How did the frequency of stimulation affect the amount of force generated by the isolated skeletal muscle when the frequency of stimulation was increased such that the muscle twitches did not fully relax between subsequent stimuli?

Muscle relaxation did not occur because of the increase of stimulation frequency to the point in which the amount of force gradually increased with each stimulus.  

This is what is known as wave summation: the overlap of muscle twitches with no relaxation of the muscle, and with the constant stimuli in a short amount of time, make this muscle contraction higher than if a single stand-alone contraction had occurred.

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