We have had success with one variation of this system using a leaf spring embedded in the carbon fiber structs attached to the calf segment. The spring system helped reduce battery consumption. However, the controller needed to be modified to throttle the motor during stance phase to prevent the motor from fighting the spring.

Please see our previous paper for an example:

[1] G. Orekhov and Z. F. Lerner, “Design and Electromechanical Performance Evaluation of a Powered Parallel-Elastic Ankle Exoskeleton,” IEEE Robotics and Automation Letters, vol. 7, no. 3, Art. no. 3, Jul. 2022, doi: 10.1109/LRA.2022.3185372.