Predictive simulation of gait at low gravity reveals skipping as the preferred locomotion strategy

Abstract

The investigation of gait strategies at low gravity environments gained momentum recently as manned missions to the Moon and to Mars are reconsidered. Although reports by astronauts of the Apollo missions indicate alternative gait strategies might be favored on the Moon, computational simulations and experimental investigations have been almost exclusively limited to the study of either walking or running, the locomotion modes preferred under Earth's gravity. In order to investigate the gait strategies likely to be favored at low gravity a series of predictive, computational simulations of gait are performed using a physiological model of the musculoskeletal system, without assuming any particular type of gait. A computationally efficient optimization strategy is utilized allowing for multiple simulations. The results reveal skipping as more efficient and less fatiguing than walking or running and suggest the existence of a walk-skip rather than a walk-run transition at low gravity. The results are expected to serve as a background to the design of experimental investigations of gait under simulated low gravity.

Figure 1

Stick figures of predicted gait patterns using the effort cost function at a locomotion speed of 1.1 m/s. (Bars indicate ground contact of the color-matching foot.)

Figure 2

Stick figures of predicted gait patterns using the fatigue cost function at a locomotion speed of 1.1 m/s. (Bars indicate ground contact of the color-matching foot.)

Figure 3

Stick figures of predicted gait patterns using the effort cost function at a locomotion speed of 2.0 m/s. (Bars indicate ground contact of the color-matching foot.)

Figure 4

Stick figures of predicted gait patterns using the fatigue cost function at a locomotion speed of 2.0 m/s. (Bars indicate ground contact of the color-matching foot.)

Figure 5

Predicted vertical ground reaction forces in simulations using the effort cost function at a locomotion speed of 2.0 m/s, where “l” indicates the left leg and “r” the right leg . Note that, for this speed and cost function, walking (W), running (R) and skipping (S) were predicted on Earth on Mars and on the Moon, respectively.

Figure 6

Predicted vertical and horizontal ground reaction forces in simulations at a locomotion speed of 1.1 m/s. The walking pattern on Earth was obtained by tracking normative data by Winter (1991). The skipping pattern on the Moon is obtained by minimization of the “effort” cost function.

Figure 7

Predicted left leg muscle activations in simulations using the effort cost function at a locomotion speed of 1.1 m/s.

Figure 8

Predicted left leg muscle activations in simulations using the fatigue cost function at a locomotion speed of 1.1 m/s.

Figure 9

Predicted left leg muscle activations in simulations using the effort cost function at a locomotion speed of 2.0 m/s.

Figure 10

Predicted left leg muscle activations in simulations using the fatigue cost function at a locomotion speed of 2.0 m/s.

Figure 11

Gait types predicted at each condition of speed and gravity acceleration. The diagram also shows the walk-run transition speeds during experiments at different simulated low gravity accelerations by Kram et al. (1997), and the walk-run transition speed predicted by dynamic similarity with a Froude number Fr = 0.5 and h = 0.897 m in Eq. 1.