Models of the electro-mechanical activity of the cardiac muscle can be very useful in computing stress, strain and action potential fields from three-dimensional image processing. We designed a chemically-controlled constitutive law of cardiac myofibre mechanics, acting on the mesoscopic scale and devoted to be embedded into a macroscopic model. This law ensues from the modelling of the collective behaviour of actin-myosin molecular motors, acting on the nanoscopic scale to convert chemical into mechanical energy. The resulting dynamics of sarcomeres, acting on the microscopic scale, is shown to be consistent with the “sliding filament hypothesis”, which was first introduced by A. F. Huxley [1].