Term that refers to how often stimulus is applied (or rate of delivery).

The image shown is an example of ____________________. This occurs if a second stimulus arrives IMMEDIATELY AFTER the RELAXATION phase has ENDED. The resulting contraction will have a slightly higher tension than the first contraction. Tension rises in stages (like a staircase, hence its name).

The image shown is an example of ____________________. This happens when another action potential (stimulus) occurs BEFORE the muscle is able to RELAX (before the relaxation phase). MORE calcium (Ca2+) is released resulting in MORE binding sites being revealed. As a result, MORE force is generated in that muscle fiber. In this case, twitches add to each other (get closer together in time).

If repeated stimuli are applied at a fast rate, then no relaxation occurs between the stimuli, muscle reaches maximum tenstion and a sustained contraction phase is obtained.

The ______________ increases, more and more motor units are stimulated, and thus the strength of muscle contraction increases.