Well I think the brain and the muscles are one? The nervous system is delicately and perfectly connected (in a healthy individual). Here is the entire pathway if you want to know where the different relay and synapses occur or what parts are involved: http://readingroom.mindspec.org/?page_id=8876 But basically it goes from the primary motor cortex to the supplementary motor area to the spinal cord to the neuromuscular junction to the muscle itself.
As for sports, I would touch on this topic: https://www.bodybuilding.com/content/3-ways-to-build-a-better-mind-muscle-connection.html So the more you keep this connection, the more it can be enhanced
Are you asking "what is the relationship between brain activity and muscle activity"? As in how do they interact, e.g. central governing model? If so, this new paper discusses some of those aspects.