Lactate as Post-Exercise Fuel. Rethinking the Cooldown

Part 1 Frogs and the 1990's

When I started my undergrad in Kinesiology, in the 90's, I remember seeing fellow students wear T-shirts that said "HIGH ON LACTIC ACID." The burning sensation in your legs was lactic acid, the bad stuff we produce when we exercise vigorously. We have since debunked this myth as we don't make lactic acid. The original research was based off Frogs which made lactic acid and it seemed to make sense as the burning sensation feels like acid. However, we have since divided the words lactic acid into "lactate" and "hydrogen ions." When glucose breaks down, lactate is produced and energy is released (ATP). Hydrogen ions are also produced in this reaction which are the bad guys making our muscles burn as our pH level drops.

So, Lactate is good then?! Yes! Lactate isn't just waste; it's a versatile molecule transported to other tissues (like the heart or liver) to be used as fuel or converted back to pyruvate when oxygen is available. The T-shirts should have said "HIGH ON HYDROGEN IONS!" But, it doesn't have the same ring.

With that short review of chemistry, then why is our exercise culture obsessed with getting rid of lactate?! The Frog myth still exists and it takes decades to change concepts.

Lactate as Fuel

Recent research indicates that lactate is far from a waste product; it is a critical fuel source used both during and after exercise to replenish energy stores. Consequently, the traditional "active cool-down" is now viewed as less essential for many athletes, as the body efficiently processes lactate even without continued low-intensity movement.

Modern exercise physiology highlights several ways the body uses lactate as a valuable substrate after physical activity:

Glycogen Recovery: After intense exercise, lactate is incorporated back into muscle glycogen, particularly in glycolytic (fast-twitch) muscle fibers.

The Lactate Shuttle: According to the "lactate shuttle theory," lactate produced in fast-twitch muscle fibers is transported to other tissues—including slow-twitch fibers, the heart, and the brain—where it is oxidized for immediate energy.

Gluconeogenesis: In the liver, lactate is converted back into glucose through the Cori cycle, which then recirculates to provide energy for the body.

Brain and Heart Fuel: Both the heart and brain preferentially use lactate over glucose as a fuel source when it is available in high concentrations after exercise.

The Cooldown

For some, it may be a tough pill to swallow that the 20 minute cooldown is NOT going to help you the next day. Some competitive sports like swimming, cycling and cross-country skiing are infamous for making their athletes do cooldowns until their lactate levels drop below a 2.0 mmol/L score. I remember being one of the testers measuring blood lactates on swimmers and telling the athlete to "go for another lap, your lactate is too high."

Natural Clearance

Lactate is flushed from muscles quickly regardless of movement; passive rest allows the body to effectively utilize the remaining lactate for energy restoration.

How long should I cooldown?

While active cool-downs may help prevent dizziness by maintaining circulation to the brain immediately after a hard stop, they offer little benefit for long-term recovery or performance readiness the next day. Once you feel like you have recovered from your effort and your blood pressure has stabilized, you are good to stop. Anywhere from 1 to 5 min. This will vary with each person, but the cooldown does not need to be another set or workout. Let your muscles and heart rest.

Last Word. Human Evolution

When investigating these concepts, I like think of the biology and evolution of humans. Does the concept we preach always make sense? I think we have been somewhat misled to think that after a hard workout, we need to go for a 15-20 min cooldown in order to "flush" the lactate out of muscle and back to the liver. If we don't do this, the lactate will pool in our legs like syrup and keep us from recovering. For the finely tuned machine humans are, that doesn't make sense. I can't see hunters and gatherers going for a short run after they made a kill to feed their families. Or how about soldiers, heading out for a hike after a hard battle to flush out the lactate so they are ready to fight the next day.

We are as superb species that knows how to recover from exercise. When we stop working out, the blood keeps pumping and systems keeping running. That extra cooldown is not going to save us!