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 1.25m/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.

FIGURE 1. Custom built force-measuring treadmill

The treadmill has dual belts that are each mounted with a separate force platform as its base to measure ground reaction forces from each leg separately. The belts are 24 inches wide and mounted flush with the floor. The distance between the two belts is 0.75 inches. Each belt is connected to a separate motor (2.5 kW). Average belt speed variation when adult subjects walk on the treadmill at 1.25 m/s is 1.8%. Details of the measuring capabilities of the force treadmill are included in Collins et al. (Collins et al., 2008).

FIGURE 2. Moment and power data for the left leg of a single representative subject

Data are normalized from left heel strike to left heel strike. All major moment and power peaks were calculated for prescribed phases of the gait cycle keeping with previous convention (Winter, 1983). Ankle plantar flexion, knee extension and hip extension moments are positive. A2 represents ankle plantar flexion power during late stance. K1, K2, and K3 represent knee extension power, both absorption (negative) and generation (positive), during early stance, midstance and terminal stance respectively. H1 represents hip extension power while H2 and H3 represent hip flexion power.

FIGURE 3. Ankle, knee and hip joint kinematic and kinetic data

Data represents the mean for all subjects (N=10) walking in each of the three conditions. Data are normalized from left heel strike to left heel strike. Ankle plantar flexion, knee extension and hip extension are positive. Ankle plantar flexion angular impulse was higher in the Increased Pushoff condition compared to the Natural Pushoff condition. There were concurrent decreases in hip flexion peak moment, peak powers, and angular impulse, and hip extension peak moment and angular impulse compared to the Natural Pushoff condition (p<0.05).