Differences in muscle function during walking and running at the same speed

Abstract

Individual muscle contributions to body segment mechanical energetics and the functional tasks of body support and forward propulsion in walking and running at the same speed were quantified using forward dynamical simulations to elucidate differences in muscle function between the two different gait modes. Simulations that emulated experimentally measured kinesiological data of young adults walking and running at the preferred walk-to-run transition speed revealed that muscles use similar biomechanical mechanisms to provide support and forward propulsion during the two tasks. The primary exception was a decreased contribution of the soleus to forward propulsion in running, which was previously found to be significant in walking. In addition, the soleus distributed its mechanical power differently to individual body segments between the two gait modes from mid- to late stance. In walking, the soleus transferred mechanical energy from the leg to the trunk to provide support, but in running it delivered energy to both the leg and trunk. In running, earlier soleus excitation resulted in it working in synergy with the hip and knee extensors near mid-stance to provide the vertical acceleration for the subsequent flight phase in running. In addition, greater power output was produced by the soleus and hip and knee extensors in running. All other muscle groups distributed mechanical power among the body segments and provided support and forward propulsion in a qualitatively similar manner in both walking and running.

Introduction

Walking and running are the two most common forms of human gait, with many of the basic kinetics and kinematics of walking and running being similar between the two modes (e.g., Nilsson and Thorstensson, 1989; Nilsson et al., 1985). However, one of the most noticeable differences is the existence of a flight phase in running, rather than the double support phase that occurs in walking, which suggests that muscles generate greater body support (defined as vertical acceleration of the body center of mass) in running. In walking, recent modeling and simulation studies have shown that body support is provided by the uni-articular hip and knee extensors in early stance, and the ankle plantar flexors as primary contributors in late stance (Anderson and Pandy, 2003; Neptune et al., 2004a). However, it is unknown whether these muscle groups provide body support in a similar manner during running.

Similarly, forward propulsion (defined as horizontal acceleration of the center of mass) in walking is provided by the hip and knee extensors in early stance and the plantar flexors in late stance (Neptune et al., 2004a; Zajac et al., 2003). However, which muscle groups are the primary contributors to forward propulsion in running is not well understood. Previous inverse dynamics-based analyses of running have suggested that the knee extensors and ankle plantar flexors contribute to forward propulsion from mid- to late stance (Ounpuu, 1990; Novacheck, 1998), but others have suggested that the hip extensors are the primary contributors (Simonsen et al., 1985; Belli et al., 2002). Analyzing EMG data, other studies have suggested that the plantar flexors contribute little to push-off in late stance, since their peak muscle activity occurs in mid-stance and then rapidly decreases in late stance (e.g., Mann et al., 1986; Reber et al., 1993). Part of the discrepancy among studies could be related to the inability of inverse dynamics and EMG-based analyses to identify individual muscle contributions to accelerations and mechanical energetics of individual body segments (Zajac et al., 2002), which is essential for assessing muscle function, since individual muscle contributions can vary greatly even within the same muscle group (e.g., ankle plantar flexors, Neptune et al., 2001; Zajac et al., 2003).

To help clarify differences in muscle function between walking and running, the overall objective of this study was to use forward dynamical simulations of walking and running at the same speed to identify how differences in task mechanics influence individual muscle contributions to the mechanical energetics of the two gait modes. Comparing muscle function at the same gait speed is particularly informative, since many of the task requirements remain the same (e.g., the net contributions to forward propulsion). The specific objective was to access whether individual muscle contributions to the body segment mechanical energetics and body support and forward propulsion remain invariant during the two gait modes.