Bioimpedance Analysis: Why Swimmers Should Ignore the Number

Bioimpedance analysis estimates body composition by passing a tiny electrical current through you and measuring the resistance. Fat, muscle and water conduct differently, so the device infers a body fat percentage from the reading. The problem for swimmers is simple and fatal: the measurement is dominated by how hydrated you are. Weigh yourself on a body fat scale before a swim and after one and you will get different numbers with the same body. Neither is the truth. Neither tells you anything useful about your swimming.

This page used to encourage you to track your body fat percentage. We now think that was poor advice, and here is what a swimming coach would rather you knew.

Why the Number Moves When You Have Not Changed

Bioimpedance readings shift with:

• How much you have drunk in the last few hours
• Whether you have exercised, and how much you sweated
• The temperature of your skin and your feet
• Time of day
• What you have eaten
• Where you are in a menstrual cycle

A swimmer is, by definition, never in a standard state. You are warm, you have sweated in the water without noticing, and you have probably drunk less than you think.

So the reading moves and you conclude something happened. Nothing happened.

The Number That Actually Affects Your Swimming

Buoyancy — and you cannot change much of it.

Your lungs are the buoyant part of you. Your legs, dense with bone and muscle, are the sinking part. Lie flat and you pivot around a point near your chest. Lean, muscular people sink more readily than soft, rounded people. Men sink more readily than women, on average. A cyclist with heavy legs will find a flat body position harder than a friend who has never trained.

None of this stops anybody swimming well. It changes only how much you tilt, and how much your head position has to do.

Which means the useful lever is not your body fat percentage. It is where you put your head.

What to Measure Instead

Stroke count and time, tracked together, cannot be faked and move when your swimming moves. That is what a coach looks at.

If You Want Body Composition Assessed Properly

See a doctor or a registered dietitian. They can order or interpret an appropriate assessment, in context, alongside your health.

A swimming school cannot do this and should not try. We will not give you a target body fat percentage, a healthy range, or a rate of change. Those numbers vary enormously between individuals and a figure that is wrong for you is worse than no figure.

And if the scale has started to govern how you feel about a session — if a reading can ruin a training week, or if food, body image and exercise have become tangled together — please speak to a doctor. Instructors see this more often than they say, particularly in teenagers and in adults returning to sport. It is not a swimming problem and we are not the right people to help.

Common Mistakes

• Weighing before and after a swim. You measured your sweat.
• Comparing your reading to a competitive swimmer’s. They train twenty hours a week and were selected for their build.
• Chasing a body fat number to improve buoyancy. Head position does more, immediately, for free.
• Daily measurement. Hydration, food volume and hormones dominate the day-to-day signal.
• Restricting food to move the number. Under-fuelled swimmers get slower, get injured, and quit.
• Letting a scale decide whether a session was good. The session was good if you swam.

What Swimming Does and Does Not Change

Swimming builds cardiovascular fitness reliably. It builds upper-back, shoulder and core endurance. It is kind to joints and comfortable in the heat, and people sustain it for decades.

It does not load bone, so it does not build bone density — add two short land strength sessions a week, which matters increasingly with age. And it does not remove fat from any chosen part of the body, because nothing does.

Where body composition changes at all, it changes across the whole body, in an order your genetics decide.

Safety

Do not restrict food to change a measurement. Do not exercise to compensate for eating.

See a doctor before starting or increasing exercise if you have a heart condition, are managing diabetes, are pregnant, take medication affecting heart rate or blood pressure, or have not exercised in years.

Swim where a lifeguard is on duty, and never alone. Never use breath-holding or hypoxic sets to make a session harder — prolonged or repeated breath-holding, especially after hyperventilating, causes shallow-water blackout, a silent, warningless loss of consciousness that has drowned strong swimmers in supervised pools.

If your shoulder or knee hurts, see a physiotherapist rather than swimming through it. And drink — you sweat in the water and never notice, which is also why the scale lies.

The Useful Thing

Most adults who have never been coached are working twice as hard as necessary, because you cannot see yourself swim. A few sessions with somebody on the deck finds in ten minutes what you would never find in a year, and it will do more for how swimming feels than any measurement.

Start with private swimming lessons or a place in group swimming classes. Learn where your head should be in how to float. Track the right numbers using stroke count and tempo. To find a pool, use swimming near me.

For general physical activity guidance, Sport Singapore publishes current advice. For body composition and health, see a doctor or a registered dietitian.

Frequently Asked Questions

Is bioimpedance analysis accurate?

It is heavily affected by hydration, temperature, food and time of day. For a swimmer — warm, sweating unnoticed, variably hydrated — it is particularly unreliable.

Does body fat percentage affect how well I float?

Body composition influences buoyancy, and you cannot change most of it. Head position influences your body line, and you can change that today.

What should I track instead?

Stroke count and time over twenty-five metres, measured together, fortnightly, rested. Neither can be faked.

Will swimming lower my body fat percentage?

We will not promise that or quantify it. Swimming is good exercise, exercise supports health, and where body composition changes it changes everywhere.

Should I get a proper body composition test?

If it matters to you, ask a doctor or registered dietitian who can interpret it alongside your health. Not a swimming coach.

The number ruins my week. What should I do?

Stop measuring, and speak to a doctor. That is a serious answer, not a dismissive one.