The lactic acid threshold (LT) is the highest intensity at which the body can recycle lactic acid as quickly as it is produced. Anaerobic metabolism, or the burning of sugar by the process of glycolysis, is so slow that lactic acid, the substance that causes muscles to burn during intense exercise, does not accumulate faster than it can be eliminated. At this intensity, you are working very hard, but you can still maintain the exercise because lactic acid levels in your blood and muscles are constant and not rising. Slightly increasing the intensity causes lactic acid to build up and leads to premature fatigue and delayed recovery. Training near LT decreases the amount of lactic acid that is produced and increases lactate clearance at a given output. At this intensity, fast twitch fibers can be trained to produce less lactic acid, and slow twitch fibers can be trained to burn more lactic acid, increasing LT and allowing you to produce more force at a higher heart rate.
Since lactic acid levels are controlled, recovery from this type of training is faster than with other high-intensity training methods, making LT training the best cost-benefit ratio of any type of training. By the time you experience “rubber legs” syndrome, a marked increase in shortness of breath and a general burning throughout your body, you have reached the point where lactic acid accumulates at a rate faster than it can be removed, It will quickly decrease your ability to sustain a steady effort. At this point, your body cannot take in oxygen or expel carbon dioxide at a fast enough rate to allow for sufficient aerobic metabolism or lactic acid buffering.
The best way to determine your LT heart rate is through a laboratory test, in which blood lactate levels are collected during exercise. However, based on the clear signs that occur in your body when you are at or very close to the LT, you can approximate your personal LT without spending money on a lab test. Due to the different muscle demands of each skill, your LT will change depending on whether you are swimming, biking, or running, so I recommend an LT test for each. Depending on where your LT is in each sport, you will be equipped with the knowledge to train at the highest intensity possible (without overtraining). When you reach your LT heart rate during a training session, you know to back off to allow your body to recover for the next day’s session.
Exercise researchers haven’t found a perfect LT field test, but here’s an example for each skill. You will need a very accurate HR or carotid/radial pulse monitor for running and cycling tests:
Swimming: Since it is difficult to monitor heart rate while swimming, this is the only test that will produce a “pace” LT instead of a heart rate LT. Warm-up with swimming and kicking 4×50. Starting slowly and gradually increasing your pace, swim 1000 yards/meters at a steady pace and as close to a race pace effort as possible. Basically, you have to swim at the fastest *constant* pace possible that you can maintain for the entire distance. Divide by 20 for your average pace of 50 and by 10 for your average pace of 100. These will be your approximate steps for any LT workout.
Running: Warm up with a jog for 10 to 15 minutes. Then, on a course or flat track, run a 30-minute time trial, following the same pace recommendations as swimming (steady and fast). Record your average heart rate for the last 20 minutes. This will be your LT heart rate for your race workouts.
Cycling: Warm up with 10-15 minutes of light cycling. After warming up, pedal for 8 minutes as steadily and quickly as possible up a small incline (2-3%), at 80-100 RPM. Record your average heart rate during the ascent, then rest 3 minutes (or descend). Repeat 1 time and record your cycling LT heart rate as an average of your two 8-minute climbs.
Finally, remember: Although LT intensity training is useful as a way to train intensely with a lower risk of overtraining, there is still significant damage to fast-twitch muscle fibers during intense efforts. Depending on your fitness levels, I don’t recommend more than two days in a row of LT training for the same skill (swim, run, or bike). But ideally, at this point in the triathlon season for most athletes, you should increase the amount of LT training for each sport each week, while gradually decreasing the volume of your base training.
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