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First published online March 14, 2008
Journal of Experimental Biology 211, 1087-1092 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.012443
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The effect of increasing inertia upon vertical ground reaction forces and temporal kinematics during locomotion

John K. De Witt1,*, R. Donald Hagan2 and Ronita L. Cromwell3

1 Wyle's Life Sciences Group, Houston, TX 77058, USA
2 NASA – Johnson Space Center, Houston, TX, USA
3 Department of Physical Therapy, The University of Texas Medical Branch, Galveston, TX, USA

* Author for correspondence (e-mail: john.k.dewitt{at}nasa.gov)

Accepted 29 January 2008

The addition of inertia to exercising astronauts could increase ground reaction forces and potentially provide a greater health benefit. However, conflicting results have been reported regarding the adaptations to additional mass (inertia) without additional net weight (gravitational force) during locomotion. We examined the effect of increasing inertia while maintaining net gravitational force on vertical ground reaction forces and temporal kinematics during walking and running. Vertical ground reaction force was measured for 10 healthy adults (five male/five female) during walking (1.34 m s–1) and running (3.13 m s–1) using a force-measuring treadmill. Subjects completed locomotion at normal weight and mass and at 10, 20, 30 and 40% of added inertial force. The added gravitational force was relieved with overhead suspension, so that the net force between the subject and treadmill at rest remained equal to 100% body weight. Ground reaction forces were affected by the added inertial force, but not to the magnitude predicted by the increase in mass, suggesting that adaptations in motion occurred. Vertical ground reaction force production and adaptations in gait temporal kinematics were different between walking and running. Peak vertical impact forces and loading rates increased with increased inertia during walking, and decreased during running. As inertia increased, peak vertical propulsive forces decreased during walking and did not change during running. Stride time increased during walking and running, and contact time increased during running. The increased inertial forces were utilized independently from gravitational forces by the motor control system when determining coordination strategies.

Key words: biomechanics, locomotion, ground reaction force, inertia, gravity, weight, mass, human


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