Motor skills, large and small, become finely tuned with stringed instruments.
String players develop nimble fingers that are capable of working independently and in concert with surrounding muscle groups. These muscles are gradually trained to make extremely subtle adjustments, reacting quickly to correct pitch or add color to sounds. A complex combination a large and small muscle motions control the rhythm, style and flowing motions of the bow.
Anyone who has seen a great musician perform has seen first hand that performing music well involves the entire body. It's not just the heat of the lights that makes these musicians sweat. They're getting a workout.