Electron micrograph of muscle Chemical analysis in the US and Britain between the 1920s and 1940s had already revealed that normal mammalian muscle is comprised of microscopic 'myofibrils' of two types of protein - actin and myosin.

How these proteins moved in relation to each other to explain muscle contraction remained a mystery. 

The most persuasive model of muscle function held that during contraction individual myofibril proteins underwent some kind of structural change and themselves contracted and relaxed in elastic fashion.