All or none principle
All-or-none principle - a nerve or muscle cell's response to a stimulus is independent of the strength of the stimulus. If that stimulus exceeds the threshold potential (threshold of an action potential), the nerve or muscle fiber will respond completely; otherwise, there is no response.
Example, when a motor unit in the biceps brachii is stimulated it contracts the same way whether a 10 lb or 50 lb dumbbell is lifted. It is actually the number of muscle cells and the size of the motor unit that determines force output.