Walking with increased ankle pushoff decreases hip muscle moments

Abstract

In a simple bipedal walking model, an impulsive push along the trailing limb (similar to ankle plantar flexion) or a torque at the hip can power level walking. This suggests a tradeoff between ankle and hip muscle requirements during human gait. People with anterior hip pain may benefit from walking with increased ankle pushoff if it reduces hip muscle forces. The purpose of our study was to determine if simple instructions to alter ankle pushoff can modify gait dynamics and if resulting changes in ankle pushoff have an effect on hip muscle requirements during gait. We hypothesized that changes in ankle kinetics would be inversely related to hip muscle kinetics. Ten healthy subjects walked on a custom split-belt force-measuring treadmill at 1.25 m/s. We recorded ground reaction forces and lower extremity kinematic data to calculate joint angles and internal muscle moments, powers and angular impulses. Subjects walked under three conditions: natural pushoff, decreased pushoff and increased pushoff. For the decreased pushoff condition, subjects were instructed to push less with their feet as they walked. Conversely, for the increased pushoff condition, subjects were instructed to push more with their feet. As predicted, walking with increased ankle pushoff resulted in lower peak hip flexion moment, power and angular impulse as well as lower peak hip extension moment and angular impulse (p<0.05). Our results emphasize the interchange between hip and ankle kinetics in human walking and suggest that increased ankle pushoff during gait may help to compensate for hip muscle weakness or injury and reduce hip joint forces.

Introduction

Insight into human gait can be gained by simple mechanical representations of walking. The simple walking model, as described by Garcia et al. (1998), consists of two rigid massless limbs connected at the “hip” by a hinge joint. The model has a point mass at the hips representing the body and a small mass at the end of each limb representing each foot. This bipedal model can be modified for different methods of actuation (McGeer, 1990; Kuo, 2002). One way to actuate the model is to apply an impulsive push along the trailing limb as it leaves the ground. This push redirects the center of mass forward and upward. A second method is to apply a torque between the limbs using a torsional spring. The torque generated by the spring pulls the swing limb forward. Comparison of these methods of powering gait indicates that there is a direct tradeoff between the impulsive push from the trailing limb and the rotational torque between the limbs. Using the hip torque alone to power gait is four times more energetically expensive than using the impulse alone (Kuo, 2002). Thus, a trailing limb impulsive pushoff is a much more economical way to generate walking in the simple bipedal model.

Although there is a clear tradeoff between a trailing limb push and a swing leg hip pull in the simple walking model, the interplay between ankle pushoff and hip torque generation may be less clear when applied to human gait. The impulsive push of the trailing limb can be analogous to ankle plantar flexion or “pushoff” in late stance. Clinicians refer to gait powered by ankle pushoff as using an “ankle strategy”, which is thought to be the preferred walking strategy for healthy young adults (Kerrigan et al., 1998; Mueller et al., 1994b). In the ankle strategy for walking, ankle plantar flexion power propels the leg into swing and accelerates body mass forward. The pushoff into swing is slightly different than in the simple walking model because the impulsive push in the model redirects the center of mass and swing occurs passively. In human gait, leg swing is not a completely passive process. Muscle activation is thought to contribute to leg acceleration and deceleration, thus increasing the energetic cost of walking (Doke et al., 2005; Gottschall and Kram, 2005).

A limitation of the simple walking model is that it does not accurately reproduce hip extension torque. The model's torsional spring pulls the swing limb forward but has no effect on the stance limb or the movement of the center of mass. This type of actuation is similar to a “hip strategy” for walking where the hip flexor muscles of the swing leg concentrically contract to pull the leg forward (Mueller et al., 1994b). In human walking, however, both the hip flexor muscles and the hip extensor muscles are active during the gait cycle. McGibbon (2003) proposes a second hip strategy in which the hip extensor muscles of the stance leg concentrically contract to posteriorly rotate the pelvis and assist with forward progression of the contralateral leg into swing. Individuals with diabetes mellitus demonstrate a higher hip flexion moment than ankle plantar flexion moment, opposite the pattern found in age-matched healthy individuals (Mueller et al., 1994a). This suggests there is a definitive tradeoff between a hip flexor strategy and an ankle strategy during human walking. Healthy elderly individuals demonstrate a greater hip extension angular impulse and a lower peak ankle plantar flexion angular impulse compared to healthy young adult subjects (DeVita and Hortobagyi, 2000). Correspondingly, this suggests a tradeoff between a hip extensor strategy and an ankle strategy.

Alteration of walking strategy has been recommended for some clinical populations. Mueller et al. (1994b) showed that instructing subjects to use an exaggerated hip strategy when walking could lead to a beneficial decrease in forefoot peak plantar pressures. Conversely, if individuals can decrease hip muscle forces by walking with increased ankle pushoff, people with hip pain may benefit from walking with an exaggerated ankle strategy. This alteration may be beneficial for patients with acetabular labral tears or idiopathic hip osteoarthritis. Patients with anterior hip pain may benefit from decreasing the anterior hip forces which may over time increase pain and lead to a tear of the acetabular labrum (Lewis and Sahrmann, 2006). Patients with idiopathic hip osteoarthritis may also benefit from decreased muscle forces (Krebs et al., 1998) as increased joint force is associated with earlier progress of disease (Recnik et al., 2007; Mavcic et al., 2004). In addition, it would also suggest that plantar flexor muscle weakness may be a cause of increased hip muscle forces during walking, leading to overuse injury of hip musculature or increased hip joint forces and subsequent joint injury.

The purpose of this study was to test whether directly altering ankle pushoff in late stance has an effect on hip muscle moments during walking in healthy young adults. We hypothesized that as subjects intentionally altered ankle pushoff, there would be an inverse relationship between peak ankle plantar flexion moment and peak hip flexion and extension moments.