What to Know About Lactic Acid and Lactate Threshold Training

Experts clear up the confusion on the burning sensation you feel in your legs during hard efforts.

BY MOLLY HURFORD | 13 June 2025

Photo: iStock

You might have heard the terms ‘lactic acid’ and ‘lactate threshold’ tossed around in cycling circles. Often, a rider talking about the feeling of lactic acid build-up refers to that heavy sense of muscular fatigue, or a ‘burning’ sensation in the legs. Here’s what you need to know.

What is lactic acid and lactate?

Lactic acid is a common misnomer that tends to be used interchangeably with lactate, but they’re not quite the same.

“Lactate is a byproduct of anaerobic glycolysis, which occurs when our body breaks down glucose for energy for muscle contraction in the absence of oxygen,” says Prof. Julia Casadio, exercise physiologist and creator of HerStrength.co.

Easy to moderate exercise requires predominantly aerobic (meaning with oxygen) pathways to produce ATP in the muscles for contraction. So, when you’re doing Zone 2 workouts or recovery rides, you’re producing lactate, but your body can clear it without you noticing.

“You’ll notice a burning sensation in your muscles, heavy breathing, and a decrease in power output.”

However, lactate accumulates at a higher rate during high-intensity efforts at (and above) your anaerobic threshold, otherwise known as Zone 4. When you start to reach anaerobic levels of activity, your body turns to glucose instead of oxygen for energy, and you lose the ability to clear lactate fast enough to keep riding at the same intensity.

Is lactate bad?

Lactate isn’t a bad thing – it just feels bad. “Traditional models of muscle fatigue considered lactate to be a metabolic waste product that limits performance,” says Casadio. “But lactate in the blood is taken up and used as fuel in surrounding muscle and organs such as the brain, heart, liver and kidneys. Lactate actually offers a fast and effective fuel source for working muscles.”

What do lactate and lactate threshold have to do with cycling performance?

Lactate can provide important information when it comes to setting your power and heart-rate training zones, because it provides a clear line that demarcates the shift from aerobic to anaerobic exercise.

How do you know you’ve hit lactate threshold?

“When you’re at or above your lactate threshold, you’ll eventually notice a burning sensation in your muscles, heavy breathing, and a decrease in power output,” says Casadio. “Another clear sign is when you feel ‘locked up’ or unable to sustain intensity.”

Can you improve your lactate threshold?

“I like lactate threshold, because with proper training, you can see improvements in less than three months,” says Evan Hoyt, an exercise physiologist at the Human Powered Health Lab. “Improving something like your VO2 max takes a lot longer.”

What are the best workouts for improving your lactate threshold?

“Just doing regular high-intensity training – one VO2 max session a week (Zone 5), one lactate threshold session a week (Zone 4) – plus endurance rides and some tempo efforts will contribute to increasing your lactate threshold,” says Si Bradeley, coach and event manager at MyWhoosh.