When working your abs, the muscular contraction is the same as any other muscle group. This guy was literally bouncing on the physioball and not working through a complete range of motion. When doing any ab work whether it be a full sit up or a crunch, to most effectively train the muscle you need to keep a few things in mind.
Full Range Of Motion.
"Partial reps, partial rewards!" The idea is to get a full contraction and extension of the muscle. It doesn't matter if it abs or arms... work the muscle through the full range of motion for a deeper contraction.
Proper Form
"Slow and steady wins the race." I see people doing ab exercises like it's a timed event. Just as with any other muscle group, perform each rep in a controlled fashion. The textbook rep should be 2 second lift, 1 second pause, 4 second lower. As you lift your torso, tighten your abdominal muscles and pull your belly button in toward your spine. By performing a rep in this manner you will engage more muscle fibers causing a greater neurological response and thus greater results.
Breathe
I know this one sounds ridiculous but you'd be amazed how meany people either hold their breath or breath improperly. If you hold your breath when doing any exercise, you may turn blue, get light headed, pass out and you don't want that! You will wake up finding me going through your pockets looking for loose change and candy... just kidding. Seriously though, proper breathing is essential for safety and to achieve maximum results. As you lift your shoulders from the floor, exhale. Inhale as you lower your torso. An easy way to remember this rule is to QUIETLY count your reps as you lift up. This breathing sequence is used for every exercise. Exhale as you lift the weight and inhale as you lower.
Focus Mentally
Again, whether it's biceps curls or trunk curls, focusing on the reps, breathing and how the targeted muscle is feeling will allow you to isolate the intended muscle(s) . I recommend putting a free hand on your abs when doing crunches to feel the muscles contracting. This will allow you to focus on the muscle working. It makes me absolutely crazy when I see a few of the women in the gym talking while lifting the weight for their set!!!! There are a few of them at the gym talking about their kids, food and other life events while in the middle of the set. Mindlessly going through the motions will diminish your results. Focus on getting a deep controlled contraction and extension in the muscle.
Work All The Muscles Of The Core
As you are aware, your body moves in various directions from the waist. When doing sit up, crunches, the abdominal machine etc, you target predominantly your rectus abdominis. Those are your abs, the muscles that run in the center of your torso from the rib cage to the pubic bone and are responsible for spinal flexion. . Your obliques run on either side of the abs and are responsible for lateral flexion and rotation. The transverse abdominals are located under the rectus abdominis are used to brace and support your trunk for activities such as plank and used for activities such as Pilates and yoga. The erector spinae muscles are located on either side of the spine and support your lower back and spinal extension.
To focus on just crunching exercises can create muscle imbalance and cause bad posture and injury. To create a strong core and muscle balance, work the entire core. Include exercises that will target all the muscles in the core. Below are listed some exercises for each targeted core muscle group...
Spinal Flexion
All variations of sit ups and crunches. These target the rectus abdominis (abs).
Spinal Rotation
Exercises such as bicycle crunches, Russian twists and standing twists. These target the obliques.
Spinal Extension
Exercises such as the Nautilus back extension, hyper extension machine and supermans. These target the erector spinae muscles
Lateral Spinal Flexion
Exercises such as side bends with dumbbells or cables or side crunches. These work the rectus abdominis, obliques and erector spinae.
Static Contraction (Isometric) and Stabilization
Exercises such as plank, side side plank, bird dogs, bridge ups etc. These work the entire core from different angles.
The key points to remember is to be conscientious about your form, work the muscles from all angles and breath properly. To achieve the strong abs and core you want, its more about quality reps not quantity reps!