Stability balls have a bad rep, but not everything done on them has to look like a circus act. The knee tuck is one exercise where the ball offers an advantage. Done correctly, it'll strengthen your core and chisel your abs.
1. Basic Knee Tuck
Your arms will support a good portion of your weight. There's minimal movement of your spine and pelvis as you pull your knees in a straight line towards your chest.
In the video, you'll only notice a small amount of movement in my spine and pelvis when my knees reach their highest point. We'll get to that in just a minute.
This basic variation is a good way to master the essentials – namely, not rolling off the ball and making an ass of yourself. It's the perfect option for developing your coordination and proprioception, but it's not ideal if you want six-pack abs.
When we're talking about core stability, what we really mean is creating more spinal stability. Essentially, the muscles that support your spine, pelvis, and hips should be strong enough to withstand forces that would otherwise cause unwanted spinal-pelvic-hip movement.
To build strength in any muscle or movement pattern, we need to be putting load through those tissues. Sure, there are neurological factors at play where an unstable ball might be useful, but the unstable ball stops you from being challenged in the right way.
"You can't shoot a cannonball from a canoe." It's a fitting analogy for both why core stability is important and why an unstable exercise environment isn't always better, especially from a muscle loading standpoint.
So, once you've mastered not looking like a clown on the ball, use other variations that challenge you in a better way. Elevated knee tucks and tucks-to-rollouts are both perfect for this.
2. Elevated Knee Tuck
This is a more efficient way to perform knee tucks compared to having your hands on the floor. Having your elbows on a box or bench essentially takes your arms out of the equation. It's easier on the wrists, and it allows more focus on your core since you'll feel more "locked-in."
Despite your elbows being bent, the bench also increases the height of your upper body relative to your lower body. Remember when I mentioned the small amount of spinal and pelvic movement at the top of the tuck?
During the basic variation, this part of the exercise is pretty useless. As your knees move towards your chest, the resistance comes closer to the movement axis (lumbar/sacral base), reducing the load through your abs. You lose tension at the top.
By increasing the height of your upper body relative to your lower (even just a little) you keep the resistance further away from the movement axis, helping maintain load through your abs at the top. Quite simply, it's a better "lower abs" exercise.
3. Knee Tuck to Rollout
This is the brainchild of Nick Tumminello. It combines the benefits of two great exercises: the knee tuck and the rollout.
In one study, Escamilla et al. tested eight ab exercises using the Swiss ball (1). From the conclusion:
"The rollout and pike were the most effective exercises in activating upper and lower rectus abdominis, external and internal obliques, and latissimus dorsi muscles while minimizing lumbar paraspinals and rectus femoris activity."
The knee tucks weren't too far behind the pikes either. Combining knee tucks or pikes with rollouts is a double-whammy for your overall core development.
- Escamilla et al. "Core muscle activation during Swiss ball and traditional abdominal exercises." Journal of Orthopedic Sports Physical Therapy, 2010.