Downward dog is one of the most basic yoga postures, regularly taken in most styles of asana (physical yoga practices). It usually starts as a quite intense posture for beginner practitioners but slowly evolves into a very comfortable and a ‘resting’ pose for more advanced yogi’s.
The main muscles that get a stretch are the muscles at the backline of the body; hamstrings and calves. The spine gets lengthened, and the chest region (pectoralis major) gets stretched. The back muscles (latissimus dorsi) and shoulder muscles (deltoids) work to stabilize the shoulder. The arms are active (triceps) and get strengthened.