The Half-Moon Advantage: Training with the Tidal Tank Bulgarian Bag

The Tidal Tank Bulgarian bag creates a unique training stimulus that builds strength, stability, and body-wide control. Thanks to its half-moon shape that guides the shifting water along a rotational path, it creates a unique dynamic rotational resistance that force you to constantly stabilize and adjust to control your body and movement.

What is dynamic resistance and why is it special?

Unlike traditional static weights - the dynamic resistance created by the shifting water in the bag creates constantly changing forces in all directions. As you can see in the graph below, A dynamic water weight like Tidal Tank may have the same total mass as a static weight, but the water can move independently inside the container, creating unpredictable forces that vary in both size and direction.

As you can see in the graph - the forces created by the moving water are not only non-linear (graph 2), and multidirectional (graph 1), but also disproportionately large respective to the actual mass of the water weight itself (graph 2),

This occurs because the instant you try to move a dead static weight, the entire mass moves together immediately (in a linear fashion), whereas in a water weight, given it’s not rigidly connected to the container walls, the water lags due to internal sloshing.

Vice versa when you try to stop a rigid weight, it stops almost instantly, but in the Bulgarian bag the water keeps moving due to inertia. This makes it slam into the container walls on the opposite side; an impact that creates reactive forces in the opposite direction of your original movement.

How does this affect our body & movement?

Effect #1:

Phase lag: There’s a time delay between your movement and the water’s movement. This creates a nonlinear feedback loop, i.e., you’re always adjusting to forces that are slightly out of phase with your input.

Due to the delay, you cannot pinpoint or predetermine what precise forces will result from your movement, nor how exactly you’ll need to move to execute the movement successfully; you’re forced to adjust and adapt on the go.

Effect #2:

The water’s movement direction is not perfectly aligned with the bag’s movement direction, this causes rotational torques, unplanned shifts, and instability.

This mismatch between how you move the bag, and how the water inside moves, makes it impossible to predict and harder to control.

After a fraction of a second, more water momentum comes into play, in different directions to those you originally moved the bag, creating unpredictable instability.

What happens when the forces are rotational?

Unlike the Tidal Tank classic, the Bulgarian bag’s half-moon shape guides the water in a rotary path, creating a unique dynamic rotational force. A-symmetrical, rotational, and constantly changing; it results in more joint torque than normal weights (accentuated by the shifting water), which forces your muscles to work harder to resist.

As the water shifts from one side to the other of the half-moon, the centre of mass (COM) shifts forwards and sideways pulling your body around with it. The more water begins to shift to one end of the bag, the stronger the pulling forces becomes, dragging you around like a merry go round.

Rotations drive human movement, whether it’s throwing a football, kicking a soccer ball, or walking upstairs, our joints rotate to create movement. When these rotations are resisted and overloaded with a dynamic weight like water, the

body learns to rotate joints more efficiently through synergies, control rotations to protect vulnerable joints and tissues, and stabilizes unnecessary rotations in joints that could otherwise cause injury.

Demonstrated in the graph below, is the directional shift in COM of the water weight (graph 1) and the resulting spike in force from the impact of the water (graph 2).

As you can see the shift of water weight causes a disproportionately large spike in force requiring your muscles to work harder (1) and more efficiently together (2) to resist the moving water, stabilize and control your body.

This builds:

1. Superior joint stability

2. Improved movement control

3. Enhanced ability to adapt movement patterns to the unpredictable demands of functional, real-world activities.

How can we apply dynamic rotational resistance in practise?

 

We can use the dynamic rotational resistance in different ways to improve our movement and strengthen our bodies.

Exercise 1: Bulgarian Bag Cradle Pitch  

For rotational power key to throw and swing performance, we can hold the Bulgarian bag diagonally across our bodies, so it aligns with the front sling (an interconnected line of muscles and fascia responsible for generating power during throwing, swinging, hitting, and punching).  
As you rotate with the Bulgarian bag, try to shift all the water from the bottom to the top of the bag, this trains your ability to transfer power from the hips to your shoulders using your core - increasing exit velocity of the ball during a throw or bat/racquet swing speed.

 

Exercise 2: Bulgarian Bag Split Jump Chop

This exercise targets the same functional connection between the contralateral (opposite) arm and leg - key to rotational movements - only this time on the back of the body. It’s responsible for generating power in kicking, sprinting, and jumping.

By simultaneously kicking back the leg and chopping the Bulgarian bag across the body, we call into action this posterior sling – enhancing kicking and running power. Fighting to overcome the initial inertia and then decelerate against the impact of the water strengthens the muscles and trains the coordination (synergistic link) between the muscles in the posterior sling.

 

Summary of the Bulgarian Bag and its unique dynamic rotational resistance

When the unstable effect of the moving water is COMBINED with the unique curved shape of the Bulgarian bag, the rotational forces that pull on your body are accentuated, making a dynamic rotational resistance that challenges the control of your body and movement.

This is why to overcome the dynamic resistance characteristic of water-based weights like the Tidal Tank Bulgarian Bag more muscles are forced to work (1), stabilizer muscles are recruited (2), and movement control is improved (superior coordination) (3) translating to superior performance in life and sport.

 

References:

 (1) More muscles are forced to work

A 2022 study by Wezenbeek and colleagues looked at muscle activation during exercises performed with a water bag – a water-filled resistance tool comparable to a normal Bulgarian bag. Using EMG analysis, they found significantly higher activation of the abdominals, glutes, hamstrings, and back muscles when compared to a control load. The increased muscular demand is directly linked to the challenge of stabilising the sloshing water throughout the movement.

Reference: Wezenbeek, E. et al. (2022). Muscle activity and perceived effort during exercises with an aquabag versus a stick. PubMed ID: 35108673

(2) "Stabilizer muscles are recruited"

Ditroilo et al. (2018) compared squats performed with a traditional barbell versus a water-filled training tube, similar in behaviour to the Tidal Tank. They observed that deep trunk stabilisers like the multifidus and external obliques were significantly more active under water-based conditions. In other words, the dynamic resistance of water forces the body to constantly adapt, engaging stabiliser muscles to maintain control.

Reference: Ditroilo, M. et al. (2018). Trunk muscle activity and balance during squats with water-filled versus stable loads. PubMed ID: 29364062

(3) "Movement control is improved (superior coordination)"

When it comes to long-term adaptation, unstable loads don’t just challenge you — they teach your body how to move better. A 2023 study by Glass & Wisneski found that participants training with a sloshing water tube developed improved neuromuscular coordination, as measured by more consistent EMG patterns and better postural control.

Meanwhile, Kang & Park (2024) used water bags in an instability training programme, showing improvements in both static and dynamic balance tests — including faster times in the TUG test and better scores on the Y-balance test. These results highlight how water-based resistance not only builds strength but refines control, which is crucial for real-world movement and sport.

References:

• Glass, S. M. & Wisneski, K. J. (2023). The effects of water-filled instability loads on neuromuscular control. PubMed ID: 37754969

• Kang, M. & Park, Y. (2024). Instability neuromuscular training using water bags improves balance and movement control. MDPI Sports Journal – Vol. 9, Issue 1