Answer:
The field involved in the study of muscles and how the body uses them during exercise is "physiology." Physiology is the branch of biology that focuses on understanding the functioning and mechanisms of living organisms and their various systems, including the muscular system. It delves into how muscles work, contract, and adapt during exercise, as well as the physiological responses of the body to physical activity. While other fields like anatomy and biology are related to this study, physiology specifically deals with the dynamic processes and functions of the body, making it the most relevant choice for understanding muscle function during exercise.