Lactic Acid Threshold (LT) is the highest 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 slow enough that lactic acid, the substance that makes muscles burn during hard exercise, does not accumulate faster than it can be removed. At this intensity, you are working very hard, but can still maintain exercise because lactic acid levels in the blood and muscles are steady, not increasing. Increasing the intensity just slightly causes lactic acid to build up and brings premature fatigue and delayed recovery. Training near LT decreases the amount of lactic acid being produced and increases lactate removal at a given output. At this intensity, the fast-twitch fibers can be trained to produce less lactic acid and the slow twitch fibers can be trained to burn more lactic acid, both of which raise the LT and allow you to produce more force at a higher heart rate.
Since lactic acid levels are controlled, recovery from this type of training is quicker than from other high-intensity training methods, therefore LT training has the best cost:benefit ratio of any type of training. When you experience “rubbery leg” syndrome, a marked increase in breathing difficulty, and a general full body burn, you have reached the point where lactic acid accumulates at a faster rate than it can be removed, which will rapidly decrease your ability to maintain a steady effort. At this point, your body cannot inhale 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 via a lab 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 near LT, you can approximate your personal LT without spending money on a lab test. Due to the varying muscular demands of each skill, your LT will change depending on whether you are swimming, cycling, or running, so I recommend an LT test for each. Based on where your LT lies in each sport, you will be equipped with the knowledge to train at the highest intensity that is possible (without overtraining). When you reach your LT heart rate during a training session, you will know to back off, so that the body bounces back for the next day’s session.
Exercise researchers have found no perfect LT field test, but here is an example for each skill. You’ll need a HR monitor or very accurate carotid/radial pulse for the running and cycling tests:
Swimming: Since monitoring heart rate during the swim is difficult, this is the one test that will produce a “pace” LT rather than a heart-rate LT. Warm-up with a 4×50 swim and kick. Starting slowly and gradually increasing pace, swim 1000 yard/meters at a constant pace and as close as possible to a race pace effort. Basically, you should be swimming at the fastest possible *steady* pace you can maintain for the entire distance. Divide by 20 for your average 50 pace and by 10 for your average 100 pace. These will be your approximate paces for any LT workouts.
Running: Warm-up with a 10-15 minute jog. Then, on a flat course or track, run a 30 minute time trial, following the same pace recommendations as the swim (steady and fast). Record your average heart rate for the last 20 minutes. This will be your LT heart rate for your run workouts.
Cycling: Warm-up with 10-15 minutes of light cycling. Following the warm-up, cycle for 8 minutes as steady and fast as possible up a slight hill (2-3%), at 80-100RPM. Record your average heart rate during the climb, then rest 3 minutes (or descend). Repeat 1x, and record your LT heart rate for cycling 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 fast twitch muscle fiber damage during hard efforts. Depending on your fitness levels, I do not recommend more than a two back-to-back days of LT training for the same skill (swimming, running, or cycling). But ideally, at this point in the triathlon season for most athletes, you should be increasing the number of LT workouts for every sport each week, while gradually decreasing the volume of your base training.
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