Soreness in the belly of the calf is mostly a pain in the gastrocnemius muscle of the calf. It is mostly related to exertion, calcium deficiency and side effects of some medications. Soreness can occur after long-distance running and other sports that are prone to muscle fiber strains. Calcium deficiency increases nerve excitability, which can induce muscle spasms, and the gastrocnemius muscle of the calf is the muscle group most prone to spasms, and is prone to spasmodic pain. Also statins have the potential to induce rhabdomyolysis leading to leg pain, and it is important to check creatine kinase during medication to determine if this change is present.