Painful Sit-up Many students say to me at their first private Functional Integration lesson something like “I know I need to strengthen my core.” (This is almost as common as “I know I have bad posture”). Sometimes they refer to their own experience, telling me that they feel weak, or that they tire easily, but more often their personal experience is simply that their back or neck hurts, and they heard from someone (often a skilled Physical Therapist or yoga teacher) that the pain in their back is due to weak “core” muscles. When I help them feel what they’re actually doing, it usually turns out that they’re tightening those “core” abdominal muscles all the time. No wonder they feel weak! If a muscle is always engaged, it has less strength to engage further – its potential is already in use.