# Active drag, part 1: what it is and why it has to be inferred (ELI5: in plain English) > Why the water's push on a swimming body is so hard to measure, and why scientists' numbers for it disagree, in plain English. The first of five posts. - Source: https://swimas.co.nz/blog/active-drag-1-what-it-is/eli5/ - The full version: https://swimas.co.nz/blog/active-drag-1-what-it-is/ - Author: V. Karpov, swimas (Swimming Art & Science), New Zealand - Published: 18 Oct 2026 - Topics: Active drag, Biomechanics; strokes: FR, BK, BR, FL How much does the water hold a swimmer back? It is a hard question, because water is at once the thing you push against and the thing in your way. You move forward by pushing on the very water that is trying to stop you. Hold a tight streamline and let someone tow you on a rope at a steady speed, and the answer is easy: the pull on the rope is the drag. This is called *passive drag*, because you are not moving your arms or legs. The moment you start swimming, that simple picture falls apart. Your body bends and rolls, your arms and legs move in different directions, and pushing forward and being held back happen at the same instant. At a steady average speed, your push and the water's drag cancel out over each stroke. All anyone can see from the pool deck is the result, your speed. How hard you pushed, and how hard the water pushed back, stays hidden. That is why the drag while swimming, called *active drag*, is never simply measured. It is worked out, and every method works it out with its own guesses about how to split the total into push and drag. On the same swimmers, two methods have given answers almost twice apart, and across studies active drag comes out anywhere from about the same as passive drag to three times as much. For a coach that raises a practical question: when a test says a swimmer's drag has gone down, has it really, or has something about the test changed? **Figure 1.** Towed, there is one force to measure. Swimming, the forward push and the drag act on the same body at once, and your motion shows only their total. Two panels side by side. Left, passive drag: a swimmer towed or gliding in a fixed streamlined posture, one tow force forward and one drag force back. One unknown force, measured directly: hang a weight and read the steady speed; repeatable, with r = 0.98 between repeats (Roberts 2003). Right, active drag: a swimming swimmer whose limbs both propel and resist, with propulsion and resistance arrows; at constant speed they sum to zero over a cycle. Two unknown forces on one body, whose motion shows only their sum; every method is a set of assumptions that lets the sum be split, hence the disagreements. ## A good streamline on a rope does not predict your swimming drag Passive drag has been measured for more than a century, and it behaves well. It also shows things a coach can use: where you hold your head changes it by 4 to 11 per cent, and arms stretched overhead instead of at your sides cut it by about 18 per cent. The catch is that how slippery you are on a rope says almost nothing about your drag while swimming. In the largest study of its kind, 797 swimmers tested by Kolmogorov's group, the two simply did not go together. Body size and shape still matter; but a gliding body and a swimming body are two different machines. While you swim, your speed also rises and falls inside every stroke. Even in swimmers from the world top ten it varies by about 8 to 9 per cent of the average in front crawl, about a third in butterfly and about half in breaststroke. Could you measure the push at the hands instead? Pressure sensors on the palms do exist, but they show what happens at the hand, not the drag on the whole body. ## Three brakes Whatever the method, the drag it is trying to catch comes from three brakes that act on anyone moving through water. **Skin friction: the water that sticks to you.** A thin layer of water clings to your skin and gets dragged along with you. Over the first hand's length or so from your fingertips it slides smoothly; further back it turns churning, and the rubbing grows. Most estimates make friction the smallest of the three brakes, from a few per cent of the total to, in one estimate, a quarter. That small slice is what shaving and swimsuits go after. Shaving does help: in a classic study shaved swimmers used 9 per cent less oxygen on the same swim and covered 12 per cent more distance per stroke. Fabrics copying shark skin cut friction by up to about 10 per cent in the laboratory, but in the pool a suit like that lowered swimmers' active drag by only 2 per cent, too little to tell from chance: a tenth of a small slice is a very small saving. Suits can help in another way, by making the water flow along the body longer before it breaks away, which lowers the next brake. **Form drag: the hand out of the car window.** Hold your hand out of the window of a moving car: flat to the wind, it is shoved back hard; edge-on, it slices through. Water does the same. It piles up against your head and shoulders, and behind you, where it cannot follow the curves of your back and hips, it breaks away into a swirling wake that pulls you back. In one computer study of a gliding swimmer, form drag came to about six times the friction. It grows with the square of speed: swim 10 per cent faster and it pushes back about 21 per cent harder. That is why a lifted head or sinking hips cost so much: while swimming, the trunk is tilted to the water at about 10 to 12 degrees, against 4 to 6 when towed, and the body becomes a wider plough. **Figure 2.** Shape matters more than size. With the same front, a flat plate leaves a big hole of churned, low-pressure water behind it, a ball a smaller one, and a long, slim shape almost none. A swimmer is the same: even a tight streamline leaves some churned water behind the neck, the lower back and the hips, and a lifted head, sinking hips and bent knees leave more. An animated figure in two parts. The water comes from the left, drawn as dotted streamlines moving with it. Above, three bodies of the same frontal height in the same stream, each with its drag coefficient for the same frontal area and a bar of that length: a plate across the flow, 1.28; a ball, up to 0.5; a streamlined section, 0.045. A shaded patch ahead of each body marks the high pressure where the water piles up (+). Behind the plate the flow breaks away at its edges and leaves a wake wider than the plate; behind the ball it breaks away a little before the widest point and leaves a wake about as wide as the ball. In both wakes, shaded and at low pressure (−), eddies are shed from each side in turn and drift away. Behind the streamlined section the streamlines close again and the wake is a thin line. Below, the same swimmer gliding in two postures, one above the other, with a legend: an ochre line for pressure higher than in the open water, a blue line for lower, a shaded pocket with a curl for separated flow, and shading for slowed water. The streamline, the arms stretched, the head between them, the body and legs straight, is drawn after a computed flow round a gliding swimmer and tufts on swimmers' suits (Sato and Hino 2010; Zaïdi et al. 2010). The water stops at the fingertips and at the crown of the head. Along the back the pressure is lower over the back of the head, higher at the nape, lower over the shoulder blades, higher in the small of the back, lower over the buttocks, higher behind the knees, lower over the calves and higher at the heels; underneath it is higher on the upper chest and lower over the widest part of the chest. Small pockets of separated flow, drawn larger than they are, sit behind the back of the head, under the chin and over the small of the back. From behind the buttocks slowed water wraps the legs, about 0.23 m deep by the shins, and trails behind the feet, with no eddies shed in turn. With the head lifted, the hips sunk and the knees bent, the body meets the water with a wider front, a larger pocket forms behind the head, the flow breaks away at the hips, and a wider, turbulent wake trails behind the legs: its edge ragged and shifting, a few large eddies rolling up off the buttocks and the knees at irregular moments, either way round, stretching and breaking up downstream among many small ones, with no row shed in turn; this posture is schematic, as no published computation or measurement shows its flow. **Wave drag: swimming uphill.** A cyclist moves inside the air; a swimmer, like a ship, moves along the boundary between water and air. Your head pushes up a bow wave, and making waves takes energy from your muscles. At easy speeds several short waves fit along your body. As you speed up they stretch, until at what boat designers call *hull speed* a single wave is as long as you are. For a swimmer 1.9 m tall that is about 1.72 m/s, roughly a 58-second 100 m pace. There you lie in the hollow between your own bow and stern waves, and to go faster you have to climb your own bow wave. How big the wave brake is at race speed is still argued: estimates run from a fifth to well over half of the total. What is clear is that it grows faster than the other two as you speed up. **Figure 3.** Your hull speed. The faster you swim, the longer your wave. When it is as long as you are, you lie in the hollow between two crests; to go faster, you have to climb your own bow wave. An animated side view of a front crawl swimmer 1.90 m tall at the surface, with the wave he makes; wave heights are exaggerated. The speed rises from 1.30 m/s (Froude number 0.30) to 1.72 m/s and holds, rises to 1.99 m/s (0.46) and holds, and falls back to 1.72 m/s. The wavelength λ, marked from crest to crest above the water, follows λ = 2πv²/g, where v is the speed and g the acceleration due to gravity: 1.08 m at 1.30 m/s, shorter than the body; 1.90 m at 1.72 m/s, equal to the body length marked under the swimmer, with the bow crest at his head, the next crest at his feet and the trough between them ("hull speed: λ = L, the swimmer in his own trough"); 2.53 m at 1.99 m/s, longer than the body, the swimmer pitched a little nose up ("climbing his own bow wave"). A scale of the Froude number v/√(gL), where L is the body's length, from 0.30 to 0.50 marks the hull speed at 0.40 and elite crawl sprinters at 0.46, with a marker moving to the current value. Waves have two weak spots a swimmer can use. They live at the surface: about half a metre down, a swimmer's wave drag falls by roughly 70 per cent, and a metre down it is almost gone, which is why swimmers stay under after the start and each turn (and why the rules limit that to 15 m). And they can be shared: half a metre behind another swimmer, the follower moves in water the leader has already set going, and the follower's drag coefficient is only about 56 per cent of the leader's. > **Aside: Why so few animals swim at the surface** > > Swimming along the surface is so wasteful that fish, penguins, seals and dolphins travel under water and come up only to breathe. A seal towed at the surface met two and a half times the drag it met under water. Fast dolphins even leap: above a certain speed, a run of jumps costs them less than swimming just under the surface. Ducklings, stuck on top, cheat in two ways: they line up behind their mother and ride her wave, or they lift out of the water and skim along at up to four times their hull speed. ## Why push and drag cannot be pulled apart without guessing When researchers filmed the water around a swimmer's hand, they saw why simple sums fail. The hand never pushes on still water. Each time it changes direction it sheds a whirl of water and starts a new one spinning the other way, and the pair shoots a jet of water backwards. In one experiment the force on a sculling swimmer's hand came out about twice what calculations for a steady flow predict. And here is the twist: when your hand pulls straight back, what gives it its grip is the water's drag on the hand. The drag on your hand is your push. Push and drag come from the same swirling water, the same movements, the same moment, and no instrument can measure them apart. So every method has to take two shortcuts: 1. **Pretend the speed is steady.** Inside every stroke you speed up and slow down, and you carry along extra water whose mass, by different estimates, is 10 to 30 per cent of your body mass. Almost every method works as if you were gliding at your average speed. 2. **Pretend drag grows with the square of speed.** In real measurements the power of speed wanders roughly between 1.6 and 2.9, because of waves and the changing tilt of the body. ## A short history For more than a century people have come at the problem from different sides: - **From 1905:** swimmers towed by boats and winches. This gave passive drag, which says little about drag while swimming. - **1974, breathing:** measure the oxygen a swimmer uses while a rope helps or holds them back, and work out the force that would make swimming free, which gives a result with active drag two to three times passive drag. - **1986, pads:** a row of fixed pads under the water that the swimmer pushes off with the hands, the MAD system. Active drag: about the same as passive drag. - **1992, two sprints:** Kolmogorov's method, one sprint free and one towing a small calibrated brake. Active drag: near passive drag, in a good third of the tests even below it. - **From 1999, on a tether:** the swimmer held by force sensors, in a flume with a current or in a pool against a brake. Active drag: about one and a half to two times passive drag. **Figure 4.** The methods over the years, above the line, and the studies that tried two methods on the same swimmers, below it. A timeline from 1930 to 2025, the scale widening after 1985. Above the line, the methods: Karpovich, passive towing (1933); Counsilman, forces in crawl (1955); di Prampero et al., energetic extrapolation (1974); Clarys, form against technique (1979); the MAD system, Hollander et al. (1986); the SPM or VPM, Kolmogorov and Duplishcheva (1992); 797 swimmers, Kolmogorov et al. (1997); flume residual thrust, Takagi (1999); assisted towing, Alcock and Mason (2007); the HRM, Kolmogorov (2008); NABA, Webb et al. (2011); MRT, Narita et al. (2017); robotic VPM and its reliability, Gonjo and Olstad (2022); the Sacilotto review and Kolmogorov's single-swim stand (2023); tethered residual thrust, Cortesi (2024). Below the line, comparisons on the same swimmers: MAD against VPM, Toussaint, Roos and Kolmogorov (2004); MAD against ATM, Formosa et al. (2012); the AIS workshop, VPM, ATM and RTM on 13 swimmers (2013); MRT against MAD, Narita et al. (2018); hand pressure sensors against VPM, Morais et al. (2023). ## What to remember Passive drag is a real force you can measure on a rope. Active drag is a worked-out number that depends on how a method splits your push from the water's drag. When two methods disagree, neither has to be wrong: often they give the same name, active drag, to two different things. The next post starts with the two oldest methods, the breathing method and the pads. ## The series 1. What active drag is and why it has to be inferred (this post) 2. [Pads and masks: the MAD system and the energetic method](/blog/active-drag-2-pads-and-masks/eli5/) 3. [Two swims, one power: the perturbation family](/blog/active-drag-3-two-swims-one-power/eli5/) 4. [Residual thrust and the hydrodynamic reaction](/blog/active-drag-4-residual-thrust/eli5/) 5. [Why the methods disagree, and what a coach can use](/blog/active-drag-5-why-they-disagree/eli5/) The sources for this post are listed in [the full version](/blog/active-drag-1-what-it-is/#references).