Issue 2: The Four Forces Working With You in Water
Issue 2: The Four Forces Working With You in Water
Water changes the forces acting on your body and on anything you move. Four are worth understanding: gravity, buoyancy, hydrostatic pressure and drag. Every aquatic movement is some combination of them.
Research Spotlight
A 2026 meta-analysis of 29 randomized trials (1,984 participants) on aquatic rehabilitation for knee joint problems, including osteoarthritis, ligament injury and post-surgical recovery, found significant improvements in symptoms and physical function compared with control conditions. The clearest gains were in balance, proprioception and muscle strength.1 The authors point to buoyancy, resistance and hydrostatic properties as the reasons water lets people move with less joint load.
That does not make water a treatment for every condition. It helps explain why aquatic therapy is so widely used when people need meaningful movement with reduced weight-bearing.
Water Science
- Gravity pulls mass down. Once part of a weighted object rises above the waterline, gravity dominates what you feel.
- Buoyancy pushes up. It reduces the apparent weight of your body and of submerged objects.
- Hydrostatic pressure surrounds an immersed body, increases with depth, and influences fluid and circulatory responses.
- Drag resists motion and rises sharply with speed, so a fast rep feels very different from a slow one.2
That's why "low impact" undersells water: it can unload your joints while still creating resistance.
Inventor's Notebook
AquaBLAST let me feel all four forces in one piece of equipment. The water inside supplies weight. Air changes buoyancy. The pool around it creates drag. And when the bag crosses the waterline, buoyancy hands off to gravity.
Myth vs. Fact
Myth: Buoyancy only makes exercise easier.
Fact: Buoyancy reduces weight-bearing, but push a buoyant object down and controlling its return becomes resistance.
Try It Yourself
Fill AquaBLAST partly with water and leave some air. Push it down below the surface and control it back up. That upward force is buoyancy working against you. Lift it up until part of the bag clears the surface, and feel the load get heavier as buoyancy drops. Move it slowly back and forth through the waterline to find the point where the feel changes. The bag never changed; the forces did.
Safety: Work in waist- to chest-deep water where you can stand firmly, near the pool wall or a rail. Keep your head above water, move with control, and stop if you feel pain or dizziness. If you have a medical condition, or you are recovering from injury or surgery, follow your healthcare or rehabilitation professional's guidance.
Where AquaBLAST Fits
AquaBLAST is designed for the pool. You choose the load by how much water you add: a few gallons for lighter technique work, up to a full bag of about 40–45 lb (20L) or 50–55 lb (25L). Out of the pool, it is suited to light land training with up to about 2 gallons of water (roughly 16 lb of water plus the bag). Save slams and throws for the pool.
Shop AquaBLAST 20L V2 → · Shop AquaBLAST 25L V2 → · 20L V1 (while supplies last) · Add the Suction Tether
More exercises: Aquatic Fitness Guide · All issues: Strength in Water® Blog
Blast Away!
— Rob Magrino, inventor of AquaBLAST
Research Notes
These studies looked at aquatic exercise in general. None of them tested AquaBLAST.
- Jiang Y, Peng Y. Effects of aquatic rehabilitation on symptoms, physical function, and quality of life in individuals with knee joint dysfunction: a meta-analysis. Front Physiol. 2026;17:1804926. doi:10.3389/fphys.2026.1804926
- Lopes TJ, Morais JE, Pinto MP, Marinho DA. Numerical and experimental methods used to evaluate active drag in swimming: a systematic narrative review. Front Physiol. 2022;13:938658. doi:10.3389/fphys.2022.938658
