Sometime in the near future humans may walk in the reduced gravity of Mars. Gravity plays an essential role in walking. On Earth, the body uses gravity to ‘fall forwards’ at each step and then the forward speed is used to restore the initial height in a pendulum-like mechanism. When gravity is reduced, as on the Moon or Mars, the mechanism of walking must change1. Here we investigate the mechanics of walking on Mars onboard an aircraft undergoing gravity-reducing flight profiles. The optimal walking speed on Mars will be 3.4 km h−1 (down from 5.5 km h−1 on Earth) and the work done per unit distance to move the centre of mass will be half that on Earth.
In contrast to swimming or flying, during which the fins or wings can slide against the surrounding medium, locomotion on a solid surface is constrained by the link between the centre of gravity of the body and the fixed point of contact of the foot on the ground. After foot contact, this link leads to a forward deceleration which must be compensated for by a subsequent forward acceleration in order to maintain a constant average speed of locomotion. This increases the cost of terrestrial locomotion.
However, this cost is contained by the transfer of kinetic energy into gravitational potential energy during the deceleration (when the body rides upwards on the leg after heel strike) and the subsequent recovery of kinetic energy from the potential energy during acceleration. The recovery of mechanical energy by this mechanism is described by
R = (W f+ W v− W cm)/(W f+ W v)
where W fis the work needed to increase the kinetic energy, W vis the work to increase the potential energy, and W cmis the work to increase the total mechanical energy of the centre of mass. With an ideal, frictionless pendulum, W cmwould be nil and R equal to 1. With walking on Earth, R attains a maximum of 0.7 at about 5.5 km h−1 (Fig. 1) near the speed at which the energy expenditure per unit distance is at a minimum2. At higher and lower speeds, R decreases and the energy cost increases.
We investigate the mechanics of locomotion on Mars in three male subjects (of weight and height, respectively, of 77 kg, 1.79 m; 92 kg, 1.93 m; 86 kg, 1.79 m) walking at different speeds in a Martian gravity (0.4 g, maintained for about 30 s) on a force platform (3 m × 0.4 m) sensitive to the force exerted in both forward and vertical directions3. The platform was fixed to the floor of a KC-135 and an A300 Airbus aeroplane during the 23rd and 24th European Space Agency parabolic flight campaigns. The force signals from the plate (also measured in ref. 4) were analysed5 to determine W cmand R.
Figure 1 shows that on Mars the maximum pendular recovery of mechanical energy R is reduced to 0.6 and occurs at the lower speed of 3.4 km h−1. In spite of the lower R, the minimum mechanical work done per unit distance to maintain the motion of the centre of mass on Mars is about one half of that on Earth. In general, walking a given distance at any absolute speed will be cheaper on Mars than on Earth. In fact, the energy consumption measured during locomotion in simulated partial gravity is less than that at 1 g (refs 6,7). A decrease in the maximum speed of walking was also observed in partial gravity simulators7,8.
On Mars, then, both the optimal walking speed and the range of possible walking speeds will be about half those on Earth. The walk-run transition on Mars will occur near the optimal walking speed on Earth, and the mechanical work done to walk a given distance on Mars will be about half of what it would be on Earth. So, energy expenditure will probably be lower and locomotion smoother on Mars.
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