritic obese with arthritic normal-weight subjects. Thus, although insightful, this study provided no information about the GRF of nonarthritic obese adults walking at a normal walking speed.

To date, the most intriguing and thorough study of the biomechanics of walking by obese subjects is that of DeVita and Hortobagyi.[8] They compared the sagittal-plane net muscle moments generated at the ankle, knee, and hip joints of nonarthritic class II and III obese adults (mean BMI = 42 kg.m-2) and normal-weight controls. The knee is the primary site of obesity-related OA,[13] yet, surprisingly, DeVita and Hortobagyi[8] found that the absolute peak knee moments when walking at 1.5 m.s-1 were identical: 64 N.m at 1.5 m.s-1 for obese (123 kg) versus normal-weight (63 kg) subjects. Although the obese subjects presumably had greater absolute GRF, they maintained their knee-joint angles in more extended positions that aligned the resultant GRF more closely to the center of the knee joint. As a result, the net muscle moment required and, by proxy, sagittal-plane knee-joint forces, were likely not greater. Although these data suggest that obese adults do not have greater knee-joint loads, it is unclear how the wide range of adiposity (BMI 30-60 kg.m-2) influenced the findings and whether their findings would be similar in a group of moderately obese adults (BMI 30-40 kg.m-2). Also, the kinematic results reported by DeVita and Hortobagyi are not consistent with those of Spyropoulos et al., so a clear consensus of the effects of obesity on lower-extremity sagittal-plane biomechanics has yet to be achieved.

Walking at slower speeds may be an effective way to reduce knee-joint loads in obese adults. Previous studies have shown that in normal-weight subjects, GRF and lower-extremity net muscle moments are smaller at slower walking speeds.[21,40] In most studies, obese adults prefer to walk slower than their normal-weight peers.[8,23] DeVita and Hortobagyi[8] reported that at a just slightly slower speed (1.29 vs 1.50 m.s-1), peak knee sagittal-plane net muscle moment in the obese subjects was reduced by 25%. However, no study has comprehensively examined the relationship between walking speed and lower-extremity joint loads in obese versus normal-weight adults.

The purpose of this study was to measure how obesity affects walking biomechanics, particularly knee-joint loads, by determining GRF and lower-extremity sagittal-plane joint moments across a range of walking speeds. We hypothesized that compared with normal-weight adults, 1) obese adults have greater absolute GRF during walking, but their GRF are reduced by walking slower; and 2) obese adults have greater absolute lower-extremity joint moments at a given walking speed, but these moments are reduced at slower walking speeds.

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