Metrics of Natural Muscle Function (2010)

Natural muscle is a spectacular actuator. Why? After millions of years, nature has evolved actuators that allow breathtaking performances. Cheetahs can run, dolphins can swim, and flies can fly like no artificial technology can. It is often argued that if human technology could mimic muscle, then biological-like performance would follow. Unfortunately, the blind copying or mimicking of a part of nature [Ritzmann et al., 2000] does not often lead to the best design, for a host of reasons [Vogel, 1998]. Evolution works on the "just good enough" principle. Optimal designs are not the necessary end product of evolution. Multiple satisfactory solutions can result in similar performances. Animals do bring to our attention amazing designs, but these designs carry with them the baggage of their history. Why should these historical vestiges be incorporated into an artificial technology? Moreover, muscle design is constrained by factors that may have no relationship to human-engineered designs. Muscles must be able to grow over time, but still function along the way. Muscles remain plastic in adulthood and can self-repair. Muscles are intimately tied to pressure in the fluid system that supports them. Muscles are involved in metabolic regulation and can even serve as a source of fuel in starvation. Finally, muscles are obviously not the only part of an animal that makes spectacular performances possible. We must understand what muscle uniquely contributes to an integrated, tuned system that includes multiple muscles, joints and sensors, a transport system for fuel delivery, and a complex control system, all of which functions through skeletal scaffolding. To design an artificial muscle is a worthy endeavor. However, we strongly urge that nature's technologies provide biological inspiration for artificial technologies. Biological inspiration should involve the transfer of principles or lessons discovered in a diversity of animals. Our knowledge of biological muscle should be able to assist us in the construction of an actuator with desired performance capacities only observed in animals. However, the performance of biological actuators should not be and has not been the single design by which we measure our success. We have and will continue to design human-made actuators that exceed natural muscle in performance in particular metrics and for specific tasks. If we are to call a human-made actuator an artificial muscle, we must detail precisely the tasks that uniquely define what muscles do. Metrics can best be compared under common conditions. To develop these appropriate tests is an ongoing challenge, because we are still discovering how muscles work in animals. Moreover, engineers have a multitude of metrics that have made relevant, direct comparisons nearly impossible. The design of an artificial muscle will require novel interdisciplinary collaborations between muscle biologists and engineers. Biologists can provide inspiration and detail about what is known at present, but engineers can reciprocate with quantitative hypotheses and novel instrumentation that will lead to new tests and discoveries of muscle function.