SLTHR: Swimming Lactate-Threshold Heart Rate

During nearly all exercise, some combination of carbohydrate and fat is used for fuel. Lactate (lactic acid) is a by-product of carbohydrate metabolism in working muscles. If too much lactate accumulates in your muscles or blood, it begins to cause discomfort or pain and to limit performance. During exercise, your aerobic system clears lactic acid from your muscles and blood. At rest and at low and moderate levels of exertion, there is a balance between lactate production and lactate removal. As exercise intensity increases, however, there is a tipping point at which the body produces lactate faster than it can be removed. This causes an accumulation of lactate in the muscles and blood, which soon limits performance. This tipping point is called the anaerobic threshold or lactate threshold. Lactate-threshold work is the highest intensity of exercise that you can continue for an extended time, and it is a very effective work range for improving your aerobic fitness. A good portion of the training detailed in the practices is performed just at and just above your swimming lactate threshold. Other training intensities called for in the practices are also expressed as a percentage of your SLTHR.

In chapter 3, you were exposed to the T-20 swim. If you gave this swim your best effort, without stopping (or with only very short pauses) and without speeding up in the last two minutes, then the IHR you took at the end of this swim is your lactate-threshold heart rate for swimming. In fact, throughout your swimming career, your best-effort T-20 (or T-30) IHR will be your best indicator of your current SLTHR. But for those of you who are in poor condition, are new to sustained exercise, or are new to sustained swimming, your perception of a best effort may be well short of your true physical ability. Your T-swim IHR will thus fall short of your true swimming lactate-threshold heart rate. That’s okay, though, because you’ll have lots of opportunity to retest yourself and to get more accurate results. As you move through the workouts, the program will call for you to do T-swims on a regular basis. Each time you do a T-swim, the goal is to improve upon previous performances. If you approach each T-swim in that manner and if your conditioning and performances improve over time, your SLTHR will increase.