HUMAN LOCOMOTION IN SUBGRAVITY.

The validity of experimental models simulating subgravity on earth is discussed. The mechanical characteristics of human locomotion in subgravity are perhaps better described by extrapolating data obtained at g = 1. Walking at g = I muscular energy is utilized substantially to lift the body (increase of potential energy) while the forward acceleration (increase of kinetic energy) is obtained mainly through the transformation of the potential energy into kinetic in the second phase of the step; kinetic and potential energy levels are thus mainly in phase opposition. The shift from walking to running takes place at a critical speed, 8.5 km/hr at I g, at which the changes of kinetic energy attain too high a value to be sustained only by the changes of the potential energy which are necessarily limited. For a higher speed to be attained the forward acceleration must be sustained directly by the muscular push, at the initial phase of the step; this involves a simultaneous increase of both potential and kinetic energy: in running, therefore, kinetic and potential energies are substantlally in phase. This investigation was supported by a grant from the Italian National Research Council. From the Istituto di Fisiologia Umana, Universit& di Milano, Milano, Italy. 1140 Aerospace Medicine 9 December 1964 Walking in subgravity, the lift of the body in the first phase of the step requires less energy: correspondingly less potential energy is available to sustain the forward acceleration of the body in the second phase of the step, and the critical speed at which walking is shifted to running will be correspondingly lower than on the earth. In gravity conditions such as the moon (0.16 g) walking should he practically impossible. Also maximal speed of running is lower on the moon because, for the lower weight of the subject, the vertical component of the force may be too low to maintain the adherence of the foot on the ground and prevent skidding; this depends on the conditions of the soil: if this is hard, a maximal speed of running of about 13 km/hr can be achieved, if it is covered by a deep layer of dust, the maximal speed will be about 5 kin/hr. A higher speed of progression can he obtained by recurring to another mechanism, namely jumping, which involves a higher vertical component of the push exerted by the limb and obviously a decrease of the frequency of the steps, because of the increased parabula time. Through jumping, similar or higher speed of locomotion as on earth, can possibly be obtained on the moon, depending on the structure of the soil. The possible utilization of the elastic energy of the contracted muscle is discussed: this, running on earth, is responsible for the 40 per cent of the work performed. Due to the lower step frequency, the acceleration at the HUMAN LOCOMOTION IN SUBGRAVITY-MARGARIA AND CAVAGNA start will attain a very low value on the moon, as cornpard with the earth conditions. As on the moon the work done against gravity is considerably reduced, the energy cost of speed maintenance per km covered and for a given speed value is much less than on earth. T HE MAIN FORCES acting in locomotion are: a) inertial as due to a change of the quantity of motion of the system relative to the surroundings and b) the body weight, P, which is constant. The wind resistance in still air is very small at all walking speeds, and can be neglected: such a resistance becomes appreciable only at speed values met in running. 1, 2 The body weight is determined by the mass and the gravitation values as P = M g: the mechanics of locomotion will be affected differently ff a given change of P is due to a change of M or of g. In fact, for a given g value, the mechanics of locomotion of a 35 kg subject is substantially the same as that of a subject of 70 kg, i.e. of a mass M twice as great, the only difference being the value of the forces, both inertial and gravitational, acting during a step cycle. The 70 kg subject at g = 0.5 will weigh 35 kg, and in this condition his mechanics of locomotion will be very different from that of the 35 kg subject at 1 g The prevalent importance of the value of g in locomotion is evident considering that the lowering of the center of gravity of the body, that takes place in the secorid phase of the step, both in walking and running, depends only on the acceleration of the gravity and it is independent of the body mass; on the other side a change of M implies proportional changes both of the body weight as of the inertial forces, F, that are responsible for the velocity changes a ( F = H 9 a) , while a change of g is reflected on a change of the body weight, P, only, the inertial forces being unaffected. The change of the relative importance of the body weight and the inertial forces is one of the main factors responsible for the change of the mechanics of locomotion at g < 1. At the lower limit, g = 0, and therefore P = 0, a condition that is met in interplanetary space, and/or in parabolic flight for a short time, locomotion will not be possible. Methods for studying locomotion in subgravity:The mechanics of locomotion in subgravity may be analyzed: a) through models that simulate the condition of subgravity on the surface of the earth, and b) by analyzing quantitatively the mechanics of locomotion at g = 1 and by extrapolating then the data to g < 1, to obtain the possible changes of the mechanics of locomotion. A real condition of subgravity can be obtained for a short t ime in parabolic flight: it is not easy however to have access to this technique. A condition of subgravity can be simulated in the Laboratory by applying to the subject a force opposing the body weight: this can be obtained by sustaining the subject by means of springs or light gas filled balloons, 4 or by immersion in water; 5, 6 in this last case the condition of subgravity may be simulated satisfactorily, only a new factor is introduced, i.e. ,the resistance to progression due to the high viscosity of the surrounding: because of this, locomotion in immersion 1 cm ~ ~ ~ , ~ ~ K m / h A B C D 1 e m i l I ~ ~ , ~ Km/h