A girlfriend of mine deeply feels that the reason she struggles in parsvakonasana and in arm balances is that her arms are just too short. She has trouble touching the floor. Too short may be an overstatement. Her arms are her arms and they are completely perfect for her lovely body. She has a long torso too which may make it more difficult to reach the floor sometimes. I got her to drop her hip and scoop her tailbone in parsvaK the other day and "suddenly" the arm length wasn't a problem.