Creatine, From the Muscle Out · Day 1 of 7

What Powers the First Ten Seconds

Berlin, 16 August 2009. Usain Bolt covers 100 metres in 9.58 seconds. Between the 60 and 80 metre marks he is moving a little over 12 metres a second, faster than any human has been timed running. And then, over the last 20 metres, he slows down. Everyone does. The fastest man alive could not hold his top speed for two more seconds.

That is not a failure of will. It is a supply problem, and the shape of it explains almost everything worth knowing about creatine.

Muscle contracts by spending a molecule called ATP, adenosine triphosphate. It is the only fuel a muscle fibre can actually use. Fat, sugar, protein — none of them power a contraction directly. They are all raw material for making ATP, and ATP is what gets spent. When a myosin head grabs an actin filament and pulls, it snaps a phosphate off ATP and uses the energy released. What’s left is ADP, adenosine diphosphate, which is useless until the phosphate goes back on.

Here is the number that matters. A kilogram of your muscle holds roughly 5 millimoles of ATP. At full sprinting effort, that is enough for about one to two seconds. Not ten. Not five. One or two. Your whole body contains something like 250 grams of ATP at any instant, and over a normal day you cycle through roughly your own body weight of the stuff. It is not a tank. It is a bucket brigade, and the bucket is small.

So why doesn’t Bolt seize up at the two-metre mark?

Because sitting alongside the ATP in every muscle fibre is a second molecule at about four to five times the concentration: creatine phosphate, also called phosphocreatine. It is creatine with a phosphate group attached to it, and that phosphate is held in a high-energy bond. An enzyme called creatine kinase takes the phosphate off creatine phosphate and puts it straight onto ADP. ADP becomes ATP again. The creatine, now bare, waits to be recharged.

The important thing about this reaction is its speed. Creatine kinase is one of the fastest enzymes in the body, and it sits right where the ATP is being spent — bolted onto the contractile machinery itself. There is no waiting. No oxygen needed, no membranes to cross, no chain of ten reactions. One step, instant, local.

The result is a buffer. During an all-out effort, the ATP concentration in a working muscle barely falls. What falls is creatine phosphate. In a 100-metre sprint it drops by something like 70 or 80 per cent. The muscle is not running on ATP so much as it is running on creatine phosphate that is constantly being converted into ATP a fraction of a second before use. The ATP level stays nearly flat because the buffer is being drained to keep it that way. This is why the system is sometimes called a spatial and temporal buffer: it holds the line while slower energy systems get organised.

The slower systems do arrive. Breaking down stored muscle sugar without oxygen — glycolysis — supplies a large share of the energy in a 100-metre race, probably around half. But it takes a couple of seconds to spin up, and it cannot match the raw rate that creatine phosphate delivers. Aerobic metabolism, which burns fat and carbohydrate with oxygen, is slower still to arrive and far slower in output, though it can run for hours. So the power curve of a human being has a distinct shape: enormous for two or three seconds, high for another five or six, then a steep drop as the creatine phosphate runs low and everything downstream has to carry the load. Bolt’s deceleration over the last 20 metres is that curve made visible.

This chemistry has been known for a long time. Creatine phosphate was identified in 1927, independently by Philip and Grace Eggleton in Cambridge and by Cyrus Fiske and Yellapragada Subbarow at Harvard. In 1934 Karl Lohmann worked out that the phosphate does not go straight to the muscle — it goes to ADP, via the enzyme. Ninety years later, that reaction is still the whole basis of the supplement industry built on creatine.

And here is the consequence. If creatine phosphate is the buffer for efforts lasting a handful of seconds, then having more of it in your muscle can only matter for efforts lasting a handful of seconds. A marathon runner spends two hours in a state where creatine phosphate is essentially topped up and irrelevant, because the rate of energy demand never exceeds what oxygen can supply. Adding more of the buffer to a system that is not using the buffer changes nothing. The evidence bears this out. Creatine supplementation shows up reliably in sprints, jumps, throws and heavy lifts, and it does not show up in endurance events.

There is a second half to this that is easy to miss, and it turns out to matter more than the first. Creatine phosphate is not just a store, it is a rechargeable store. After a sprint, the depleted creatine gets its phosphate back from ATP made by the mitochondria using oxygen. That recharging has a half-time of roughly 20 to 30 seconds and takes several minutes to complete. So if you sprint, rest 90 seconds, and sprint again, your second sprint is limited by how much creatine phosphate you managed to rebuild in that gap.

That is where a bigger pool actually earns its keep. Not in one heroic effort, but in the fifth one. In the last two reps of your third set, when the buffer is partly refilled instead of nearly empty. The measurable benefit of creatine in a laboratory is usually not a faster single sprint. It is the sprint after the sprint after the sprint.

Which means the supplement is not really doing anything to your muscle. It is doing something to your rest intervals.