This is a great exercise to facilitate dynamic thoracic extension with help from a Swiss ball. Not everyone can get into a quadruped position comfortably, or maintain an optimal position to help facilitate thoracic extension, so here is a seated alternative! There are a few key notes to point out here... (1) Proper sitting posture with an anterior tilt/ lumbar extension will help PROMOTE thoracic extension mobility, try it with a posterior tilt and see what happens (2) Swiss ball height: not too low and not too high, otherwise you're going to move less in your thoracic spine (3) Apply some pressure into the ball and reach your fingers away from you to activate your serratus anterior to encourage ideal mechanics at your shoulder and get that extra extension and posterior tipping of your scapula. This is a great way to help mobilize the lats as well! Assess your overhead shoulder ROM before and after this and see how it feels!
Exercise Library