The All or None Law is a physiological principle which relates to the response to stimulus in excitable tissues. It was first established for contraction of heart muscle by Henry P Bowditch in 1871. It was later found that the all or none law was also applicable to skeletal muscle contraction.
“An induction shock produces a contraction or fails to do so according to its strength; if it does so at all, it produces the greatest contraction that can be produced by any strength of stimulus in the condition of the muscle at the time.” – Henry Bowditch
In other words, an action potential (stimulus) will either cause complete activation of the muscle fibre or none at all. The action potential must meet or pass a certain threshold (usually between 55mV-65mV) in order for a contractile response to occur. The variants in contractile muscle strength come from the number of fibres recruited and the increase in individual muscle fibre size and strength due to an increase in the number and size of myofibrils.