I think this may be the key:
Muscle-Specific Bass Playing

When you play the bass, be careful to use only the muscles necessary to perform the specific tasks involved. As the body tires, the tendency is to overuse these muscles and to involve muscles that aren't necessary for playing. By building muscle-specific awareness, you can conserve energy, reduce the risk for injury, and allow your body to be more physically fit to play the bass.
from international bassist