The lats, or Latissimus Dorsi, is one of the biggest muscles in your upper body. This powerful muscle helps pull your arms closer to your body. If it becomes too tight it can effect your posture by pulling your shoulders down or curving your lower back.
The Anatomy - The latissimus Dorsi is a broad, flat sheet of muscle that starts at your lower back in several places. They go all the way up into the middle of your spine and extend out to your shoulder, where they bunch together and twist into a large bundle and insert into a groove in your upper arm bone close to your armpit. It is the only muscles that connects your spine to your upper body. In many ways, The Lats are the Psoas of your upper body. Your Psoas connects your lower body to your spine, you