The influence of sagittal trunk leans on uneven running mechanics.

The role of trunk orientation during uneven running is not well understood. This study compares the running mechanics during the approach step to and the stepdown of a 10-cm expected drop, positioned halfway through a 15-m runway, with that of the level step in twelve participants at a speed of 3.5 m/s while maintaining self-selected (17.7±4.2°; mean±S.D.), posterior (1.8±7.4°) and anterior (26.6±5.6°) trunk leans from the vertical. Our findings reveal that the global (i.e., the spring-mass model dynamics and centre-of-mass height) and local (i.e., knee and ankle kinematics and kinetics) biomechanical adjustments during uneven running are specific to the step nature and trunk posture. Unlike the anterior-leaning posture, running with a posterior trunk lean is characterized with increases in leg angle, leg compression, knee flexion angle and moment, resulting in a stiffer knee and a more compliant spring-leg compared with self-selected condition. In the approach versus level step, reductions in the leg length and stiffness through the ankle stiffness yield lower leg force and centre-of-mass position. Contrariwise, significant increases in the leg length, angle and force, and the ankle moment, reflect in a higher centre-of-mass position during the stepdown. Plus, the ankle stiffness significantly decreases, owing to a substantially increased leg compression. Overall, the stepdown appears to be dominated by centre-of-mass height changes, regardless of having a trunk lean. Observed adjustments during uneven running can be attributed to anticipation of changes to running posture and height. These findings highlight the role of trunk posture in human perturbed locomotion relevant for design and development of exoskeleton or humanoid bipedal robots. [Abstract copyright: © 2020. Published by The Company of Biologists Ltd.]