Calcium is needed during the process of muscle contraction, without calcium muscle contraction is not possible. When slightly deficient the ability to fully contract will decrease. When an adequate amount of calcium is available there will be more muscular endurance.  Below is an article from livestrong.com by Laura Niedziocha explaining the importance of calcium for proper muscle contraction.

http://www.livestrong.com/article/504840-why-is-calcium-important-to-muscle-function/