An Introduction To Stress Levels

Note: this article contains data originally published at Acadian Endurance and Coaching Science Abstracts.


USA Swimming has adopted a set classification system that has seven levels:

  •     Recovery (REC)

  •     Endurance Level 1 (EN1)

  •     Endurance Level 2 (EN2)

  •     Endurance Level 3 (EN3)

  •     Sprint Level 1 (SP1)

  •     Sprint Level 2 (SP2)

  •     Sprint Level 3 (SP3)

Other sports and other countries use slightly different classifications but most systems break out along similar lines. Each category aims at a slightly different set of adaptations, as a result the set types have characteristic lengths, rests and intensities.



The bulk of any program in terms of yards/meters, and to a lesser extent in terms of time spent, consists of endurance sets as these types of sets are meant to lead mostly to aerobic adaptations. 


REC: Warm-Up / Recovery (Score Index = 0)

Recovery swims are done at very easy paces and the rest intervals during the set are usually not critical. In an acute situation, a recovery set is used after a hard set or at a meet after a race. In this use the purpose is to help rid your muscles of lactate and other byproducts of effort. It has been shown that active recovery in the form of easy swimming is more effective than passive rest at lactate removal. In the long run recovery sets are used to loosen up or help get rid of stiffness in the days following a hard workout or competition. It is meant to help you recover for the next workout.


EN1: Aerobic Base (Score Index = 1)

EN1 type sets are done at an intensity level around 90 to 95% of critical pace for the stroke you are swimming. This can vary widely with the amount of rest and length of the swim however, longer repeat distances with short rest mean you can swim a little slower and get this adaptation. Longer rest for shorter repeats mean you need to swim a bit faster, closer to your critical pace. The duration of the sets needed to induce adaptations is over 16 minutes and rests are short, generally 20 seconds or less to keep your body from recovering. You want to keep your hr up for an extended time.


EN2: Anaerobic Threshold (Score Index = 2)

EN2 type sets are done very near critical pace. A little faster than critical pace for shorter repeats and a little under critical pace for longer repeats. EN2 sets are the most effective way to develop your aerobic metabolism. Minute for minute, the best bang for your buck in aerobic developments, the stress is moderate and they can usually be performed multiple times in a week without causing problems for the athletes.

The typical set length for EN2 sets is 16 to 40 minutes. Depending on the set structure, rests can be from 5 to 40 seconds, or even up to one minute on a very long repeat.


EN3: Max VO2 (Score Index = 6)

EN3 sets are done at or very close to VO2Max. This is where your energy metabolism is at full tilt, the firehose is open, the drain is draining and overflow drain is draining as fast as possible. In fact it seems like this should be all we ever do. But like everything there is a downside, the EN3 sets are high stress and can be done in somewhat limited quantities. If we were to continue to do them they would bring on stagnation more quickly than other types of sets.

EN3 sets are also usually 16 minutes long but there is significant amounts of rest in them, up to 1 to 1 ratio of work to rest. Since your hr often gets very close to its max, this type of set affects your heart function. The amount of plasma in your blood goes up, maximum stroke volume is improved, and since you are working near VO2max, your VO2max level is improved.



Work on critical paces has shown that doing a lot of sprint work, the SP1 and SP2 levels can be detrimental to your aerobic fitness. So at the extremes, if all you ever did was sprint work, your aerobic metabolism would get worse and you could actually end up slower in your chosen event for it. The mechanisms once again are not exactly clear, if nothing else, sprint work takes a lot of time in your workout. Thirty minutes spent on an SP1 set would not be unusual, but that is 30 minutes not spent developing your aerobic metabolism. There are other more in depth mechanisms used to explain why, but they are all offshoots of, if you do less aerobic work for one reason or another, your aerobic metabolism won’t work as well. And included in that “One reason or another,” is a 40 minute set of sprint work.

This is one of the major reasons why coaches tend to switch emphases a bit as the year goes on, too much sprint work early in the season could deteriorate your aerobic metabolism and leave you a step behind come the big race.


SP1: Lactate Tolerance (Score Index = 8)

SP1 sets are also called lactate production sets, these sets are intense but due to the set structure with shorter intervals and longish rests, the lactate levels don’t get too terribly high. On these you should have some lactate type fatigue setting in during the set but not so much that your arms start to burn and really struggle. You should be able to maintain a pretty consistent pace through the set. SP1 sets are high stress and can be taken in small doses for a good chunk of the season.

SP1 sets have intervals of 25 yards to possibly out to 200 yards. The speeds are at about 95% of max speed and the total time of an SP1 set would be 20 minutes or more. However rest durations on SP1 sets are equal to or twice the duration of the work interval. So of the 20 minutes half or more of the time would be taken up by rest between swims. These types of sets are usually done earlier in the season than the very hardest work that is the SP2 work.


SP2: Lactate Peak (Score Index = 10)

SP2 sets or lactate tolerance sets are the hardest sets a swim group will do. They are similar to the distances and speeds of competition events. The idea behind a lactate tolerance set is to get accustomed to the type and amount of pain you might experience in a race. In this way you increase the size of your bathtub. It is thought that your muscles get better at buffering acid and that your brain resets the “STOP!” point on how much you will take and still swim.

SP2 set distances are usually directly relevant to your chosen event. An SP2 sets for a 200 person would consist of 150’s to 300’s, for a 100 person it would be 75’s to 150’s. These swims can be done straight through or perhaps broken up with 5 to 15 second rests in the repeat. The rest intervals usually involve active recovery and are 2 to 5 times as long as the swim interval. The effort is 100% for these sets and the lactate levels induced are quite high.


SP3: Alactate (Score Index = 4)

While the SP3 sets are the fastest ones we do, they are not the most stressful. The intervals on SP3 work top out at 25 yards and the rest is complete, a work to rest ratio of 1 to 5 to 1 to 8. Since these sets are lower stress these are actually included both early and late in the season.

Early in the season, before your fitness has really come around you can still work on SP3 without inducing a lot of stress, the work on your turnover and coordination is quite effective and this type of work continues to a degree all through the year. SP3 sets also become a mainstay in the taper, in a taper the general idea is to reduce the training loads so your body gets into the supercompensated state, but you also want to do some fast swimming as well to keep your feel for the water, the SP3 sets can fit the bill late in the taper.



The stress values for each workout are derived by taking 1/100th of the yardage for each set and multiplying it by the stress index for the energy level assigned.  If a given workout is in meters, then the stress value is increased by either 5% (for SCM) or 10% (for LCM).

Every workout is calculated as to how stressful it is as opposed to how many meters or yards were swum.  The stress value for each workout is derived from the concepts in the “Coaches’ Quarterly”, September 1995, by Rich Sharp, Ph.D., ICAR Director (and adapted from Sharp, R.L. Prescribing and evaluating interval training sets in swimming: a proposed model. Journal of Swimming Research.  9:36-40, 1993).

Sharp, R. L. (1993). Prescribing and evaluating interval training sets in swimming: a proposed model. Journal of Swimming Research, 9, 36-40.

Unless appropriate paces and intensities of work are prescribed for individuals, some swimmers may under-work while others will overwork. The task is to prescribe optimal training activities which involve the correct mix of aerobic endurance, aerobic power, lactate tolerance, and sprint ability. Each of those forms requires different intensities, duration of repetitions, and rest intervals.

Sprint ability. This is one’s maximum velocity and is a function of muscle fiber type, level of creatine phosphate in the muscles, activity of creatine kinase in muscles, maximum muscle power, and neuromuscular recruitment patterns. A swimmer has to develop the skill of reaching maximum velocity as soon as possible in a race, to maintain maximum velocity for as long as possible, and develop the ability to call upon sprint ability in the middle and at the end of longer (>30 sec) races.

Lactate Tolerance. When muscles contract they produce lactic acid because of incomplete oxidation of carbohydrate used as fuel. After its formation, it immediately splits to form lactate and hydrogen ions (H+). The H+ ions alter the acidity of the blood, lowering its pH value depending upon their concentration. This reaction is why the terms lactic acid and lactate often are used interchangeably. Thus, the pH of blood is a measure of the amount of H+ in the body. When the H+ ions are allowed to accumulate, the pH in the muscles falls, that is, the environment in the muscles increases in acidity. A normal resting measure of pH is 7.0 whereas in very strenuous work that predominantly uses anaerobic energy sources the level can drop to a value of 6.3. As the acidity level changes (the pH level is lowered), the muscles become weaker, often tighter, and contractile force is reduced. As blood and muscle acidity increase, so does the feeling of fatigue.

At low intensities of exercise, for example, ANThreshold training, the rate at which lactic acid is produced is balanced by the rate at which it can be removed from muscle and blood. However, as a swimmer speeds up, for example, at aerobic capacity speeds and faster, the use of carbohydrate as fuel is greatly increased, and the production of lactate is greater than the ability of the lactate-removal mechanisms. Thus, after a certain intensity of work, that is, swimming at a particular speed for a minimum duration, lactate accumulates.

Resting or normal activity levels do not tax the capacity to remove lactate. Exercise can increase the production of lactate from 3-5 times above the resting level without any appreciable change in a muscle’s pH. This is because the body has buffers which combine with the H+ ions and remove them from bodily fluids. The greater the amount of buffer capacity, the greater can be the intensity of work before H+ ions accumulate and lower the blood pH. The buffering capacity of muscle determines its ability to tolerate lactate before the pH is altered noticeably. Fast twitch muscle fibers have a greater buffer capacity than slow twitch fibers. Buffer capacity can be increased through training. It is very helpful to assess a swimmer’s ability to tolerate lactate accumulation because it will indicate the changes derived from training designed to increase the amount of anaerobic work that can be sustained.

Aerobic power. This is a person’s maximum ability to use oxygen. It is the upper limit or ceiling for aerobic endurance. Endurance athletes have a high capacity but it does not differentiate between them. It is a requirement for achieving an elite status but is not related to performance among an elite homogeneous group.

Aerobic endurance. This is a measure of an athlete’s ability to perform prolonged, continuous exercise and depends upon physiological, biomechanical, nutritional, and psychological factors. The best measure currently available is the lactate or anaerobic threshold. It determines the maximum speed a swimmer can sustain without experiencing progressive accumulation of lactate in the blood. However, there are no pool races that use this capacity. Thus, its contribution to race quality is questionable. Rather, it serves as the basis for a general conditioned state.

Two reasons justify aerobic endurance training. It contributes to accelerated recovery from fatiguing work and it extends one’s ability to tolerate the demands of lactate tolerance, aerobic power, and speed training. This form of training may be the easiest and most efficient way of improving a swimmer’s stroking economy which in turn, means that a swimmer can swim at faster speeds before reaching lactate threshold.