Walking is a motor task requiring coordination of many muscles. Previous biomechanical
studies, based primarily on analyses of the net ankle moment during stance, have concluded
different functional roles for the plantar flexors. We hypothesize that some of the
disparities in interpretation arise because of the effects of the uniarticular and
biarticular muscles that comprise the plantar flexor group have not been separated.
Furthermore, we believe that an accurate determination of muscle function requires
quantification of the contributions of individual plantar flexor muscles to the energetics
of individual body segments. In this study, we examined the individual contributions
of the ankle plantar flexors (gastrocnemius (GAS); soleus (SOL)) to the body segment
energetics using a musculoskeletal model and optimization framework to generate a
forward dynamics simulation of normal walking at 1.5 m/s. At any instant in the gait
cycle, the contribution of a muscle to support and forward progression was defined
by its contribution to trunk vertical and horizontal acceleration, respectively, and
its contribution to swing initiation by the mechanical energy it delivers to the leg
in pre-swing (i.e., double-leg stance prior to toe-off). GAS and SOL were both found
to provide trunk support during single-leg stance and pre-swing. In early single-leg
stance, undergoing eccentric and isometric activity, they accelerate the trunk vertically
but decelerate forward trunk progression. In mid single-leg stance, while isometric,
GAS delivers energy to the leg while SOL decelerates it, and SOL delivers energy to
the trunk while GAS decelerates it. In late single-leg stance through pre-swing, though
GAS and SOL both undergo concentric activity and accelerate the trunk forward while
decelerating the downward motion of the trunk (i.e., providing forward progression
and support), they execute different energetic functions. The energy produced from
SOL accelerates the trunk forward, whereas GAS delivers almost all its energy to accelerate
the leg to initiate swing. Although GAS and SOL maintain or accelerate forward motion
in mid single-leg stance through pre-swing, other muscles acting at the beginning
of stance contribute comparably to forward progression. In summary, throughout single-leg
stance both SOL and GAS provide vertical support, in mid single-leg stance SOL and
GAS have opposite energetic effects on the leg and trunk to ensure support and forward
progression of both the leg and trunk, and in pre-swing only GAS contributes to swing
initiation.