Musculoskeletal modelling is building computer models that represent the jointed skeleton, muscles and tendons of a living animal. The models are used to run simulations (goal-directed activities solved by computers) in which they recreate the forces and movements associated with activities like jumping and running. Musculoskeletal models of humans and other animals are used to understand how the muscles, skeletons and joints work together to support and move the body.
Why do we need to create complicated computer models and simulations to understand movement when we could just analyse the real animals themselves? The anatomy of movement is very complicated. Animal limbs have many joints and even more muscles (over 40 in the human leg) that interact in complex and unexpected ways, and so trying to puzzle out how all of the many components work together from anatomy alone is very difficult. We have the technology to measure muscle activity patterns and forces during movement, but this requires sensors to be placed on or in the muscles themselves. As the majority of limb muscles are so-called ‘deep’ muscles (underneath other muscles), there is currently no way to attach sensors to every muscle without disabling the limb.
Recording data from living animals can therefore only give us an incomplete picture of how the complex mechanical system of muscles, bones and joints functions during movement. A musculoskeletal model, however, copies the anatomy of the living animal, with each virtual muscle given the same size, power and attachments as their counterparts in the real animal. When the model is made to perform a simulation of movement, the function that each virtual muscle (or tendon, or joint, etc.) performs should then be close to the function the equivalent component performs in the real animal. The activities and functions of the various parts, as well as the way they cooperate as a whole, can then simply be read out from the computer once the simulation has finished running.
Models and simulations have been used to analyse and treat many problems with limbs in humans – analysing causes of injuries in sport, aiding injury rehabilitation, studying and treating cerebral palsy and other neuromuscular problems, designing prosthetic limbs, and more. How and why are we using them to study movement in extinct animals?
All that typically remains of extinct animals are skeletons. We aim to make those skeletons move again. For that, we need to add muscles to them. Thankfully, muscles often leave characteristic bumps and scars where they attach onto the bones. From careful examination of the limb anatomy of living relatives (birds and crocodiles, in our case), we have previously been able to determine which muscles are associated with which bumps and which scars on the bones. This allows us to reconstruct the muscular anatomy of extinct pseudosuchians and dinosaurs with reasonable confidence. Over evolutionary time, the bones of pseudosuchians and dinosaurs changed shape and muscle scars moved. This gives us a good idea of how the muscular anatomy of their limbs evolved. However, knowing where a muscle is does not mean that you know what it does. What do these anatomical changes mean for how movement evolved?
This is where the musculoskeletal modelling comes in. We are building musculoskeletal models of living birds and crocodiles that can faithfully simulate the movements and forces they use when running, jumping, turning, standing and walking. This will allow us to understand the function that each muscle does during these activities. Combining this knowledge of muscle function with our reconstructions of muscle anatomy allows us to build moving musculoskeletal simulations of extinct pseudosuchians and dinosaurs. The evolutionary changes in limb anatomy that have taken place in both of these groups can then be understood in terms of changes in limb forces and movements.
Just as knowledge of living anatomy allows us to reconstruct anatomy in extinct animals, detailed knowledge of limb function in living birds and crocodiles will allow us to estimate those functions in pseudosuchians and dinosaurs. Musculoskeletal modelling and simulation give us an approach for obtaining that knowledge. We then can compare how different living and extinct animals work(ed) and how their behaviours evolved along with anatomical changes. This may answer why dinosaurs became so successful after the Triassic period, whereas pseudosuchians (except the crocodile lineage itself) did not.