Here we discuss exoskeletons that act in parallel with the
human lower limb for load transfer to the ground. Perhaps
an in-series leg exoskeleton like the SpringWalker (Figure 1c) increases the metabolic cost of running
because the limb length of the human plus machine is
substantially increased, thereby increasing both the work
at the hip to protract the leg during the aerial phase and
the overall energetic demand to stabilize movement, overcoming
any potential advantage of extending limb length.
Additionally, with an in-series leg exoskeleton device, the
ground reaction forces are still borne by the human leg. In
contrast, with a parallel mechanism, body weight could
be transferred through the exoskeleton directly to the
ground, decreasing the loads borne by the biological
limbs and lowering the metabolic demands to walk, run,
and hop. Furthermore, such a parallel exoskeleton would
not increase limb length, thereby not increasing the overall
energetic demand to stabilize movement.