In addition to the complexities of the metabolism, the body also has multiple human energy systems. Much like NASCAR, Formula 1 or drag racers rely on different types of engines and fuel to maximize performance, the body uses certain types of fuel for short, powerful bursts of energy and a completely different fuel source for long term endurance events. Think Powerlifter vs. Ultra-Marathoner. The desired adaptations such as an increase of muscle tissue size, are not actually caused by the dumbbell in your hand, the number of reps, sets, or even the types of exercises, frequency or amount of protein you eat. Sure, all these variables play a role in program development, but all of these variables pale in comparison to the impact of the energy system that is being trained.

Before you start any new training program it is important to have a general working knowledge of why certain types of exercise, at certain intensity variables, for certain periods of time result in completely different physiological adaptations. This ensures that your program design, and all your hard work is moving you closer to your goals.

For instance, why are marathoners normally very slim, yet bodybuilders with equally low body fat have muscles on top of muscles?

These completely different physiological adaptations are caused by certain types of exercise regimens which use unique, corresponding energy systems. The body has three main energy systems—the phosphagen, the glycolytic, and the oxidative systems. I’m not going to get too involved in science mumbo jumbo here, but I do want to give you a brief explanation of each system’s primary responsibility and how the body physiologically response to each system differently.

First, some foundational knowledge about energy production. All energy used during physical activity is derived from the nutrients in the foods we eat: Carbohydrates, Fat and Protein. However, the body cannot derive energy directly from food consumption. The food we eat must be metabolized, or broken down and converted into usable energy sources such as glucose, amino acids and fatty acids. These nutrients are then used to produce adenosine triphosphate (ATP), which is the most essential source of usable energy needed for all cellular function in the human body. Our body can create, store and release ATP through one of three different pathways.

The following definitions of the three metabolic pathways are adapted from National Strength and Conditioning Association (NSCA)

Phosphagen (Immediate Fuel)

This system uses creatine phosphate (CP) and has a very rapid rate of adenosine triphosphate (ATP) production. CP is used to reconstitute ATP after it’s broken down to release its energy. The total amount of CP and ATP stored in muscles is small, so there is limited energy available for muscular contraction. It is, however, instantaneously available and is essential at the onset of activity, as well as during short-term, high-intensity activities lasting from 1 to 15 seconds, such as a 100-meter sprint, Olympic weightlifting, or bodybuilding.

Anaerobic Glycolysis (Moderate Release, uses carbohydrates)

Anaerobic glycolysis does not require oxygen and uses the energy contained in glucose for the formation of ATP. This pathway occurs within the cytoplasm and breaks glucose down into a simpler component called pyruvate. As an intermediate pathway between the phosphagen and aerobic system, anaerobic glycolysis can produce ATP quite rap- idly for use during activities requiring large bursts of energy over somewhat longer periods of time (15-seconds to 2 minutes max) or during endurance activities prior to steady state being achieved. Events that emphasize anaerobic glycolysis are the 800-meter sprint, 400-meter swim, boxing, MMA, and wrestling.

Oxidative System (Slow Release, relies on Carbs and Fats)

This pathway requires oxygen to produce ATP because carbohydrates and fats are only burned at a significant rate in the presence of oxygen. This pathway occurs in the mitochondria of the cell and is used for activities requiring sustained energy production. Aerobic glycolysis has a slow rate of ATP production and is predominantly utilized during longer-duration, lower-intensity activities such as hiking or long-distance running, after the phosphagen and anaerobic systems have fatigued. It is important to remember that all three of these systems contribute to the energy needs of the body during physical activity. These systems do not work independently of each other, but rather dominate at different times, depending on the duration and the intensity of the activity.

Let’s take a deeper look at program designs that affect different energy systems and the type of athlete who would benefit most from the specific regimen. The we will examine the physiological adaptations that result from this type of training.

The phosphagen system (ATP and PC) lasts approximately 1-15 seconds. Athletes who rely on the phosphagen system for their sport such as bodybuilders, powerlifters, sprinters, or Olympic weightlifters benefit by using loads varying from 65–100% of their 1 repetition max (1RM). They may perform resistance training with repetitions ranging from 1–12. Notice the repetition number is always within the time constraints of the energy system to achieve desired physiological adaptation. This is not a coincidence. Sets normally range from 15–30 seconds and rest intervals following each set from 30 seconds (hypertrophy) to 5 minutes (maximum strength).

The physiological adaptation of training primarily within the Phosphagen system causes hypertrophy, or an increase in size of the working muscles. This is especially true when training to volitional failure, or the point in the set where you can’t do one more rep without cheating it up by using momentum. The use of heavier weights
and repeated sets with little variation in exercises creates adaptations in the nervous system that increase dimensions of strength (e.g. stability strength, strength endurance, and absolute strength). Due to an increase in muscle size, this type of training has profound effects on the metabolism. Larger, more active muscles require more energy to sustain themselves.

If you need a refresher, Check Out Part 1 on Metabolism.

Next is Anaerobic Glycolysis, a pathway providing energy for bouts of effort lasting from 15 seconds to 2 minutes.  Athletes that benefit from this optimized system include mixed martial artists, boxers, wrestlers, and certain middle-distance sprinters to name just a few. These athletes typically train with relatively light loads or weight (30%–65% of their 1RM), much higher repetition ranges (15–failure), and set ranges from 10–30. Typically exercise bouts using the Anaerobic system last between 20 and 60 minutes. This athlete benefits by taking shorter rest breaks (10–30 seconds) and often using methods such as super-sets (pairing two exercises and alternating repeatedly in immediate succession) or circuit training (multiple exercises repeated in succession). Athletes who train in this energy system tend to focus on workout designs that include comprehensive exercises and focus on whole-body development and utilization, as opposed to isolated movements that target individual muscles or muscle groups. The effects of this type of training include at least some level of hypertrophy and an increase in aerobic and anaerobic metabolism, which—along with proper nutrition—can reduce body-fat percentage. The nervous system responds to this type of training incredibly well. The inclusion of a range of whole-body exercises increases both neural and myofascial adaptations. This type of training fused with the phosphogen system creates more functionality in the body and movement systems, which in turn create higher levels of GPP, hypertrophy and strength.

Lastly, the Oxidative System. Athletes who rely primarily on this energy system include long-distance runners, swimmers, and triathletes. These athletes tend to train with low- or no-load bodyweight exercises or simply perform the event itself with few rest breaks and often incorporate heart-rate training as a means of tracking cardiovascular response to exercise. The oxidative system lends itself to a higher propensity for fat metabolism and cardiac conditioning. The relatively limited breakdown in muscle tissue in these types of exercises (running, swimming) offers an opportunity to increase volume of exercise by increasing frequency and intensity (HR). This type of training is also responsible for creating minimal amounts of hypertrophy in Type I muscle fibers.

All three energy systems have benefits and all three must be represented in effective periodized programming. Each one is responsible for completely different physiological adaptations, none more important than the others when raising general physical performance (GPP). Once the goal has been established to increase strength and size, a priority must be placed on the phosphogen (ATP & PC) energy system.


  1. The energy system used during resistance training is responsible for corresponding physiological adaptations.
  2. Work capacity and recovery time must be increased in all three systems. Specificity of any singular energy system is necessary for attainment of specific performance goals but can and will result in stagnation and overtraining if not varied. Periodized programming should incorporate all 3 energy systems to develop the full potential of the strength athlete.
    • Phosphogen (ATP & PC) energy system: Develops speed, explosive and absolute strength and hypertrophy of type II muscle fiber.
    • Glycolytic energy system: Develops, stability strength, strength endurance, and specific work capacity.
    • Oxidative energy system: Improves recovery time and work capacity, increases fat metabolism
  1. Multiple studies within the strength training community (Klemp et al., Schoenfeld et al., Burd) support that variables such as time under tension (e.g., the energy system used), the level of fatigue (high or low) and training volume are the determining factors for creating both hypertrophy and strength. There is no correlation in such gains with reps, sets, or even exercise choices.