Training explained · 6 min read

Why a 10kg Weight Doesn’t Always Feel Like Just 10kg

We tend to judge resistance training by one number: how heavy is it? But the way a load behaves can change the problem completely.

September 2026

Lifestyle image: Kari Alfonso / Pexels

Pick up a dumbbell and you know roughly what is going to happen next.

The weight stays where the manufacturer put it. If you move your hand, the load moves with you. That predictability is one of the reasons dumbbells are so useful.

But it also hides something we rarely think about.

A 10kg object with a fixed centre of mass and a 10kg object with a moving centre of mass are not identical training experiences.

The kilograms have not changed. The behaviour of the load has.

The overlooked variable: what the load does while you move

Fitness programmes already manipulate exercises in dozens of ways without changing the number on the weight.

Slow the tempo. Change your stance. Add range of motion. Hold the load further from your body. Use one side instead of two.

Each change alters the task.

Instability is another variable.

Lifestyle image: Kampus Production / Pexels

What happens when the weight moves inside the object?

Water is a simple example because it refuses to stay perfectly still.

Accelerate the container and the water shifts. Change direction and it reacts. Stop suddenly and the load still has movement inside it.

Your body therefore has two jobs: move the load and control the movement within it.

Researchers have measured the difference

A 2022 study examined functional exercises performed with a water-filled aquabag and reported changes in muscle activation during several movements compared with a more stable condition.

The study did not test the Swell Fit-Bag and it does not prove that water resistance should replace traditional weights. It supports the narrower point that load stability can change the muscular demands of an exercise.

Read the study on PubMed

So where does this become useful?

For people who enjoy heavy barbell work, the answer is not “throw away your barbells”.

For people training at home, travelling, working on control or simply looking for a fresh stimulus, moving resistance creates another option.

That is especially useful when you care about controlled strength, core involvement, balance, carries, lunges, rotations, mobility and functional movement.

Where Swell Fitness comes in

A purpose-built bag with moving water inside

The Swell Fit-Bag can be filled with between 1 and 10 litres of water. One litre is approximately one kilogram.

Unlike a sandbag or dumbbell, the load is free to move inside the bag as you train.

That makes familiar exercises feel less predictable, particularly when you slow them down and focus on control.

“But surely 10kg is still only 10kg?”

Yes.

And that is exactly the point.

Swell Fitness is not claiming that 10kg somehow becomes 20kg or 30kg. If maximal strength is the goal, you need progressively heavier resistance.

The argument is simply that weight is not the only variable that creates a training challenge.

There is a practical advantage too

Once the workout ends, you can empty the water and the weight disappears.

That makes the same training tool easier to store at home, take outside or pack for travel than a fixed 10kg object.

The current Swell Fit-Bag also includes Start with Strength workout support so the product comes with somewhere practical to begin.

Disclosure: This is sponsored editorial content presented by Swell Fitness. Lifestyle photography is illustrative and does not depict Swell Fitness customers or imply endorsement by the people shown.

Swell Fitness products and workouts are for general fitness, movement and educational purposes only. Train within your ability.