Lactate is a fuel. Your heart, your brain and your slower muscle fibres burn it for energy during exercise, and your liver turns some of it back into glucose (Brooks, 2018). The old picture of lactic acid as a poison that pools in your legs has been wrong for decades, and it still leads runners to the wrong fixes.
There are three different sensations that runners blame on lactic acid. Each one has a different cause.
The burn during a hard effort
This is the one closest to the old story, but the villain is different. When you run hard, above roughly your threshold pace, your muscles break down energy faster than they can fully process it. Lactate and hydrogen ions both build up. It is the hydrogen ions, not lactate, that make the muscle more acidic, and the lactate-producing reaction actually uses up hydrogen ions rather than releasing them (Robergs, Ghiasvand and Parker, 2004).
You feel this in a 5K, the last kilometre of a 10K, or a steep climb you attack too hard. Back off and it fades within a minute or two, because the body clears it quickly once supply and demand come back into balance.
Heavy legs at 30 km in a marathon
This one isn’t lactate at all. Marathon pace sits below threshold for almost everyone, so lactate stays fairly steady for most of the race. What runs low late in a marathon is stored carbohydrate. Modelling of marathon runners shows that many simply don’t carry enough glycogen in their muscles and liver to cover 42 km at their target pace without eating on the way (Rapoport, 2010). Add muscle damage from thousands of foot strikes and you get the lead-legs feeling at 30 km.
The fix is carbohydrate during the race and a sensible first half, not anything to do with lactate. In Malaysian heat, add your fluid and sodium plan to that list. We cover it in How Much Sodium Do You Lose Running in Malaysian Heat?
Soreness the next day
Also not lactate. Blood lactate is back to normal within about an hour of stopping. The soreness that peaks one to three days after a hard or hilly run is delayed onset muscle soreness, caused by small amounts of muscle damage and the repair that follows (Cheung, Hume and Maxwell, 2003). Downhill running causes the most. See Recovery After a Marathon or Ultra for what helps.
What this changes in your training
- Threshold work is worth doing. Training around threshold pace improves how well your body uses and clears lactate, which is why tempo runs and cruise intervals are a staple of marathon training.
- “Flushing out” lactic acid is unnecessary. A cool-down jog feels good, but lactate clears on its own.
- Fuel the long race. Late-race slowing in a marathon is mostly a fuelling and pacing problem, so solve it with carbohydrate and a controlled start.
References
- Brooks GA (2018). The science and translation of lactate shuttle theory. Cell Metabolism, 27(4), 757-785.
- Robergs RA, Ghiasvand F, Parker D (2004). Biochemistry of exercise-induced metabolic acidosis. American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, 287(3), R502-R516.
- Rapoport BI (2010). Metabolic factors limiting performance in marathon runners. PLoS Computational Biology, 6(10), e1000960.
- Cheung K, Hume P, Maxwell L (2003). Delayed onset muscle soreness: treatment strategies and performance factors. Sports Medicine, 33(2), 145-164.
