THE EFFECTS OF GRAVITY ON HUMAN LOCOMOTION REPERTOIRE: COST OF TRANSPORT & BODY CENTRE OF MASS ANALYSIS

Human legged locomotion has been widely studied from both mechanical and bioenergetics points of view, however some aspects are still unaddressed and this thesis aimed to analysed some of them. One of the two methods for calculating muscular work during locomotion, which is an interesting parameters that can describe locomotion and subjective featuring, concerns the body centre of mass (BCoM) movements. The BCoM is the ideal point of the body where all forces act, and especially in a multi segment body as the human body, it is much easier and useful to calculate and follow its trajectory as the movement of the whole body. In order to compute BCoM two methods can be used: a double integration of the ground reaction forces, the forces exerted by feet when in contact to the ground, based on Newton’s second law, which is considered the gold standard, and called Direct Dynamics; and the weighted mean of segments centre of mass (COM) obtained by motion analysis, called Inverse Dynamics. Segments mass and COM location are based on anthropometric tables, which are scaled on subjects’ lengths; this is an approximation and assumes that segments are rigid, introducing potential errors. Even if there is not a complete 3D validation of Inverse Dynamics as a function of speed in the human gaits, Inverse and Direct Dynamics are often used interchangeably. In the first part of the thesis Inverse and Direct Dynamics in the human locomotion repertoire were compared in order to analyse different models, based on different anthropometric tables, and validate Inverse Dynamics. BCoM trajectory in walking, running and skipping is well described by Inverse Dynamics models employing a whole body marker set, where the main body segments are considered for BCoM calculation. On the contrary, simplified estimation models employing few markers, such as just one marker on the trunk or the mean of the pelvis, poorly match Direct Dynamics trajectory. Same results come from the further analysis of muscular work, where whole body models better describe and match Direct Dynamics data. Some interesting observations emerged from these analyses: i) two anthropometric tables with quite different segments definition reach the same results; ii) whole body models of Inverse Dynamics well matched Direct Dynamics values, validating this methods, whereas poor models should not be employed anymore; iii) the difference between Inverse and Direct Dynamics in the same gait is almost speed independent highlighting a systematic error, and among gaits it shows the same trend; iv) race walking BCoM trajectory cannot be described with any Inverse Dynamics models, therefore only ground reaction forces should be used for computation. Skipping is the third paradigm of human locomotion. Differently from walking and running, it was only investigated on level ground, addressing the much expensive cost of transport as the reason for its under use in day life activity; conversely it was displayed by astronauts of Apollo missions on the Moon. In the second part of this thesis the mechanics and bioenergetics of skipping on gradient was investigated since Ed Mitchell during Apollo 14 mission explicitly said “That nice skipping gait that I liked was very easy to do going downhill”. Gradient range was ±15%, the range of gradient presents on the Moon. On Earth skipping cost is higher than walking and running at all gradients and it decreases with speed, differently from the other gaits no minimum was found during downhill skipping, and it is impossible to skip at positive gradient steeper than 5% due to muscular demand and consequent fraction of metabolic power. When analysing mechanical parameters, the work done by muscles to move BCoM (WEXT) and the work done to accelerate limbs with respect to BCoM (WINT), skipping changes are similar to running with WEXT decreasing with downhill gradient and increasing speed, whereas WINT increases with speed. These results show that skipping on gradient can be performed on Earth only downhill due to the great metabolic demand. However, skipping cost of transport is always higher than walking and running at the same slopes. Based on these findings and astronauts’ choices, we could expect that gravity plays an important role on skipping and locomotion cost of transport, which are analysed and discussed in the third part of this thesis. Low gravity locomotion can be studied on Earth with different methods, the gold standard is the parabolic flight, since with the adequate angle of the airplane parabola it is possible to obtain gravity levels ranging from hypo-gravity (including 0 g) to hyper-gravity. However the time available at low gravity simulation is only about 30 seconds, which is too short for metabolic measurements. The second most used method is based on the body weight suspension, where subjects are unloaded of the desired body weight by the suspension of the body via bungee cords or springs. We re-vamped the Margaria’s low- gravity ‘cavedium’ with a treadmill and two bungee cords free to stretch until 16 m and let subjects walk, run and skip on a range of speed with Moon and Mars gravity, in order to study cost of transport and biomechanical parameters. Walking range of speed decreases with gravity and cost of transport decreases by 18% in hypo-gravity; higher decrements are shown in bouncing gaits, running and skipping. On the Moon their cost is the same and comparable with terrestrial walking values. Being on Earth was almost 40% higher than running, skipping shows the best decrease and a threefold gain in operative speed. This means that on the Moon human can skip three times faster than on Earth with the same metabolic power, whereas running gain is only twofold. Mechanically these cost changes can be explained by a reduction in pendulum-like recovery of energy in walking that needs a higher muscular work, whereas in skipping it is not shown. Moreover WEXT is lower in low gravity and a greater reduction of WINT in skipping compared with running can partially explain the major reduction in skipping cost. Another interesting aspect related to gait mechanics regards stability, and when the surface is slippery, as on the Moon due to regoliths, balance during support phase becomes an important issue. Skipping, compared to running, involves a shorter stance phase and also a double support, in which the trajectory of the flight can be adjusted. Moreover higher vertical forces on the ground and a greater angle at take off let the foot to be less slippery when pushing the body forward. Based on this biomechanical and bioenergetics analyses it can be concluded that human locomotion on hypo-gravity planets will be a bouncing gait and probably skipping could be preferred to running. Secondly the decrease of skipping cost up to walking values on Earth can explain the astronauts’ choice of skipping during Apollo missions.