One truly amazing thing about the body is that it will allow you to compensate for bad posture with more bad posture. At first it could just be a forward head, a locked out knee or a rib cage popping forward. Next,your body must adapt by repositioning another part of your body. One muscle lengthens another part shortens to create imbalances in our bodies. Eventually you get an achey feeling, a tight feeling or a full blown injury.
As part of the "posture police" you might think I am immune to this..unfortunately, I am not. All of us have muscle imbalances to different degrees. It is one of the reasons I love Pilates and what it can do for my clients and myself.
Next time you are in class, make a really concentrated effort to keep your body in alignment throughout the class. "spine tall, abs up and in, pelvis in neutral, ribs in line with the hips, shoulders gently down, ears in line with shoulders, etc etc!!!