BIOMECHANICS OF TERRESTRIAL LOCOMOTION: ASYMMETRIC OCTOPEDAL AND QUADRUPEDAL GAITS

The main goal of this dissertation is to investigate the biomechanics of octopedal and quadrupedal locomotion in terrestrial animals, common determinants, advantages and limits, in particular of the asymmetric gaits. Two different approach have been chosen: i) a kinematic study of a terrestrial spider, the Brazilian giant tawny-red tarantula, an octopods predator species that hide in burrows, ambush and rapidly bounce the prey with a sprint, and ii) a comparative study of the two types of gallop of the cursorial terrestrial mammals. Eight-legs locomotion has been one of the first travelling modes on land, and spiders display one of the most versatile locomotor repertoire: they move at slow and fast speed, forward-backward-sideways, they climb and even jump, both on firm terrain and from the water surface. Spiders can walk in the two senses at the same speed, just by reversing their diagonal footfall scheme. They turn on the spot like an armoured tank, with opposite direction of the two treads of limbs. Also, the high number of limbs ensures an increased locomotor versatility on uneven and rough terrains, particularly in the likely unawareness of each endpoint location on the ground. The aims of this first part were: i) identifying the principal octopod gaits, ii) calculating the mechanical external and internal work at the different speeds/gaits, iii) assessing any tendency to exchange potential and kinetic energy of the body centre of mass, as in pendulum-like gaits, and iv) evaluating how spiders’ mechanical performance and variables allometrically compare to other species. Another question was: can the octopod gaits be considered as different combinations of two quadrupeds’ locomotion? In this investigation we used inverse dynamics to study the locomotor performance of a terrestrial spider. 9 reflective markers have been placed on the tip of the 8 legs and on the cephalothorax, and their position recorded at a frequency of 50 Hz and digitized through a motion analysis system. Data have been processed using LabView (National Instruments, USA) specific development. The 3D trajectories of the body centre of mass in local coordinates, as during locomotion on a treadmill, have been calculated by applying a mathematical method based on the Fourier analysis of the three coordinates of the centre of mass (COM) over time. Two main gaits, a slow and a fast one characterised by distinctive 3D trajectories of COM, have been identified. The calculated total mechanical work (= external+internal) and metabolic data from the literature allowed estimating the locomotion efficiency of this species, which resulted less than 4%. Octopod gait pattern due to alternating limb support, which generates asymmetrical COM trajectories and a small but consistent energy transfer between potential and kinetic energies of COM, can be considered as formed by two subsequent quadrupeds, where the first two pairs of feet (1 and 2) are the fore and the hind feet of the first quadruped, and the third and fourth pairs are the fore and hind feet of the second quadruped. The two quadrupeds are almost in phase, being the first and third pairs synchronised in their movements as well as the second and fourth. Octopedal locomotion exhibits two main gaits, neither of which incorporating a flight phase, characterised by a consistent limb pattern and a small but remarkable energy recovery index. Gallop has been chosen as model of asymmetric cursorial locomotion in quadrupeds. In transverse gallop the placement of the second hind foot is followed by that of the contralateral forefoot, while in rotary gallop is followed by the ipsilateral forefoot, and the sequence of footfalls appears to rotate around the body. The question are: why two models of gallop? Are they specie-specific? Which are the biomechanical determinants of the choice between transverse and rotary gallop? Aims of this part of the research were: i) assess, when possible, the specie-specificity of the gallop type in different cursorial mammal species, ii) phylogenetically classify the investigated species, iii) Made a comparative analysis based on morphological, physiological and environmental differences. 351 filmed sequences have been analysed to assess the gallop type of 89 investigated mammal species belonging to Carnivora, Artiodactyla and Perissodactyla orders. 23 biometrical, ecological and physiological parameters have been collected for each species both from literature data and from experimental measures. Most of the species showed only one kind of gallop: transverse (42%) or rotary (39%), while some species performed rotary gallop only at high speed (19%). In a multivariate factorial analysis the first principal component (PC), which accounted for 40% of the total variance, was positively correlated to the relative speed and negatively correlated to size and body mass. The second PC was correlated to the ratio between autopodial and zygopodial limb segments. Large size and longer proximal limb segments resulted associated to transverse gallop, while rotary and speed dependent species showed higher metacarpus/humerus and metatarsus/femur length ratio and faster relative speeds. The maximum angular excursion resulted proportional to the maximum Froude number, and significantly higher in rotary galloper. The gait pattern analysis provided significant differences between transverse and rotary gallop in fore and hind duty factor, and in duration of the fore contact. Our results assessed that a typical gallop gait is adopted by a large number of mammal species, and indicated that the gallop pattern depends on diverse environmental, morphometrical and biomechanical characters. Even if mammals and spiders can be considered far and different worlds, we can recognize common pattern of locomotion. The quadruped gaits have been modelled as the combination of two biped gaits with some difference in the phase-cycle, in the same way, we described the octopods gaits as the combination of two quadruped gaits in series. In conclusion, this work shed light on some aspects of octopedal and quadrupedal asymmetric gaits, opening to the raising of new questions and new perspective of research.