The compensatory strategies among lower limb muscles and its effect on metabolic cost when hip assisted by passive elastic exotendon

The unique kinesiologic characteristics of human hip joint plays a dominant role in walking. Although many studies are carried to reveal the musculoskeletal mechanisms of human walking, how to improve metabolic energy efficiency still remains substantial challenges. In this paper, we constructed a passive elastic exotendon attached to the hip joint and tried to reveal the compensatory biomechanics of muscles related to the lower limb joints. Through the muscle-driven simulations in OpenSim, we studied the net joint moments of lower limbs and changes of muscle activity in each different scenarios. It is found that adding hip exotendon not only changes muscle activity crossing the hip but also changes muscle activity crossing the knee and the ankle. Furthermore, we explored the metabolic energy consumption responding to the different design parameters of the hip elastic exoskeleton. The simulations show that the metabolic cost of walking is reduced by 3.5% compared to normal walking when the ratio of peak moment provided by hip exotendon to the normal hip peak flexion moment is in 0.7-1.0. When the ratio is greater than 1.6, the metabolic cost of walking will increase. These muscle compensatory strategies and precise metric of metabolic consumption from lower muscles revealed by the simulation are difficult to study via experiments. This work provides an alternative way to understand the kinesiologic behavior of lower limb assisted by passive exotendon, which help to guide the design of unpowered exoskeleton devices and experiments.