If it was 100% efficient, then air resistance would be the main loss. But the losses from something like: (motion → hydraulic cylinder → hydraulic motor → electric motor → power electronics → energy storage → power electronics → electric motor → hydraulic motor → hydraulic cylinder) are pretty substantial.
Springs are more efficient, and animals use protein elasticity to store some energy while walking/running.
How efficient could walking be with effective regenerative breaking in each step?
If it was 100% efficient, then air resistance would be the main loss. But the losses from something like: (motion → hydraulic cylinder → hydraulic motor → electric motor → power electronics → energy storage → power electronics → electric motor → hydraulic motor → hydraulic cylinder) are pretty substantial.
Springs are more efficient, and animals use protein elasticity to store some energy while walking/running.