Active drag, part 2: pads and masks. The MAD system and the energetic method

The first post ended on a fundamental problem: while you swim, your propulsion and the water’s drag cancel each other out, and no instrument can tell them apart. This post looks at the two oldest ways around this dilemma. One listens to your breathing; the other gives your hands something solid to push against so that the force can be weighed. Both are clever, yet their answers could hardly be further apart: by the breathing method, active drag comes out at 1.6 to 3 times passive drag; on the pads, it is roughly the same as passive drag. Understanding why this happens tells you a lot about every method that came after.

The breathing method: how much help would make swimming effortless?

Picture riding a bike into a strong headwind. The pedals are heavy, you are gasping, your legs are filling with lead. Then a friend catches up and starts pushing you from behind. Lightly at first, and breathing gets a little easier. Then harder… At last they push so hard that you can stop pedalling altogether and your speed does not drop: at that point, your friend’s push is exactly equal to the air’s drag against you.

The breathing method, first used in 1974, applies this exact logic to the pool. You swim several lengths at one and the same speed. A rope runs from you over a pulley to a weight: in some swims it pulls you along, in others it holds you back, and the weight is changed from swim to swim. A mask connected to a gas analyser measures how much oxygen you consume at each setting, showing how hard your body is working. The more help you get, the less oxygen you use.

If you plot oxygen consumption against the force on the rope, the points fall close to a straight line. Now extend that line down to the amount of oxygen you use at rest. The point where it meets that baseline represents the exact amount of help at which swimming would cost you no extra effort (the friend’s push that lets you stop pedalling). That assisting force is taken as your active drag.

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A chart of oxygen uptake, 0 to 4 litres a minute, against an external horizontal force added at constant speed, from minus 100 newtons (assisting) to plus 50 (resisting). Measured points over the range from about minus 30 to plus 40 newtons lie on a straight line rising with the force. Extended down to the resting uptake, a dashed line, the line meets it at minus 80 newtons: the assisting force at which swimming would cost nothing, read as minus the active drag. Notes: the method assumes the efficiency stays the same across loads; van der Vaart et al. (1987) warned that “even small systematic errors are greatly magnified as a result of extrapolation”; it works at aerobic speeds only, where oxygen uptake is steady; its results come out at about two to three times passive drag.

The method has two real strengths: you swim almost normally with nothing in your way but the mask, and it ties drag directly to energy expenditure, which is what an endurance coach cares about most.

Its weak spot is the line itself. The measured data points cover only a small stretch, while the answer lies far beyond them. It is like guessing where a road ends from its first few hundred metres: if the road starts to bend, you end up somewhere else entirely. The line remains straight only if your body converts energy into speed equally well at every load: whether the rope is pulling you or holding you back, with a light weight or a heavy one. And there is good reason to doubt that: in other forms of movement, efficiency rises as you work harder. The Dutch scientists who raised this issue warned that over such a long extrapolation even the smallest error, repeated in every measurement, grows many times over by the end of the line. A laser pointer turned by a fraction of a degree hardly moves at your hand, but its dot shifts half a metre on a far wall. Nobody has been able to check this doubt, because a swimmer’s efficiency cannot be measured on its own in a pool.

There is a speed limit, too. Oxygen consumption has to rise to a certain level and stay there for a while, which only happens at easy, aerobic speeds, up to about 1.4 metres per second. If you swim any faster, the body starts working on anaerobic credit, meaning the mask no longer shows the whole bill. To reach race speeds, one research group decided to add a blood-lactate correction, but that correction has been validated mainly by the group itself.

Ultimately, the breathing method yields some of the highest numbers of any method: two to three times passive drag in early studies, and about 1.6 to 2.1 times in more recent ones.

This method has a simpler cousin, planimetry (literally, measuring areas), that does not even need a gas analyser. Look at a swimmer from the front, against the direction they are moving: their outline is the area with which the body meets the water. When towed, the body lies almost flat, tilted by only 4 to 6 degrees. While swimming, the legs and hips drop, the trunk tilts by 10 to 12 degrees, and the outline seen from the front grows. Researchers compared these areas on video and assumed that drag grows by as much as the area does. The results land in the same range as the breathing method’s, but they track speed much less closely than measured drag does, and they ignore the moving limbs and surface waves.

The pads: the MAD system

Why can’t the force of a stroke simply be measured while someone swims? The hand pushes against water, and water is a hold that moves: it gives way and slips out from under the hand. In 1986, Dutch researchers proposed replacing the water under the hand with a fixed hold: then every push lands on a solid object, and that object can be connected to a force sensor.

Their Measuring Active Drag (MAD) system works like this. A 23-metre rod is mounted 0.8 metres underwater along the swimming lane. Fitted to the rod are 15 or 16 pads spaced about 1.3 metres apart, with a force sensor at the end. Think of monkey bars laid flat underwater. You swim freestyle using only your arms, with a pull buoy held between your legs, pushing off each pad in turn instead of catching the water.

At a constant speed, you neither accelerate nor decelerate, meaning your average push on the pads equals the water’s drag. That push is what the sensor reads: a real force, measured directly. By repeating the swim across 10 to 16 speeds, from about 1.0 to 2.1 metres per second, you get your entire drag curve rather than just a single number.

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An animation of the MAD system in four parts. At the top, the rod as a whole: 23 m long with 16 pads and a force transducer at its end, and the swimmer’s place along it. Below, a painting of the swimmer seen from the side, the water cut away at the surface and the view going along with him: arms only, his legs together on a pull buoy, he swims over pads 1.35 m apart on a rod 0.8 m under the surface; each hand in turn lands on the upright face of a pad ahead of the shoulder, presses on it while the body passes over and leaves it behind the shoulder, the pad being pushed lit and an arrow showing the push; the arm then recovers over the water to the next pad. Third, the force the pads pass to the transducer: a pulse for each push, peaking at about 100 newtons in the slowest swim and 275 in the fastest, and a dashed line for its running mean, which at constant speed is the active drag. Fourth, a drag-speed chart: five swims at 1.2, 1.4, 1.6, 1.8 and 2.0 metres a second each add a point, from about 36 newtons at 1.2 to about 97 at 2.0, and a curve D = A·v^n is fitted through them, where D is the drag, v the speed, and A and n are fitted constants.

The MAD system has proved remarkably reliable. Tested on two consecutive days, the same swimmers’ results differed on average by only about 1.5 newtons, roughly the weight of a medium-sized apple. Even changing the distance between the pads by 16 per cent, which altered stroke rate by 14 per cent, did not change the measured drag. The system’s readings come out roughly equal to passive drag: in one study, 53 newtons at 1.48 metres per second. Furthermore, the curves vary from swimmer to swimmer more than textbook rules suggest: drag does not simply grow with the square of speed, as most models assume.

That reliability comes at a price, though: on the MAD system you do not swim the way you normally do.

• Arms only. It works solely for front crawl and isolates the arms, even though the legs contribute up to 12 per cent of a swimmer’s power.
• Solid contact. A hand pressing against a fixed pad is not the same as a hand catching water that gives way. Moreover, the start of each stroke, from water entry until the hand reaches the pad, is not measured at all.
• Fixed geometry. The pad spacing locks in your stroke length and, consequently, your stroke rate at any given speed. At an equivalent percentage of top speed, this rate differs from free swimming by about 14 per cent. It is like crossing a stream on stepping stones: you get across, but you can no longer walk naturally. Every step has to be fitted to the stones.
• Altered technique. Swimmers tend to lift their heads to spot the next pad, alter their elbow bend, and at top speeds struggle to turn their arms over fast enough.

Why the two disagree

The breathing method says active drag is 1.6 to 3 times passive drag; the pads say it is roughly equal to passive drag. Both cannot be right about the same thing, yet the field has lived with this discrepancy for forty years. Three explanations are on the table:

1. The road bends. If the true relationship between oxygen uptake and assisting force is curved rather than linear, extending a straight line will overshoot the mark, making the breathing method’s estimates too high.
2. The pads take something away. Solid pads, floating legs, an unmeasured catch and a fixed stroke length may each strip away some of the drag present in normal swimming. When the same swimmers were tested in a flume (a pool with a flowing current), their arms-only drag came out 19 to 47 per cent higher than their MAD drag.
3. Two different questions. The breathing method asks what it costs to move the entire swimmer, including the energy lost to the water thrown backwards by the arms and legs. The pads simply ask how hard the hands push against fixed points. They may well be measuring two different quantities under the same name, which brings us right back to the core theme of the first post. (The fourth post in this series will look at a school of thought that treats them in exactly this way.)

What to remember

The MAD system measures a genuine mechanical force and delivers repeatable results from day to day, but only for an arms-only freestyle with an altered stroke. The breathing method links drag to metabolic energy, but only at easy speeds, relying on a long extrapolation that cannot be directly verified. Neither tells you the true drag of a swimmer’s full, natural stroke at race pace.

To get at that drag, researchers turned to a new idea: swim twice, once freely and once against a small, known load, and compare the two. That is the subject of the next post.

The series

1. What active drag is and why it has to be inferred
2. Pads and masks: the MAD system and the energetic method (this post)
3. Two swims, one power: the perturbation family
4. Residual thrust and the hydrodynamic reaction
5. Why the methods disagree, and what a coach can use

The sources for this post are listed in the full version.