Abstract
The role of trunk orientation during uneven running is not well understood. This study compared the running mechanics during the approach step to and the step down for a 10 cm expected drop, positioned halfway through a 15 m runway, with that of the level step in 12 participants at a speed of 3.5 m s-1 while maintaining self-selected (17.7±4.2 deg; mean±s.d.), posterior (1.8±7.4 deg) and anterior (26.6±5.6 deg) 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 by increases in leg angle, leg compression, knee flexion angle and moment, resulting in a stiffer knee and a more compliant spring-leg compared with the self-selected condition. In the approach step versus the level step, reductions in leg length and stiffness through the ankle stiffness yield lower leg force and centre-of-mass position. Contrariwise, significant increases in leg length, angle and force, and ankle moment, reflect in a higher centre-of-mass position during the step down. Plus, ankle stiffness significantly decreases, owing to a substantially increased leg compression. Overall, the step down 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 the design and development of exoskeleton or humanoid bipedal robots.
Highlights
Running is a low cost and easy accessibility form of physical activity that enhances health and increases longevity (Lee et al, 2017)
Unlike the anteriorleaning 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 springleg compared with self-selected condition
These findings highlight the role of trunk posture in human perturbed locomotion relevant for design and development of exoskeleton or humanoid bipedal robots
Summary
Running is a low cost and easy accessibility form of physical activity that enhances health and increases longevity (Lee et al, 2017). The popularity of running continues to grow worldwide. In 2019 alone, for instance, millions of recreational participants around the world covered 1.3 billion total miles with an average distance of 4.1 miles per run (Strava, 2019). Running outdoor in a natural environment often entails a frequent negotiation of the terrain irregularities such as variations in ground compliance, slipperiness, or substrate height. In running over changing surfaces, human runners appear to use springmass dynamics to help passively stabilise their locomotion (Blickhan, 1989; McMahon and Cheng, 1990). The spring-mass model (Blickhan, 1989) is comprised of a mass-less spring and the body (represented by a point mass), and is described by: the leg stiffness (kleg), leg orientation (θTD)
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