Utilising rigid body mechanics to measure physiological outcomes as motion is all well and good, although it is hard to fathom the meaning of joint extension angles to handlebar reach without gaining an understanding of muscle physiology. Muscles have an optimal operating condition of speed and contraction length which defines our physical limits to produce power along with our respiratory system. Therefore, to develop a complete understanding of bicycle fit and the limitations of endurance, the mechanism behind muscle contractions are now explored.