Elsevier

Gait & Posture

Volume 17, Issue 2, April 2003, Pages 159-169
Gait & Posture

Winner of the 2001 GCMAS Best Paper Award
Individual muscle contributions to support in normal walking

https://doi.org/10.1016/S0966-6362(02)00073-5Get rights and content

Abstract

The purpose of this study was to quantify the contributions made by individual muscles to support of the whole body during normal gait. A muscle's contribution to support was described by its contribution to the time history of the vertical force exerted by the ground. The analysis was based on a three-dimensional, muscle-actuated model of the body and a dynamic optimization solution for normal walking. The results showed that, in early stance, before the foot was placed flat on the ground, support was provided mainly by the ankle dorsiflexors. After foot-flat, but before contralateral toe-off, support was generated primarily by gluteus maximus, vasti, and posterior gluteus medius/minimus; these muscles were responsible for the first peak seen in the vertical ground-reaction force. The majority of support in midstance was provided by gluteus medius/minimus, with gravity assisting significantly as well. The ankle plantarflexors generated nearly all support in late stance; these muscles were responsible for the second peak in the vertical ground-reaction force. The results showed also that centrifugal forces act to decrease the vertical ground-reaction force, but only by minor amounts, and that resistance of the skeleton to the force of gravity is no larger than 1/2 body weight throughout the gait cycle.

Introduction

The vertical ground-reaction force is measured routinely in gait labs, and its characteristic shape is well known for normal gait. Saunders et al. [1] proposed six kinematic determinants to explain the smooth and energy-efficient trajectory that the body's center of mass undertakes during normal bipedal gait. Because the trajectory of the center of mass is determined by the time history of the resultant ground-reaction force, they suggested that the pattern of the ground-reaction force is an important indicator of how well the body is able to execute the six determinants. Saunders et al. [1] further suggested that irregularities observed in the ground-reaction force may be valuable to clinicians in the diagnosis and treatment of gait pathologies.

It has been decades since the work of Saunders et al. [1] was published, yet we still do not have a comprehensive and quantitative picture of how muscles contribute to the vertical ground-reaction force and therefore to support, even during normal gait. Considerable insight in this area has been offered by a number of researchers, but findings have been limited because the analyses have largely not been based on estimates of muscle forces. Winter [2] based his observations on similarities between the shape of the ground-reaction force and the sum of the net extensor moments applied at the hip, knee, and ankle. Sutherland et al. [3] inferred the role of the ankle plantarflexors by administering a tibial nerve block and observing the resulting changes in the ground-reaction force. Perry [4] used detailed analyses of kinetic and EMG data to form hypotheses about the roles of individual muscles. Mochon and McMahon [5], [6] and Pandy and Berme [7], [8], [9] used simplified dynamic models of the body to study the effects of various gait determinants on the vertical ground-reaction force, but their models did not include the influence of muscles.

More recently, Kepple et al. [10] used a dynamic analysis to quantify the contributions of joint moments to support of the upper body. Their analysis did not address the contributions made to the support of the whole body, and, because their model did not have muscles, they were not able to examine the roles of individual muscles. Neptune et al. [11] did use a muscle-actuated simulation of gait to investigate the roles of the ankle plantarflexors; however, they too examined the contributions to support of the upper body rather than the whole-body center of mass. In addition, Neptune et al.'s [11] model was two-dimensional, which meant that they were unable to examine the actions of muscles outside the sagittal plane.

Despite limitations, these studies have established some expectations for how some muscles contribute to the vertical ground-reaction force. There is broad consensus that the second maximum observed in the ground-reaction force is due largely to the forces exerted by the plantarflexors during late stance, also referred to as push-off or role-off [2], [3], [4], [5], [7], [8], [9], [10], [11], [12]. However, an explanation for the shape of the ground-reaction force during early stance (or weight acceptance) and midstance has been less definitive. It is generally accepted that the hip and knee extensors make important contributions during early stance [2], [6], [8], [9], [10], but the relative importance of muscles like the vasti, gluteus maximus, hamstrings, and rectus femoris has not been established. Mochon and McMahon [6] hypothesized that the abductors likely make substantive contributions to support during midstance, but Kepple et al.'s [10] results do not substantiate this claim. With regard to the influence of other muscles known to be active during the gait cycle, such as tibialis anterior, adductor magnus, iliopsoas, and erector spinae [4], few quantitative data are available to evaluate the contributions these muscles might make to support. In addition, we note that the skeleton likely offers some contribution to support in resistance to gravity, but this contribution also has yet to be quantified.

We believe an important next step in our understanding of basic gait mechanics is establishing a direct connection between the actions of muscles and the well-known shape of the vertical ground-reaction force. The analyses performed in the present study are based on a three-dimensional, muscle-actuated model of the body and a detailed dynamic optimization solution for normal gait [13], [14]. The specific questions we address are: (1) how do muscle forces, gravitational forces, and centrifugal forces (i.e. forces arising from rotations of the body segments) contribute to the vertical ground-reaction force generated during normal gait?, and (2) which muscles contribute most significantly to the time histories of the vertical ground-reaction force and the vertical acceleration of the center of mass?

Section snippets

Musculoskeletal model

The body was modeled as a 10 segment, 23 degree-of-freedom linkage [15]. The first 6 degrees-of-freedom were used to define the position and orientation of the pelvis relative to the ground. The remaining 9 segments branched out in an open chain from the pelvis. The head, arms, and torso were represented as a single rigid body that articulated with the pelvis via a ball-and-socket joint located at approximately the third lumbar vertebra. Each hip was modeled as a ball-and-socket joint, each

Results

Muscles made the largest contribution to support, accounting for 50–95% of the vertical ground-reaction force generated in stance (Fig. 1, Muscle+Ligaments). The passive transmission of force through the joints and bones in resistance to gravity accounted for 20–50% when the foot was flat on the ground, but made much smaller contributions before foot-flat and after heel-off (Fig. 1, Gravity). Centrifugal and inertial forces contributed little throughout stance, except for a brief period after

Discussion

Although the issue of support has been addressed previously, most notably by Winter [2], Mochon and McMahon [5], [6], Pandy and Berme [7], [8], [9], Perry [4], Kepple et al. [10], Meinders et al. [26], Riley et al. [12], and Neptune et al. [11], detailed knowledge of how individual muscles generate support has not emerged until now. Each of the above studies, except Neptune et al. [11], considered only the contributions of the net joint moments to support. The analysis presented by Neptune et

Acknowledgements

Funding and support for this project was provided by the Whitaker Foundation, NASA Grant NAG5-2217, NIH Grant R01 HD38962 to Scott L. Delp, NASA/Ames Research Center, and the Center for High Performance Computing at The University of Texas at Austin. The authors also gratefully acknowledge the critical reading of this manuscript by Allison S. Arnold, Ph.D.

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