Are there drills/exercises to build flexibility in the arms? I find myself tapping to kimuras/americanas that don't even seem that tight. My arms are very inflexible in that position: imagine laying on your back with your elbows bent at 90 degrees. If I pull my hands back towards my head/shoulders (like an americana from mount) I can't touch my knuckles to the mat. Would it be a good idea to let training partners put me in "soft" americanas to stretch the muscles or get the arms used to being in that position? I am wondering if that would stretch a muscle and make it more flexible, or if that would cause joint damage. Are there any other ideas to build flexibility in this area?