Active spring muscle model - a new phenomenological model of skeletal muscle mechanics

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Understanding Machina Carnis, how our muscles mechanically generate movement, is one of the fundamental research questions in human movement science and also has significant implications in many application areas including, to name a few, physiotherapy, sport/exercise science and humanoid robotics. Notwithstanding the scientific achievements over the centuries in elucidating the molecular mechanism of the muscle contraction in micro/nanoscopic levels, one would be surprised to find that our understanding of the muscle becomes substantially elusive when it comes to the actual force production. As a highlighting example, we do not yet have any model that fully explains muscle's behaviour when it is being stretched against an external force (called eccentric contraction). A well-known fact is that the muscle works as an efficient "brake" during eccentric contraction, actively stabilising itself against a stretch consuming a just minimal amount of metabolic energy. Eccentric contraction is known to make a critical contribution to the muscle's mechanical efficiency, but currently existing muscle models offer very limited explanations on eccentric contraction and little effort has been put forward to rationally recognize this issue and to develop an alternative model that has a wider explanatory scope.This "inconvenient truth" in muscle mechanics has been particularly overlooked in the upper-layer, musculoskeletal modelling studies, where simple muscle models are highly preferred in order to efficiently simulate the behaviour a large group of muscles. Despite numerous problems of the conventional muscle models in predicting dynamic contractile behaviour of the muscle, including eccentric contraction, the eighty years old Hill-type muscle model is predominantly used as a standard phenomenological model of the musculoskeletal simulation studies. This is not because the researchers in this area are unaware of its weaknesses, but because there is no alternative model that can yet replace the Hill-type muscle model.For these reasons, the proposed study aims to build and validate an effective alternative to the Hill-type muscle model. The key insight is on the recently proposed titin-based muscle contraction theories, collectively called the active spring model that shows great potential for elucidating many unexplained dynamic muscle behaviours. In addition to the traditional sliding-filament mechanism between the actin and the myosin filament, the active spring model highlights the mechanical role of titin, an additional spring-like filament that connects those filaments, in regulating the stiffness of the active muscle. It is important, however, that the proposed study does not aim to develop a purely explanatory, microscopic model of which the mechanical and parametric simplicity is often sacrificed, but aims to develop a simple and reliable phenomenological model that can be readily used by upper-layer musculoskeletal researchers. By developing such a model, the study is expected to bridge an eighty-years standing gap between muscle and musculoskeletal studies.The proposed study will take an integrative approach to achieve this goal. The study will first focus on building a model of single fiber/muscle under a controlled in vitro setup. To ensure the reliability as a general-purpose muscle mechanics model, rigorous validations will be conducted under various dynamic contractile situations, including eccentric contraction and naturalistic locomotion-like stimulation patterns. After that, the model will be further validated in the in vivo human experiment, focusing on predicting the mechanics of leg muscle during locomotion, by incorporating novel non-invasive techniques that estimate the architectural and mechanical changes of the working muscle. As a pathway to impact, the model and the simulation code will be open to general musculoskeletal modellers/researchers via OpenMuscle.org, an open-source muscle project website.