Human walking in the real world: interactions between terrain type, gait parameters, and energy expenditure

Humans often traverse real-world environments with a variety of surface
irregularities and inconsistencies, which can disrupt steady gait and
require additional effort. Such effects have, however, scarcely been
demonstrated quantitatively, because few laboratory biomechanical measures
apply outdoors. Walking can nevertheless be quantified by other means. In
particular, the foot’s trajectory in space can be reconstructed from
foot-mounted inertial measurement units (IMUs), to yield measures of
stride and associated variabilities. But it remains unknown whether such
measures are related to metabolic energy expenditure. We therefore
quantified the effect of five different outdoor terrains on foot motion
(from IMUs) and net metabolic rate (from oxygen consumption) in healthy
adults (N = 10; walking at 1.25 m/s). Energy expenditure increased
significantly (P < 0.05) in the order Sidewalk, Dirt, Gravel,
Grass, and Woodchips, with Woodchips about 27% costlier than Sidewalk.
Terrain type also affected measures, particularly stride variability and
virtual foot clearance (swing foot’s lowest height above consecutive
footfalls). In combination, such measures can also roughly predict
metabolic cost (adjusted , partial least squares regression), and even
discriminate between terrain types (10% reclassification error). Body-worn
sensors can characterize how uneven terrain affects gait, gait
variability, and metabolic cost in the real world.