When a single nerve impulse stimulates a muscle cell, that cell will quickly contract and relax, in what is called a muscle twitch. A single twitch can be recorded electronically on what is called a myogram. Examination of a myogram shows three distinct periods within a single twitch: Latent, Contraction, and Relaxation periods.
The Latent Period is a brief period from the time of stimulation until the actual contraction begins. During this period the impulse travels down the SR, calcium ions are released and the myosin heads bond with the actin filaments.
The Contraction Period is the time during which the muscle cell actually shortens. It is during this time that the myosin heads change shape, drawing actin filaments inward towards the center of the sacromere.
A muscle twitch can be altered by the frequency of impulses (number per second). Contractions called summation are produced when the frequency of impulse is so rapid the muscle doesn't have a chance to relax completely before the next contraction. Summation produces a greater strength of contraction by adding (summation!) together many separate contractions. If the frequency of stimulation is so great that there is no time for relaxation, the muscle remains in a state of contraction called tetanus.
These aspects of muscle cell contraction are all part of how muscles work within the body. All muscular activities are the result of a series of rapid stimulations from the brain. The strength at which a muscle contracts is dependent upon the strength of the stimulus. However not every muscle cell responds to the same degree of stimulation. A muscle cell only contracts when it receives a stimulus of sufficient strength. The smallest stimulus strength to cause a contraction is called the threshold stimulus. When a muscle receives a stimulus at the threshold, it will contract at it's maximum strength. Even if the stimulus is well above the threshold, the contraction strength will be the same. This is called the All-or-None law. This means a muscle cell will contract at it's maximum strength or not at all.
This law applies only to individual muscle cells and not to a whole muscle. Muscle cells within any given muscle have varying threshold levels which allow a muscle to contract with varying degrees of strength. If the stimulus is at threshold for only 30% of the cells, only those 30% will contract. If the stimulus is greater, above threshold for 80% of the cells, then those 80% will contract, giving the muscle a stronger contraction. The greater the need for strengh for any task, the greater the stimulus from the brain: Thus the ability to lift a pen or a brick!