Walking with increased ankle pushoff decreases hip muscle moments
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.
Section snippets
Subjects
Ten healthy subjects (3 male, 7 female, age 27.0±7.2 years (mean±SD), height 1.69±0.09 m and mass 63.2±11.3 kg) participated in the study. Written informed consent as approved by the University of Michigan Medical School Institutional Review Board was obtained from all subjects prior to any testing.
Instrumentation
We used a motion capture system (120 Hz) and reflective markers to record the kinematics of the ankle, knee, and hip joints (low pass Butterworth filtered at 6 Hz with zero phase lag to remove movement
Results
The simple instructions to and short practice by the subjects led to marked changes for the increased ankle pushoff condition but the instructions were not successful in producing substantial changes in the decreased pushoff condition (Table 1). Repeated measures ANOVAs revealed significant differences among the three conditions for all variables except maximum hip flexion and extension angles, ankle dorsiflexion angular impulse, ankle plantar flexion peak power, knee extension peak power
Discussion
The results of this study support the hypothesis that changes in ankle pushoff are inversely related to the changes in the internal net hip muscle moments. When the ankle plantar flexion angular impulse was higher in the increased pushoff condition, both hip flexion and hip extension muscle moments and angular impulses were lower. In addition, hip flexion peak powers were also significantly lower when the plantar flexion angular impulse was increased. These findings agree with the prediction
Conflict of interest
The authors affirm that they have no financial affiliation or involvement with any commercial organization that has direct financial interest in any matter included in this manuscript.
Acknowledgments
The authors would like to thank the members of the Human Neuromechanics Laboratory for their assistance. This work was supported by the Medical Rehabilitation Research Training Program at the University of Michigan which was funded by the National Institutes of Health (NIH), the National Institute of Child Health and Human Development (NICHD), the National Center for Medical Rehabilitation Research (NCMRR) Grants 5-T32-HD007422-17, and by F32-HD055010 and R01-NS45486.
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