Many coaches and trainers want to claim that spinal flexion exercises like stability-ball crunches and reverse crunches are universally dangerous, which they believe is a claim based on Dr. Stuart McGill's research. They're wrong. Earlier this year, Dr. McGill co-authored a paper on the crunch. Here's a snippet:
"If the ability to bear heavy loads is important to a client, it may be better to choose abdominal exercises with high muscular loads such as push-up position walkouts, rollout planks, or stir the pot. If flexibility is more important to the client, the personal trainer may want to select full-range curl-ups and crunches, and reduce heavy loading.
"If maximal muscular development is the primary goal, including the crunch and/or its numerous variations, together with other exercises, may help to enhance desired results. However, personal trainers should consider the entire exercise program, including cumulative tissue loading considerations and weigh the tradeoff between mobility and load-bearing ability."
In other words, spinal flexion exercises are no different than any other resistance training exercise. All exercises can induce stress, which causes tissue adaptation. From a back perspective, loading enhances tissue resiliency in general, but there's a tipping point when you exceed your capacity. That's the individual nature of training, and exactly what's meant by training smart!
The Right Way
Lie supine on a stability ball with the ball in the arch of your lower back and hold a weight plate, dumbbell, or medicine ball directly above your chest, arms outstretched.
Do a crunch, reaching towards the sky while holding the weight. Pause for one or two seconds at the top of each rep. Don't sit all the way up (with your torso perpendicular to the floor). That removes the tension from the abs.
Slowly reverse the motion, allowing your abs to stretch over the curvature of the ball. Don't allow your neck to hyperextend in the bottom position; keep it in a fairly neutral position throughout.
To focus on your abs when doing ball crunches, the ball shouldn't move under you at all. Instead, keep your knees bent at roughly a 90-degree angle throughout, and flex and extend your spine over the ball in a controlled manner. If the ball is rolling back and forth, it's primarily your knees (bending and extending) that are driving the motion, not your abs.