If anyone tells you that he can hit the reset button between events in a triathlon, you can tell him science doesn't agree. Whether your going from a swim to a cycle or a squat to curls, your performance on the second exercise will be diminished, according to a new study from Nottingham Trent University in Britain. Physiologist Michael Johnson and his team showed that 10 minutes of high-intensity arm cycling significantly reduced performance on a "leg-cycling" time trial, compared to athletes who didn't arm cycle. More surprising, the work leg muscles were able to do seemed to depend on by-products of the exercise, like lactic acid, and not just depleted energy.
This means the level of performance and speed you can maintain strongly depends on how much any part of your body performed above the lactate threshold. How much of a "sprint finish" you have, therefore, would depend on how much lactic acid (among other chemicals released in fatigued muscles) is in the bloodstream, rather than how much energy you have to burn, according to Johnson.
No matter how much energy you have stored in certain muscles, if you've overworked other areas of your body, you're going to feel the effects. While these muscles seemingly have the physical energy to work fresh, researchers speculate that much of what slows you down is actually occurring in your brain. "It is possible that accumulation in muscles is 'seen' by the brain, which subsequently regulates the intensity of "drive" sent to the muscles in order to prevent metabolite accumulation exceeding some critical limit," says Johnson. "Thus, maybe all exercise does indeed start and end in the brain."
This might give you more reason to take it easy during the swim or your warm-up run. "I don't think it is possible to go fresh into the next event of the triathlon," says Johnson. "The fatigue process begins as soon as exercise starts, and it is very likely that the swim impacts on the cycle, thus the cycle would be quicker without the preceding swim."