Using a linear mass-spring model of the body and leg (T. A. McMahon and G. C. Cheng. J. Biomech. 23: 65-78, 1990), we present experimental observations of human running under simulated low gravity and an analysis of these experiments. The purpose of the study was to investigate how the spring properties of the leg are adjusted to different levels of gravity. We hypothesized that leg spring stiffness would not change under simulated low-gravity conditions. To simulate low gravity, a nearly constant vertical force was applied to human subjects via a bicycle seat. The force was obtained by stretching long steel springs via a hand-operated winch. Subjects ran on a motorized treadmill that had been modified to include a force platform under the tread. Four subjects ran at one speed (3.0 m/s) under conditions of normal gravity and six simulated fractions of normal gravity from 0.2 to 0.7 G. For comparison, subjects also ran under normal gravity at five speeds from 2.0 to 6.0 m/s. Two basic principles emerged from all comparisons: both the stiffness of the leg, considered as a linear spring, and the vertical excursion of the center of mass during the flight phase did not change with forward speed or gravity. With these results as inputs, the mathematical model is able to account correctly for many of the changes in dynamic parameters that do take place, including the increasing vertical stiffness with speed at normal gravity and the decreasing peak force observed under conditions simulating low gravity.
[1]
A A Spady.
Prototype of a New Lunar-Gravity Simulator for Astronaut Mobility Studies
,
1969,
Human factors.
[2]
R Margaria,et al.
Jumping on the moon: power output at different gravity values.
,
1972,
Aerospace medicine.
[3]
G. Cavagna.
Force platforms as ergometers.
,
1975,
Journal of applied physiology.
[4]
T. McMahon,et al.
Reflex stiffness of man's anti-gravity muscles during kneebends while carrying extra weights.
,
1979,
Journal of biomechanics.
[5]
S. Andreassen,et al.
Regulation of soleus muscle stiffness in premammillary cats: intrinsic and reflex components.
,
1981,
Journal of neurophysiology.
[6]
G. Cavagna,et al.
The determinants of the step frequency in running, trotting and hopping in man and other vertebrates.
,
1988,
The Journal of physiology.
[7]
R. M. Alexander,et al.
Elastic mechanisms in animal movement
,
1988
.
[8]
R. Kram,et al.
A treadmill-mounted force platform.
,
1989,
Journal of applied physiology.
[9]
T. McMahon,et al.
The mechanics of running: how does stiffness couple with speed?
,
1990,
Journal of biomechanics.