Belief determines muscle capability

My massage therapist taught me to stretch my shoulders back at my desk, alternating the stretch with effort. My desk is a corner unit. I can stretch my arms wide on the edges of the desk. I was doing isometrics, pushing against the desk. After a while I realized my chair has wheels and it is a simple matter to push my chair back. And with the next push, the chair went back away from the desk. No more isometrics. My belief dramatically affected what my muscles were capable of.