AUTHOR=Carrard Apolline , Fontana Elisa , Malatesta Davide 

TITLE=Mechanical Determinants of the U-Shaped Speed-Energy Cost of Running Relationship

JOURNAL=Frontiers in Physiology

VOLUME=Volume 9 - 2018

YEAR=2018

URL=https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2018.01790

DOI=10.3389/fphys.2018.01790

ISSN=1664-042X

ABSTRACT=Purpose. The aim of this study was to investigate the relationship between the energy cost of running (Cr) and speed and its mechanical determinants by comparing running in normal [100% body weight (BW)] and reduced (20% and 60% BW) gravity conditions at several speeds (2.25, 3.17, 4.08 and 5.00 m∙s-1) with experienced runners.

Methods. Twelve experienced runners (24.6±5.4 yr) ran on an AlterG treadmill in a partially randomized order at the four running speeds and at the three gravity conditions in order to assess Cr, spatiotemporal parameters, spring-mass characteristics and elastic energy (EL) during running.

Results. For the three gravity conditions, the speed-Cr per kg of body mass relationship was curvilinear (significant speed effect: P<0.001) and was significantly downward shifted with gravity (100%>60%>20% BW; P<0.001). EL, expressed in J∙step-1, was significantly higher at 100% BW than at 60% and 20% BW and at 60% BW than at 20% BW (significant gravity effect: P < 0.001) with a significant increase in EL per step depending on speed for the 3 gravity conditions (P < 0.001). EL, expressed in J∙kg-1∙m-1, was significantly downward shifted with gravity (100%>60%>20% BW; P<0.001), with no significant speed effect (P = 0.39).

Conclusions. Our findings showed that, for the three gravity conditions, the speed-Cr relationship was curvilinear, and the optimization of the stretch-shortening cycle and muscle activation in the muscle-tendon unit may be involved to explain these U-shaped relationships, especially at normal terrestrial gravitational conditions (100% BW). The U-shaped speed-Cr per kg of the body mass relationship was shifted downward in hypogravity conditions, which was characterized by decreased EL compared to 100% BW. These mechanical mechanisms may contribute to the disproportional decrease in Cr per kg of body mass relative to gravity.