Muscle contributions to support and progression over a range of walking speeds

Abstract

Muscles actuate walking by providing vertical support and forward progression of the mass center. To quantify muscle contributions to vertical support and forward progression (i.e., vertical and fore-aft accelerations of the mass center) over a range of walking speeds, three-dimensional muscle-actuated simulations of gait were generated and analyzed for eight subjects walking overground at very slow, slow, free, and fast speeds. We found that gluteus maximus, gluteus medius, vasti, hamstrings, gastrocnemius, and soleus were the primary contributors to support and progression at all speeds. With the exception of gluteus medius, contributions from these muscles generally increased with walking speed. During very slow and slow walking speeds, vertical support in early stance was primarily provided by a straighter limb, such that skeletal alignment, rather than muscles, provided resistance to gravity. When walking speed increased from slow to free, contributions to support from vasti and soleus increased dramatically. Greater stance-phase knee flexion during free and fast walking speeds caused increased vasti force, which provided support but also slowed progression, while contralateral soleus simultaneously provided increased propulsion. This study provides reference data for muscle contributions to support and progression over a wide range of walking speeds and highlights the importance of walking speed when evaluating muscle function.

Introduction

Many individuals with neuromuscular impairments walk slowly (Turnbull et al., 1995; Abel and Damiano, 1996; Goldie et al., 1996; Dingwell et al., 2000). Evaluating a patient's gait requires discriminating between deviations caused by pathology and walking speed. Several studies identified how walking speed influences joint kinematics (Murray et al., 1984; Kirtley et al., 1985; Holden et al., 1997; Stansfield et al., 2001b; van der Linden et al., 2002; Nymark et al., 2005; Schwartz et al., 2008), ground reaction forces (Andriacchi et al., 1977; Jansen and Jansen, 1978; Vaughan et al., 1987; Stansfield et al., 2001a; Schwartz et al., 2008), and muscle activity (Murray et al., 1984; Shiavi et al., 1987; Hof et al., 2002; den Otter et al., 2004; Nymark et al., 2005; Cappellini et al., 2006; Schwartz et al., 2008). However, the mechanisms by which muscles modulate the accelerations of the mass center over a range of walking speeds are not well understood.

Several studies have examined how muscles provide support and progression (Pandy, 2001; Neptune et al., 2004; Liu et al., 2006) at a typical walking speed in unimpaired adults. Using a computer simulation of overground walking, Liu et al. (2006) found that gluteus maximus, vasti, and dorsiflexors slowed the body mass center during early stance; gluteus medius, soleus, and gastrocnemius propelled the mass center forward during late stance. The same muscles modulated vertical acceleration of the body mass center. Their findings agreed with those of other researchers (Pandy, 2001; Anderson and Pandy, 2003; Neptune et al., 2004). A drawback to all of these previous simulation studies, however, is that each analyzed only one simulation at one walking speed, making it difficult to generalize the results to the larger population who walk at various speeds.

Neptune et al. (2008) recently analyzed two-dimensional computer simulations of walking at five speeds and found that vertical support of the trunk was provided by gluteus maximus, vasti, soleus, and gastrocnemius, while forward propulsion of the trunk was provided by soleus and rectus femoris. Neptune et al. (2008) were able to precisely control subject walking speed using a treadmill; however, there are differences between overground and treadmill walking (Murray et al., 1985; Lee and Hidler, 2008).

We examined the mechanisms that modulate vertical (support) and fore-aft (progression) accelerations of the body mass center at different overground walking speeds. We quantified how the contributions of individual muscles to mass center accelerations vary with walking speed by creating and analyzing 32 three-dimensional, muscle-actuated simulations of walking, representing eight different subjects walking at four speeds.