Energy Systems for Dummies
By Scott Rabalais

Freddie Finless showed up for practice, expecting a few easy sets. Instead, after a brief warm-up swim, his coach offered three timed swims to the team. Freddie and friends swam the following events, and here were Freddie’s results:


25-yard freestyle -- 15.0 seconds (15 seconds per 25 yards)
100-yard freestyle -- 1:12.4 (18.1 seconds per 25 yards)
500-yard freestyle -- 7:24.0 (22.2 seconds per 25 yards)

Freddie’s energy expenditure in the three time trials followed three distinct pathways in energy metabolism. Let’s take a closer look at the work on the cellular level to determine how Freddie used his energy to complete his time trials.

ATP – Where It All Begins

ATP, or adenosine triphosphate, is the sole source of potential chemical energy in the body. When Freddie called his body to work, ATP decomposed into ADP (adenosine diphosphate) and phosphoric acid, plus energy. There are various energy forms in the body that recycle ADP back to ATP, thus allowing for the further creation of energy to be used by the body. Physical energy is created primarily by the continual recycling of ATP, rather than an increased store of ATP, and how the ATP is recycled determines the swimmer’s ability to work.

Basically, there are three ways that ATP is recycled (the three metabolic pathways):

1) Creatine Phosphate – Short and Fast

The process of recycling ATP from creatine phosphate (CP), which is contained in the cells, provides high intensity work, but has a short duration and low efficiency. In the 25-yard freestyle, Freddie primarily used CP to provide energy for his swim. However, the CP system works for a maximum of only 10-15 seconds, and would take significant recovery time to restore itself. The use of oxygen was a minimal factor in this swim, and Freddie actually completed it in a single breath.

2) Anaerobic – The Middle Distance Metabolism

When Freddie swam his 100-yard time trial, he was forced into slowing his rate of speed due to the length of the swim. Instead of using CP to recycle his ATP stores, glycogen (glucose) was called into play to allow Freddie to complete the 72-second swim at his quickest pace. Like creatine phosphate, glycogen is stored in the muscle cells, but it recycles ATP at a slower, though longer-lasting, rate than CP. Even though Freddie took several breaths per length during his 100-yard swim, the metabolism that provided the energy for swim did not require oxygen (anaerobic or non-oxidative).

3) Aerobic Metabolism – Going the Distance

The process of using oxygen to recycle ATP primarily from glycogen is called aerobic metabolism. Though the process is slower and creates less power than the CP and anaerobic metabolisms, it is very efficient and long lasting. Freddie’s 500-yard freestyle, which required the use of his energy for over seven minutes at a reduced pace, was made possible with the use of his aerobic energy system.

Increasing Your Capabilities

An important purpose of swim training is to develop the body’s ability to metabolize in each of the three systems: CP, anaerobic and aerobic. Commonly, swimmers will specialize in either sprint, middle distance or distance events, corresponding to their ability to metabolize effectively in the respective system. Though rare, some swimmers do have wide-ranging energy-producing capabilities that allow them to achieve great results in events from short to long distances.

Swimming’s scientific community has identified various training zones that are defined by a number of factors. One the most basic level, there are two energy zones, anaerobic (sprint or speed) and aerobic (endurance). The anaerobic and aerobic levels can be further categorized into additional levels, with five and seven levels both commonly accepted. These levels are characterized by energy supply, duration of exercise, swimming distance, lactate amount and heart rate.

The table below shows a summary of five training zones. The values shown in the chart are only estimates and may vary based on the individual genetics, training backgrounds muscle composition and fatigue levels, among other factors. Also, there is no precise division or boundary between zones as displayed in the table.

Energy System/Supply



Lactate Level

Heart Rate

Perceived Exertion




None to Low

60% max



10 minutes +



60-75% max


Mix Aerobic/Anaerobic

3-10 minutes



75-85% max



30 sec – 3 min



85% max and higher

Very High

Creatine Phosphate

0 – 30 sec





Factors Determining Energy Zones

Duration and Distance – Obviously, the longer the exercise, both in terms of time and distance, the more oxygen is called into play. In the recovery and aerobic systems, the transport of oxygen to the cells is of paramount importance for the continual recycling of ATP.

Lactate Level – During aerobic exercise, lactic acid is produced in the cells in relatively small quantities, not enough to impair performance. As the work intensity increases and insufficient oxygen is supplied to the muscle, lactic acid enters the bloodstream (ouch!). The point at which this happens is called the anaerobic threshold or lactate threshold. Note in the table that as the intensity of the swim increases (and duration/distance decreases), lactate is produced at a higher rate.

Heart Rate – A rule of thumb for calculating one’s maximum heart rate is 220 beats per minute minus swimmer’s age. For example, a female, age 40, may have a maximum heart rate in the area of 180 beats per minute. However, various factors play into each individual’s maximum heart rate, and various stress tests can be used to determine a more accurate value.

Again, note that as the swimmers move from the easier aerobic swims to the more intensive anaerobic swims, the heart rate correspondingly increases. At the CP level, the swims are so short that heart rate and lactate production are insignificant factors.

Perceived Exertion - This column shows the relative energy output in each zone. Aerobic swims, though challenging, do not have the same intensity of physical effort per duration as the anaerobic (non-CP) swims. While the challenge of endurance swimming is to remain near the lactate threshold for long periods, the short, anaerobic swims demand a high degree of exertion and force the swimmer beyond the lactate threshold. Swims using the CP metabolism are typically of high exertion, though lacking in extreme challenge due to their short duration.

Sample Swims and Sets

Recovery 2000 yards at low exertion

Aerobic 4 x 500 yards with 1:00 rest at moderate exertion

Mix Aerobic/Anaerobic 6 x 150 with 3:00 rest at high exertion

Anaerobic 4 x 100 with 5:00 rest at very high exertion

Creatine Phosphate 12 x 15 with 2:00 rest at high exertion

Training Applications

Through the course of a typical swim workout, Masters swimmers are likely emphasizing both aerobic and anaerobic energy systems. The general warm-up at the beginning of practice and the warm-down at the end of the session should be recovery or light aerobic swimming. Short kick or sprint sets enhance anaerobic capabilities, as do quick turn sets with extended rest. Drill or stroke technique sets are frequently practiced at a moderate pace in the aerobic zone. Sets swum in a descending manner, such as 10 x 50 on 1:00, slowest 50 first, increasing pace through the tenth 50, may cover multiple energy zones.

For swimmers with competitive aspirations, it is wise to identify specific events and distances for which to train. Then, devise a training plan that includes an adequate amount of training in the corresponding training zone to enhance performance ability. For example, if a swimmer desires to improve or excel at a 200-yard event, occasional training should be devoted to the Mix Aerobic/Anaerobic zone.

It is advisable to lay an aerobic foundation in one’s training before attempting the more strenuous anaerobic levels. While swimming with CP metabolism is not particularly taxing, swimming in the anaerobic or aerobic/anaerobic zones should be preceded with aerobic work in which gradually higher heart rates are experienced.

Lap swimmers who wish to maximize their fitness benefits are advised to swim in each energy zone at least once a week, with an overall emphasis on aerobic swimming. Timed 50-yard swims or a few 15-yard sprints not only raise the fitness level, but also add variety to the swimming program. So, not only is it healthy, it’s fun. Just ask Freddie!

Scott Rabalais, fitness editor for SWIM, is a collegiate and Masters coach in Savannah, Ga. He serves as Vice President of USMS.