The human body has multiple muscle types and muscle functions, and not all of them are controlled by the same parts of the brain. Some muscles are voluntary muscles that a person has intentional control over, and other muscles are completely involuntary. They contract and relax without a person's conscious input. The wave contractions of peristalsis that move food through the digestive tract are a good example of involuntary muscle movements. These involuntary muscles are controlled by the brain stem. The brain stem is located under the cerebrum and in front of the cerebellum. It will control the movement of the diaphragm which is the muscle in charge of inhaling and exhaling. The brain stem also controls the beating of the heart muscle.
If this question is asking about larger muscle movements that are under a person's conscious control, like throwing a baseball, then the part of the brain responsible for these movements is the cerebrum. The cerebrum is the largest part of the brain, and it is divided into a left and right side. As it is the largest part of the brain, it is responsible for more than only voluntary muscle movements. The cerebrum is also in charge of interpreting touch, reasoning, and even speech.
The final part of the brain involved with muscle movements is the cerebellum. The cerebellum helps to coordinate the muscle movements as well as provide a person with the ability to maintain balance. Throwing a baseball involves a lot of different muscles doing individual movements, and the cerebellum coordinates those movements to allow the ball to be thrown as intended.