What is a good way to determine my lactate threshold with a heart rate monitor?

— Jim A Wheeling, Illinois

You've got a few choices, but first a definition of lactate threshold for those not in the know. As exercise intensity increases, your body needs to produce more energy to keep you moving. The faster you go, the more this additional energy is achieved without using oxygen, a process called anaerobic metabolism. A consequence of an increase in anaerobic metabolism is an increase in lactic acid, or lactate, in the bloodstream. Now lactate, in and of itself, isn't terrible stuff, it's just that the more of it you've got in your blood, the sooner you're going to have to stop what you are doing and sit down. Your lactate threshold is the point (it can be measured by heart rate or speed) at which you've got more lactate in your blood than you do at rest, but not so much more that you can't continue to exercise at the same intensity. Lactate threshold is important to endurance athletes because it sets the speed limit for any prolonged exercise session or race. A good training program will increase your lactate threshold, allowing you additional velocity without having to back off to recover.

So, how to find it? Fortunately, your body sets off all manner of bells and whistles to let you know that you've stepped out of your comfort zone and had better have something chasing you to justify travelling at such a rapid pace. One of the key determinants that you've crossed the lactate threshold line is your rate of breathing; it will go from being rhythmical and deep to shallow and choppy, as if the oxygen in the air has just been turned off. You may also be blessed with tingling extremities and an acidic or rusty taste in your mouth as you venture above lactate threshold. These pleasant effects, combined with a perception that you really can't be doing this much longer, are a good sign that you've entered the high lactate zone. Use your HR monitor to determine at what HR these effects occur and you'll have a good estimate of your lactate threshold.

If you have the need to do something more objective (and less painful) to determine your threshold, try this: After a 15-20 minute warm-up, do a 45-minute time-trial on level terrain. The key is to find a pace that you can sustain for the entire period, without slowing down. Start at a pace that you know you can sustain for an hour, and increase your speed slightly every couple of minutes until you reach a speed that feels manageable for the remaining time (30-40 minutes). This pace, and associated HR, are good values to use for your lactate threshold. Keep in mind your threshold isn't static, it should improve with training and you'll want to continue to monitor it every six to eight weeks and adjust your high intensity training days so that you train at or near you new threshold value.