Understanding Core Strength: The Water Bottle Concept

Dave Hedges
Jul 16
5 min read

Updated: Aug 8

The Importance of Core Strength

Over the weekend, I had the pleasure of visiting Galway as a guest of the remarkable Sarah Smith, owner of Galway Kettlebells. During my time there, I conducted a bodyweight training workshop.

Teaching this workshop is always a joy. Each time I run it, I am impressed by the level of questions that participants ask. However, this time, I received a particularly thought-provoking question.

A 17-year-old martial artist, a bright young lad who is relatively new to the fitness and strength conditioning world, asked me, “So why exactly is core strength so important?”

Typically, I am asked about how to develop core strength. This was the first time someone asked me the fundamental question: “Why?”

To answer this, we first need to agree on what exactly the “core” is. I personally use three definitions depending on the context.

Definition 1 – The Water Bottle Concept

This idea came to me while teaching a kettlebell seminar to a group one day. I asked the attendees, who were mainly young fitness instructors, for their opinions on how to define the “core.”

Needless to say, I received a lot of blank stares, followed by stumbling descriptions and a lot of pointing at the stomach. At that moment, I grabbed my water bottle and used it to illustrate a simple view of the core as a singular unit rather than a jumble of parts.

Here’s a brief overview of how the speech goes:

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A plastic water bottle, even when empty, can support a good portion of my body weight without any issue. Assuming I can balance on it, it can support my entire body weight with some deformation occurring.

A sealed empty plastic bottle supporting a 10kg plate

Now, if I put the tiniest hole in the bottle or simply unseal the lid, it will collapse under a fraction of that weight.

But if we remove the lid…

How does the sealed bottle hold me up while an open one collapses? After all, it is the same bottle made of the same thin plastic.

It’s the internal air pressure that supports my weight. The walls of plastic merely prevent the air from escaping, ensuring there is sufficient pressure to support me.

As soon as the air finds a way out, through a weak spot in the plastic, the bottle collapses. This is almost exactly how our abdomen works when we generate high levels of force.

The air pressure in the torso stiffens the body, allowing the hips and shoulders to use it as a platform to push from.

The water bottle analogy even helps us understand the anatomy:

The front side represents our Rectus Abdominis, or “6 pack.”
Directly opposite this on the back of the body is the Erector Spinae.
The sides represent our obliques.
The label illustrates the Transverse Abdominis nicely as it wraps around the bottle, albeit on the outside rather than the inside.
The base of the bottle is the Pelvic Floor.
The lid represents the Diaphragm.

When all these elements work together, we are strong. Individually, they are pretty much useless.

How Does This Help Us?

This analogy shows us how the core functions as a unit, stiffening to protect the body and transfer force from one end to another.

For our martial artist, this means that when his fist lands, the core stiffens so that the force is transferred not just into, but through his opponent, with minimal recoil reverberating back through himself.

Definition 2 – From the Hips to the Shoulders

I don’t use this definition as much as the water bottle concept, but I find it useful for getting contact athletes and fighters to reconsider their training needs.

This definition is a simplified version of the next one that follows.

To illustrate this, I like to use two pens and an elastic band.

Put one pen through the band and hold it steady. Now, insert the other pen and start twisting. After a few twists, hold the top pen steady and release the bottom one. What happens?

That’s right, the bottom pen spins as the band unwinds. This is exactly how a Thai boxer throws a kick: wind the top so that the bottom whips around.

BOOM!

If the bottom pen represents our hip and the top pen represents our shoulder, then the band is our core.

What connects the hip to the shoulder? A whole host of muscles, including everything discussed in the water bottle concept, plus the Glutes, Lats, Rhomboids, Traps, and more.

When you observe a thrower, whether in shot put or baseball, you can see how they rotate their hips so the torso twists like our elastic band visual. As the torso reaches its maximum stretch, it snaps the shoulder through, whipping the arm out and propelling the ball at incredible speed toward the target.

Every muscle involved in that stretch can be considered part of the core. It’s not just your abs; it’s the entire connection between the hips and shoulders.

Definition 3 – The Spine

This is the "real" core. It consists of 33 bones, 24 of which can articulate against the bones above and below them. The spine can flex, extend, and rotate, essentially moving in every plane. It also protects our spinal cord and serves as an anchor point for a vast amount of muscle.

Let’s use throwing a punch as an example:

A punch starts from the ground. We extend our rear ankle and knee, which pushes our rear hip forward.

This all happens quickly, with each joint accelerating the next. Assuming our abdominal muscles have enough elasticity and strength, the hip turning while the shoulder remains stationary will torque the midsection. The spine will twist, causing many muscles that attach to it to either lengthen (stretch) or shorten (contract).

The spine then unwinds, releasing that stretch and literally slingshotting the shoulder forward, throwing out the arm and potentially knocking out the opponent.

If you remember nothing else from this, just remember the word “slingshotting.” I encourage you to use that word in a conversation today.

Drop me a comment letting me know how you get on.

The Role of the Spine in Strength Training

The flexion and extension of the spine in the sagittal plane (front to back) is used by strongmen and kettlebell lifters during presses and jerks to efficiently propel weight overhead with a whip-like action.

Combine forward flexion with rotation, and you have a tennis serve. Reverse it, and you have a suplex throw.

In essence, the spine is the core. Muscles are designed to move joints, and the spine has 24 articulating vertebrae, along with the sacroiliac joint, the atlas, and others.

That’s a lot of joints, all of which need to be controlled by muscular contraction.