Biology subjects

Weihmann, T.

Publications and source records attributed to Weihmann, T..

3 recordsLinked to original sources

bioRxiv2021

The smooth transition from many-legged to bipedal locomotion: Gradual ground force reduction and its impact on total ground reaction forces, body dynamics and gait transitions

Most terrestrial animals move with a specific number of propulsive legs, which differs between clades. The reasons for these differences are often unknown and rarely queried, despite the underlying mechanisms being indispensable for understanding the evolution of multilegged locomotor systems in the animal kingdom and the development of swiftly moving robots. Moreover, when speeding up, a range of species change their number of propulsive legs. The reasons for this behaviour have proven equally elusive. In animals and robots, the number of propulsive legs also has a decisive impact on the movement dynamics of the centre of mass. Here, I use the leg force interference model to elucidate these issues by introducing gradually declining ground reaction forces in locomotor apparatuses with varying numbers of leg pairs in a first numeric approach dealing with these measures impact on locomotion dynamics. The effects caused by the examined changes in ground reaction forces and timing thereof follow a continuum. However, the transition from quadrupedal to a bipedal locomotor system deviates from those between multilegged systems with different numbers of leg pairs. Only in quadrupeds do reduced ground reaction forces beneath one leg pair result in increased reliability of vertical body oscillations and therefore increased energy efficiency and dynamic stability of locomotion. Significance statementThe model grants access to the effects of gradual ground force reduction on total ground reaction forces, body dynamics and gait transitions.

biophysics↗

bioRxiv2020

Rules of nature's Formula Run: Muscle mechanics during late stance is the key to explaining maximum running speed

The maximum running speed of legged animals is one evident factor for evolutionary selection--for predators and prey. Therefore, it has been studied across the entire size range of animals, from the smallest mites to the largest elephants, and even beyond to extinct dinosaurs. A recent analysis of the relation between animal mass (size) and maximum running speed showed that there seems to be an optimal range of body masses in which the highest terrestrial running speeds occur. However, the conclusion drawn from that analysis--namely, that maximum speed is limited by the fatigue of white muscle fibres in the acceleration of the body mass to some theoretically possible maximum speed--was based on coarse reasoning on metabolic grounds, which neglected important biomechanical factors and basic muscle-metabolic parameters. Here, we propose a generic biomechanical model to investigate the allometry of the maximum speed of legged running. The model incorporates biomechanically important concepts: the ground reaction force being counteracted by air drag, the leg with its gearing of both a muscle into a leg length change and the muscle into the ground reaction force, as well as the maximum muscle contraction velocity, which includes muscle-tendon dynamics, and the muscle inertia--with all of them scaling with body mass. Put together, these concepts characteristics and their interactions provide a mechanistic explanation for the allometry of maximum legged running speed. This accompanies the offering of an explanation for the empirically found, overall maximum in speed: In animals bigger than a cheetah or pronghorn, the time that any leg-extending muscle needs to settle, starting from being isometric at about midstance, at the concentric contraction speed required for running at highest speeds becomes too long to be attainable within the time period of a leg moving from midstance to lift-off. Based on our biomechanical model we, thus, suggest considering the overall speed maximum to indicate muscle inertia being functionally significant in animal locomotion. Furthermore, the model renders possible insights into biological design principles such as differences in the leg concept between cats and spiders, and the relevance of multi-leg (mammals: four, insects: six, spiders: eight) body designs and emerging gaits. Moreover, we expose a completely new consideration regarding the muscles metabolic energy consumption, both during acceleration to maximum speed and in steady-state locomotion.

zoology↗

bioRxiv2018

Leg force interference in poly-pedal locomotion

The examination of gaits and gait-changes have been the focus of movement physiology and legged robot engineering since the first emergence of the fields. While most examinations focussed on bipedal and quadrupedal designs many robotic implementations rely on the higher static stability of three or more pairs of legs. Nevertheless, examinations of gait-changes in the biological models, i.e. poly-pedal arthropods such as insects or arachnids, are rare. Except for the well-known change from slow feedback controlled walking to a fast, feedforward controlled running gait, no changes are known or are deemed to be of low significance.\n\nHowever, recent studies in fast moving spiders, mites and cockroaches have revealed an additional gait change also for the transition from intermediate to high running speeds. This change is similar to gait transitions as found in quadrupedal vertebrates.\n\nAccordingly, the present approach aims to extend available theory to poly-pedal designs and examines how the number of active walking legs affects body dynamics when combined with changing duty factors and phase relations. The model shows that higher numbers of active leg pairs can prevent effective use of bouncing gaits such as trot and their associated advantages because significantly higher degrees of leg synchronisation are required. It also shows that small changes in the leg coordination pattern have a much higher impact onto the COM dynamics than in locomotor systems with fewer legs. In this way, the model reveals coordinative constraints for specific gaits facilitating the assessment of animal locomotion and economization of robotic locomotion.\n\nSignificance StatementThe present model approach enables to assess the impact of different numbers of walking legs onto movement dynamics and gait choice in terrestrial legged locomotion. The models results are indicatory for research in legged locomotion regardless whether biological examples or legged walking machines are considered. The approach is suitable for all numbers of pairs of walking legs larger than two and is focussed on symmetrical gaits as found in straight and continuous locomotion. The model fills a gaping gap as the impact of phase shifts among the legs in the coordinated sets of legs typical for poly-pedal animals and robots on overall body dynamics are not considered sufficiently in existing dynamic model approaches.

zoology↗