How long and hard you can work out depends partly on a trigger that makes muscles call it quits. Here’s how to train your body to do more.

Portrait of Tammy Strobel

How long and hard you can work out depends partly on a trigger that makes muscles call it quits. Here’s how to train your body to do more.


There are different ways to measure, but an important one that doesn’t get much buzz is called your lactate threshold. As your body breaks down carbs for exercise energy, it’s also cranking out lactate as a by-product. “Your lactate threshold is the intensity of exercise at which lactate begins to accumulate in the blood at a faster rate than it can be removed, causing muscle fatigue,” says Dr Pamela Peeke, author of Body for Life for Women. “Having a high threshold means you can work at a higher intensity for a longer time before lactate levels become intolerable.” The good news: You can train yours upwards.


Chances are, you’ll never hit a lab to precisely pin down your tipping point: That requires you to ride a stationary bike while increasing the intensity of exercise every three to five minutes until you reach exhaustion, with pinprick blood tests taken intermittently to find where lactate becomes too much, Dr Peeke says. But you’ll know it when you feel it – a burning sensation in your muscles; that ache or even sudden nausea that causes you to stop. Basically, once you get to the point at which the amounts of lactate being produced and cleared are equal (known as LT2, or your second lactate threshold), you’ve reached your “highest sustainable intensity,” and 

it’s only a matter of time, as lactate rapidly increases shortly thereafter, says researcher Samantha Hoffmann, a lecturer at Deakin University in Australia. In Hoffmann’s recent study, most female athletes tested were able to last at LT2 for at least 30 minutes, but you could range anywhere from 20 to 60 minutes depending on your fitness level.


Improving your LT2 is a different animal from building your VO2 max or your aerobic capacity as measured by the maximum volume of oxygen that you can take in and use during exercise. “You could be really good at running for hours, but if I ask you to do five squats, 10 push-ups, and 20 sit-ups, you might be able to do only one round before your legs feel like lead,” says Ben Lauder-Dykes, a trainer at Fhitting Room in New York City. That’s why lactate threshold is a key part of your fitness, Dr Peeke says. 

To improve your LT2 using HIIT workouts, Lauder-Dykes recommends going at 80 per cent effort for 80 per cent of the time. (Try one of Lauder-Dykes’s routines at “You want 

to work close to your lactate threshold but not go all out, because when you hit that red line, it takes longer to recover. The workout as a whole should be the challenge, not each interval,” he says. “The more often you’re at or close to that lactate threshold, the more your body adapts, and you’ll increase the amount of effort you’re able to give before you start to get fatigued.”