Muscle contraction and relaxation Ramdagram Nose Trachea Resp System Ps=Pa·Pbs 0 Diaphragm Intercostals muscle. external internal intercostals muscle Accessory muscle: sternocleidomastoid, scalene muscleMuscle contraction and relaxation Diaphragm Intercostals muscle: external, internal intercostals muscle Accessory muscle: stenocleidomastoid, scalene muscle