The skeletal muscles most affected by breast cancer and its treatment are: pectoralis major: serratus anterior coraco-brachialis trapezius. (Don’t forget that chemo and radio-therapy affect the heart (cardiac) muscle. ) Why? and What to look for? Pectoralis major: The pec major suffers a lot. Even conservative surgery leaves scars. During wound healing, a protective arm … Read more Muscles most affected by breast cancer and its treatment.