The stimulus interval has a profound effect on the contraction force of the muscle. In your own words explain what happened when the muscle was stimulated in rapid succession. At which interval did the volunteer's muscle twitches begin to merge together (summate)

The skeletal muscle is stimulated by means of action potentials that are caused by the neuromotor plate. These action potentials are one after the other and can become cumulative once the muscle fibers pass the refractory phase of the action potential.

The refractory stage is that stage in which action potentials are not admitted, therefore where the muscle fibers do not accept stimuli to contract.

Once this refractory stage is over, more action potentials can be admitted, which would be summative and the muscle fiber would contract with greater power.

Explanation:

If a muscle is repeatedly stimulated, the contraction will be weaker or the muscle will fatigue more easily, triggering muscle failure.

If the muscle is stimulated with a longer pause between contraction and contraction, the action will be performed with more power.