Why does the metabolic cost of walking increase on compliant substrates?

Walking on compliant substrates requires more energy than walking on hard
substrates, but the biomechanical factors that contribute to this increase
are debated. Previous studies suggest various causative mechanical
factors, including disruption to pendular energy recovery, increased
muscle work, decreased muscle efficiency and increased gait variability.
We test each of these hypotheses simultaneously by collecting a large
kinematic and kinetic data set of human walking on foams of differing
thickness. This allowed us to systematically characterise changes in gait
with substrate compliance, and, by combining data with mechanical
substrate testing, drive the very first subject-specific computer
simulations of human locomotion on compliant substrates to estimate the
internal kinetic demands on the musculoskeletal system. Negative changes
to pendular energy exchange or ankle mechanics are not supported by our
analyses. Instead, we find that the mechanistic causes of increased
energetic costs on compliant substrates are more complex than captured by
any single previous hypothesis. We present a model in which elevated
activity and mechanical work by muscles crossing the hip and knee are
required to support the changes in joint (greater excursion and maximum
flexion) and spatiotemporal kinematics (longer stride lengths, stride
times and stance times, and duty factors) on compliant substrates.