muscle's response beyond its expected maximal response level is the result of chemical changes that occur when a muscle is subjected to frequent stimulations. The chemical changes cause an increase in muscle fiber irritability and are the result of the condition of increased temperature that is produced by frequent contractions.
When the stimulations do not allow the muscle to relax at all between stimuli, individual contractions fuse together into a single sustained contraction called a tetanus. Like the contractions produced during the condition of summation, a tetanic contraction is produced by adding the contractions of the individual stimuli together. A tetanic contraction is a muscle's greatest response, and most human movements involve tetanic responses due to the rapid fire of nerve impulses produced by most stimuli. A muscle cannot sustain a tetanic contraction for more than a short period of time before fatigue sets in. Fatigue is caused by the build up of lactic acid and by the depletion of the muscle's energy reserves.
