Predictive Musculoskeletal Simulations of the Emu Resolve the Avian Grounded Running Paradox

Birds and (adult) humans utilize very different running styles. Unlike
humans, birds adopt “grounded running” at intermediate speeds – a
transitional running gait where at least one foot always maintains
ground contact. This is a paradox: animals tend to minimize locomotor
energy expenditure, but birds apparently select grounded running
despite incurring higher energy costs.
Walking is a vaulting gait, the center of mass (COM) is at its highest
point during midstance. In contrast, running is a bouncing gait, the COM
is at its lowest during midstance. Grounded running thus combines
running mechanics with kinematic features related to walking. Current
explanatory hypotheses for avian grounded running focus on two
aspects of bird anatomy: very long hindlimb tendons that store elastic
energy, and muscle-length tunings that (combined with forward COM
placement) result in crouched leg postures. Their relative contributions
to grounded running are untested, because they cannot be isolated
in real birds.
We performed predictive gait simulations of the emu (Dromaius
novaehollandiae), allowing us to decouple the effects of tendon
elasticity and postural tuning. We demonstrate that grounded running
actually represents an energetic optimum for birds, resolving the
paradox. The avian body plan prevents (near) vertical leg postures
while running, making the running style used by humans impossible.
Under this anatomical constraint, grounded running is optimal if the
muscles produce the highest forces in crouched postures, as is true
in most birds. Anatomical similarities between birds and non-avian
dinosaurs suggest that, as a behavior, avian grounded running first
evolved within non-avian theropods.