The Science of… Swimming

Swimming is a battle against two primary forces: gravity and drag. The force that helps us win the battle against them is buoyancy – the reason we stay afloat. This comes down to a principle called Archimedes’ Principle. The principle states that an object submerged in a fluid is acted upon by an upward buoyant force equal to the weight of the fluid it displaces. In simple terms, when you enter water, your body moves some of it away. If the water you move weighs more than you do, you float.

Our bodies are made up of muscle, bone, fat, and air. Muscle and bone are denser (heavier) than water, but fat and the air in our lungs are less dense (lighter). This is why taking a deep breath and filling your lungs with air can significantly increase your buoyancy, making it easier to float.

It is also why some people feel more buoyant than others. A person's body composition, including factors like body fat, lung capacity, and muscle density all play a role. Since fat is less dense than muscle, people with a higher body fat percentage often find it easier to float.

This is one of the reasons why babies are natural swimmers. In addition to having a higher body fat percentage, newborns are born with reflexes that make them hold their breath and kick when underwater. These are survival mechanisms, often called the "diving reflex" or "bradycardic response," include responses that cause their heart rates to slow and their glottis (the opening to the windpipe) to close when their face is submerged in water, preventing them from inhaling water. They also have natural motor reflexes that make them move their arms and legs in a swimming-like motion when in the water.

It's important to note though, that these are reflexes, not intentional swimming, and a baby should never be left unsupervised in water. Most babies also lose these reflexes as they grow, which is why early water play can help children build confidence and comfort in the pool.

Beyond survival and fun, swimming is one of the best exercises for the human body. Water supports your weight, reducing impact on joints, while still providing resistance to strengthen muscles. That’s why it’s often recommended for people recovering from injuries or living with arthritis.

Are you a water baby? Let us know whether the sea has called to you this summer or whether you’re more of a beach whale! Hit us up in the comments or on socials @LitLabLondon

---

Sources:

Faíl, L.B., Marinho, D.A., Marques, E.A., Costa, M.J., Santos, C.C., Marques, M.C., Izquierdo, M. and Neiva, H.P. (2021). Benefits of aquatic exercise in adults with and without chronic disease—A systematic review with meta‐analysis. Scandinavian Journal of Medicine & Science in Sports, 32(3), pp.465–486. doi:https://doi.org/10.1111/sms.14112.

Into the Swim, The Science of Swimming: How Does Our Body Move in the Water? intotheswim.com

Pedroso, F.S., Riesgo, R.S., Gatiboni, T. and Rotta, N.T. (2011). The Diving Reflex in Healthy Infants in the First Year of Life. Journal of Child Neurology, 27(2), pp.168–171. doi:https://doi.org/10.1177/0883073811415269.

Swim Australia (2014), "Swim Australia FAQs" Archived from the original on 16 October 2014. Retrieved 17 August 2025.

Find your science tribe.

Sign up to the Lit Lab London Science and Sip mailing list for the weekly blog series, 'the Science of…', Science and Sip events and experiments to try at home!

Related posts