Most likely it's the latisimus muscle. A great way to stretch the lat muscle is to grab something like a doorway, and lean back. It’s probably easiest to grab one side at a time, just grab one side of the doorway with one arm.
You can stretch both sides too. Grab a pole or something you can reach across like a secure desk… You can also reach up to the top of the doorway with both hands, and lean in.
Additional Reading Resources:
- A Deep Tissue Home Massager for painful muscles and knots
- When to use hot and cold for pain relief
- The Power of Biofreeze, a natural pain relief gel
- Neck Massage can help more than just your neck pain
- How to Make Tumeric Tea