Biomechanics is the study of how animals move. It’s a very broad field, including concepts such as how muscles are used, and even how the timing of respiration is associated with moving. Biomechanics can date its beginnings back to the 1600s, when Giovanni Alfonso Borelli first began investigating animal movements. More detailed analyses by pioneers such as Etienne Jules Marey and Eadweard Muybridge, in around the late 1800s started examining the individual frames of videos of moving animals. These initial attempts led to a field known as kinematics – the study of animal movement, but this is only one side of the coin. Kinetics, the study of motion and its causes, and kinematics together provide a very strong tool for fully understanding the strategies animals use to move as well as why they move the way they do.
One factor that really changes the way an animal moves is its body size. Small animals tend to have a much more z-shaped leg posture (when looking at them from a lateral view), and so are considered to be more crouched as their joints are more flexed. Larger animals on the other hand have straighter legs, and if you look at the extreme (e.g. elephant), they have very columnar legs. Just this one change in morphology has a significant effect on the way an animal can move.
We know that the environment animals live in is not uniform, but is cluttered with many different obstacles that must be overcome to successfully move and survive. One type of terrain that animals will frequently encounter is slopes: inclines and declines. Each of the two different types of slopes impose different mechanical challenges on the locomotor system. Inclines require much greater work from the muscles to move uphill against gravity! On declines, an animal is moving with gravity and so the limbs need to brake to prevent a headlong rush down the slope. Theoretically, there are many ways an animal can achieve successful locomotion on slopes, but, to date, there has been no consensus across species or animals of differing body sizes as to whether they do use similar strategies on slopes.
From published literature we generated an overview of how animals, ranging in size from ants to horses, move across slopes. We also investigated and analysed how strategies of moving uphill and downhill change with body size, using a traditional method for scaling analyses. What really took us by surprise was the lack of information on how animals move down slopes. There was nearly double the number of studies on inclines as opposed to declines. This is remarkable given that, if an animal climbs up something inevitably it has to find a way to come back down, either on its own or by having their owner call the fire department out to help!
Most animals tend to move slower up inclines and keep limbs in contact with the ground longer; this allows more time for the muscles to generate work to fight against gravity. Although larger animals have to do more absolute work than smaller animals to move up inclines, the relative stride length did not change across body size or on inclines. Even though there is much less data in the literature on how animals move downhill, we did notice that smaller animals (<~10kg) seem to use different strategies compared to large animals. Small animals use much shorter strides going downhill than on level terrain whereas large animals use longer strides. This difference may be due to stability issues that become more problematic (more likely to result in injury) as an animal’s size increases.
Our study highlights the lack of information we have about how size affects non-level locomotion and emphasises what future work should focus on. We really do not have any idea of how animals deal with stability issues going downhill, nor whether both small and large animals are capable of moving downhill without injuring themselves. It is clear that body size is important in determining the strategies an animal will use as it moves on inclines and declines. Gaining a better understanding of this relationship will be crucial for demonstrating how these mechanical challenges have affected the evolution of the locomotor system and the diversification of animals into various ecological niches.
Image credit: Mountain goat, near Masada, by mogos gazhai. CC-BY-2.5 via Wikimedia Commons.