Final answer:
To crack your upper back between your shoulder blades, you can perform stretching exercises that target the scapular muscles and relieve tension in the upper back.
Step-by-step explanation:
To crack your upper back between your shoulder blades, you can perform a simple stretching exercise. Start by standing upright and interlocking your fingers behind your back. Move your elbows back as if you are elbowing someone standing behind you, and simultaneously spread your elbows apart. This movement targets the scapular muscles and helps to relieve tension in the upper back.
Another exercise you can try is to lift your arms at shoulder level and bring them forward until they are crossed in front of your chest. Repeat this motion several times, focusing on stretching the upper back muscles.