PT  - JOURNAL ARTICLE
AU  - Delyle T. Polet
AU  - Ryan T. Schroeder
AU  - John E. A. Bertram
TI  - Reducing gravity takes the bounce out of running
AID  - 10.1101/123745
DP  - 2017 Jan 01
TA  - bioRxiv
PG  - 123745
4099  - http://biorxiv.org/content/early/2017/04/28/123745.short
4100  - http://biorxiv.org/content/early/2017/04/28/123745.full
AB  - In gravity below Earth normal, a person should be able to take higher leaps in running. We asked ten subjects to run on a treadmill in five levels of simulated reduced gravity and optically tracked center of mass kinematics. Subjects consistently reduced ballistic height compared to running in normal gravity. We show that this trend is partially explained by considering maximum vertical speed during the stride (MVS). Energetically optimal gaits should balance energetic costs of ground-contact collisions (favouring lower MVS), and step frequency penalties such as leg swing work (favouring higher MVS, but less so in reduced gravity). Measured MVS scaled with the square root of gravitational acceleration, following energetic optimality predictions and explaining why ballistic height does not increase in lower gravity. While it may seem counterintuitive, using less “bouncy” gaits in reduced gravity is a strategy to reduce energetic costs, to which humans seem extremely sensitive.Summary Statement As gravity decreases, humans reduce peak vertical speed in running to optimally balance energetic costs of ground-contact collisions and frequent steps, contributing to lower vertical displacement during the non-contact phase.