Answer :
If a muscle cell is stimulated to contract, allowed to partially relax, then stimulated again, its force of contraction the second time will be higher.
A single motor unit could be gradually stimulated with increasingly higher frequency action potentials to increase the contraction the muscle produces. Wave summation is the name for this phenomena. A condition known as tetanus would eventually result from the frequency of action potentials becoming so high that there would be no time for the muscle to relax between each successive stimulus.
Motor units activate asynchronously when a muscle contracts, which means that one contracts first, then another does so a split second later before the first has time to relax, and so on. Therefore, the entire muscle contraction is very smooth and controlled rather than a quick movement.
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