Differences in muscle function during walking and running at the same speed
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.
Section snippets
Musculoskeletal model
A 2D bipedal musculoskeletal model and forward dynamical simulations of walking and running at the preferred transition speed (PTS; i.e., the speed where a voluntary transition from walking to running occurs) were generated to quantify individual muscle contributions to the body segment mechanical energetics and body support and forward propulsion. The musculoskeletal model has been previously described in detail (Neptune and Sasaki, 2005; Sasaki and Neptune, 2005) and will be described briefly
Results
The generated walking and running simulations at the PTS (1.96±0.17 m/s: group average and s.d.) matched the group-averaged kinematics and ground reaction forces, with most tracking variables within ±2 s.d. (Fig. 2: Walking, Running). The corresponding muscle excitation patterns also compared well with the human subject EMG linear envelopes (Fig. 3).
The segment power analysis revealed that the only muscle that exhibited a distinct functional difference between walking and running was SOL. In
Discussion
The overall objective of this study was to use forward dynamical simulations to identify differences in muscle function between walking and running at the preferred transition speed (PTS). The specific objective was to access whether individual muscle contributions to body segment mechanical energetics, and support and forward propulsion remain invariant when walking and running at the same speed. The simulations successfully emulated the salient features of kinesiological data collected from a
Acknowledgements
The authors are grateful to The Whitaker Foundation for financial support of this work, Julie Perry, Dr. Brian Davis and Dr. Ton van den Bogert for help with the data collection and Dr. Felix Zajac for his insightful comments on a previous version of the manuscript.
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