Background: Walking and running are the most common means of locomotion in human daily life. People have made advances in developing separate exoskeletons to reduce metabolic rate of walking or running. However, the combined requirements of overcoming fundamental biomechanical differences between the two gaits and minimizing the metabolic penalty of exoskeleton mass make it challenging to develop an exoskeleton that can reduce the metabolic energy for both gaits. Here we show that the metabolic energy of both walking and running can be reduced by regulating the metabolic energy of hip flexion during the common energy consumption period of the two gaits using an unpowered hip exoskeleton.
Methods: We analyzed metabolic rates, muscle activities and spatio-temporal parameters from 9 healthy subjects (mean s.t.d; 24.9 ± 3.7 years, 66.9 ± 8.7 kg, 1.76 ± 0.05 m) walking on a treadmill at the speed of 1.5 m×s -1 and running at speed of 2.5 m×s -1 with different spring stiffnesses. After obtaining the optimal spring stiffness, we recruited the participants to walk and run with the optimal stiffness spring at different speeds to demonstrate the generality of the proposed approach.
Results: We found that the optimal exoskeleton spring stiffnesses for walking and running were 140 N×m Rad -1 and 210 N×m Rad -1 respectively, corresponding to 8.2% ± 1.5% (mean ± s.e.m, two-sided paired t-test: p < 0.01) and 9.1% ± 1.3% ( p < 0.01) metabolic reductions compared to walking/running without exoskeleton. The metabolic energy within tested speed range can be reduced with the assistance except for low speed walking (1.0 m s -1 ). Participants showed different changes in muscle activities with the assistance of proposed exoskeleton.
Conclusions: This paper first demonstrated that metabolic cost of walking and running can be reduced using an unpowered hip exoskeleton to regulate metabolic energy of hip flexion. The design method based on analyzing the common energy consumption characteristics between gaits may inspire future exoskeletons that assist multiple gaits. The results of different changes in muscle activities provided a new insight of human response to the same assistive principle in different gaits (walking and running).