If you want to improve flexibility in your upper back, consider the physio ball lat stretch. This stretch is classified as an active isolated stretch (AIS). AIS stretching involves stretching one muscle at a time, while contracting the opposing muscle. For this stretch, the lat muscles are being stretch, while the shoulders are flexed. Generally, AIS stretches are only supposed to be held for 2 seconds (this is not a typo!), but repeated for up to 10 times. Instructions for the physio ball lat stretch are listed below:

First, kneel behind a physio ball. Try to sit back on your heels. Extend your arms on the ball face up. Slowly roll the ball forward. As you do so, keep your hips stationary and lower your chest toward the floor. Once you are fully stretched out, try to lift your hands upward for 2 seconds. Repeat for 8 to 10 repetitions.