Training-Intensity Zones

Beneficial training adaptations will result from swimming at your lactate threshold, but beneficial adaptations will result from training at both lower and higher intensities as well. In fact, whenever you exercise at a heart rate greater than 70 percent of your SLTHR for at least 10 minutes, you can be sure that you will benefit from your efforts. Below 70 pecent of your SLTHR, any benefits will be negligible.

Above 70 percent of SLTHR, different training intensities produce different effects. Exercise intensity determines the training adaptation as well as the sensations you can expect during that training. There are five generally accepted training zones that are delineated by the different percentages of SLTHR. The workout levels in this book include work in each training zone.

Easy- or Active-Recovery Zone: IHR 70 to 80 Percent of SLTHR, RPE 9 to 11

This is the lowest intensity of exercise that has some fitness benefit. Beginners and people who have not exercised for a long time should start in this zone. It is a good recovery zone for people who need a break from hard training. It is also the correct intensity for recovery swims between intense exercise bouts and for cool-downs at the end of practice sessions. Easy work allows you to recover from hard work more rapidly than total rest does. Easy aerobic training stimulates circulation and growth-hormone release, which speeds the healing of tissues that have been damaged by hard training. The main source of fuel used by the muscles at this intensity is body fat. Work in this zone is perceived as very light to light and is comfortable for long durations.

Low-Aerobic Zone: IHR 80 to 90 Percent of SLTHR, RPE 11 to 13

This is a good zone for long, slow distance swimming. Exercise done in this zone improves the ability of your heart to pump blood and of your muscles to utilize oxygen. The body becomes more efficient at metabolizing stored body fat, still the main source of fuel for your muscles. Training in this zone is an effective way to overload endurance muscle fibers, and training above this intensity is less beneficial for this purpose.

This intensity is perceived as easy to moderate, depending on your fitness level—comfortable enough that you may not feel like you are actually training. Because of this, the most difficult part of following a heart-rate training program is keeping the intensity low on easy days and in long workouts. Performing basic aerobic workouts at too high of an intensity reduces the

effectiveness of harder workouts on subsequent days. Working too hard on easy days is a primary cause of overtraining.

High-Aerobic Zone: IHR 90 to 95 Percent of SLTHR, RPE 13 to 14

This zone is effective for improving overall cardiorespiratory fitness. Working here improves your ability to transport oxygenated blood to the muscle cells and carbon dioxide out of the cells. Glycogen (glucose) and fat are both utilized for fuel here, but as you do more training at this level, your body will burn less glucose and more stored fat. Since lactic acid is a by-product of glycogen metabolism in working muscles, you will experience some effects of lactic acid. But because this work is not very intense, your aerobic system is able to clear the lactic acid from the muscles before enough accumulates to cause pain or limit performance. This zone is also somewhat effective for increasing muscle strength. Training in this zone can be perceived as moderate and comfortable to somewhat hard and somewhat uncomfortable, depending on your fitness level.

                      Lactate-Threshold Zone: IHR 95 to 100 Percent of SLTHR, RPE 14 to 15

Lactate-threshold exercise is the highest intensity of exercise that you can continue for an extended time. Training near your lactate threshold increases your lactate threshold, which means that you’ll be able to maintain a faster swimming pace for a longer time. The closer to your actual lactate-threshold heart rate you train, the greater this training effect. Training in this zone is perceived as difficult but tolerable: your muscles will feel tired, your breathing will be heavy, and you will be uncomfortable.

Red-Line Zone: IHR Above SLTHR, RPE 16 and Up

In this zone, you push your body to its limit. Only train in this zone if you are very fit and have your physician’s blessing. It is possible to stay in this zone only for a short time since lactic acid accumulates rapidly, limiting performance. Training in this zone improves your body’s ability to buffer and clear lactic acid. It also improves the contractility of your fast-twitch muscle fibers, which increases speed. Your body can tolerate a relatively small amount of this type of work, but a little goes a long way. This intensity of training is perceived as extreme: your muscles will burn, your breathing will be very heavy, you will be in pain, and you may feel like you are going to vomit.