The main effect of the warm-up period of athletes, as the muscle contractions increase in strength, is to (also known as the treppe effect).

Explanation:

Make more calcium ion available and also improve the efficiency of enzymatic system, this warm up period also helps to increase the muscular performance of athletes and bring it to its peak.

The Treppe effect is also known as the "stair case effect", it is the process of trying to increase contractions vigorously which was a result of identical stimuli being applied on a muscle that is under rest. It is a result of even and balanced increase in muscular tension when there is increase in the amount of motor units that are active. Time when there is no stimulation will lead to contraction in the muscle which makes Treppe to occur (make more calcium ion available).

the answer is anaerobic activities