In part two of energy systems, we talk about the Alactic Phosphocreatine (ATP-PC) energy system and its role in high power physical activities.  The second most powerful energy system is the anaerobic lactic energy system, also know as the glycolytic energy system. The anaerobic lactic system runs without requiring oxygen and burns glucose (carbohydrates) as its preferred fuel. It's not as powerful as the ATP-PC system, but it can produce a relatively high proportion of Adenosine Triphosphate (ATP) for around 30seconds before quickly starting to plateau as energy production decreases significantly between 30-90seconds. 

The primary role of the lactic system is to generate higher levels of force and power over longer periods of time than the phosphocreatine system.

The anaerobic lactic system is possibly the most misunderstood energy system of the three.  Most coaches associate the anaerobic lactic energy system with high levels of fatigue and lactate production, which is a byproduct of the anaerobic lactic system. Continued anaerobic metabolism will lead to fatigue due to the large changes in the cellular environment that impairs energy production and muscle contractibility, but this is not the only contributing factor and simply training an athlete to fatigue doesn't train the anaerobic lactic system to improve. The cause of fatigue for heavy, explosive lifting will be significantly different from the marathon runners fatigue.  Fatigue never has a single cause.

Coaches and athletes need to understand that there are many factors that contribute to a process called glycolysis which significantly impacts energy production.  The lactic energy system produces ATP by breaking down glycogen through:

  1. Anaerobic Glycolysis (without oxygen)

  2. Aerobic Glycolysis (with oxygen) - this contributes the glycolitic energy production and hence aerobic fitness levels have a significant role to play

Lactate is always produced as a by-product of carbohydrate metabolism, both aerobically and anaerobically, but lactate only accumulates when the aerobic energy system can not keep up with the rate at which lactate is being produced.  

Steps of glycolysis: 

Pay attention to the magnesium demands on energy production, a good reason why athletes must supplement with magnesium to improve performance.

There are four key steps involved in the anaerobic glycolytic system.  Steps of the anaerobic glycolytic system:

  1. Initially, stored glycogen is converted into glucose.  Glucose is then broken down by a series of enzymes. (Investment phase)

  2. 2 ATPs are used (Investment phase) to fuel glycolysis and 4 ATPs are created (Payoff phase) so the body gains 2 ATPs to use for muscular contraction. 

  3. The breakdown of glucose to synthesise ATP results in the creation of a substance called ‘pyruvate’ and hydrogen ions.  Without aerobic input, the muscle becomes increasingly acidic as more hydrogen ions are created. 

  4. Because this system is ‘anaerobic’ there isn’t enough oxygen to break down pyruvate and synthesise anymore ATP, the changes in the cellular environment that impairs energy production. 

The anaerobic glycolytic process results in Pyruvate binding with some of the hydrogen ions and converting them into a substance called lactate (completely different to ‘lactic acid’). Pyruvate is a byproduct of ATP and it can be used to fuel either the aerobic metabolism (Krebs Cycle and Electron Transport Chain (ETC)) we will talk about these in future posts) or it can be used to produce lactate.  Lactate is always being produced, but when the aerobic energy system is functioning at a high level relative to the anaerobic demands, lactate is quickly oxidised back to pyruvate with can then be used to fuel further anaerobic metabolism. 

This is where it can get a little confusing, but to keep it simple let's just say that the moderate levels of energy produced by the anaerobic lactic system can be supported by the aerobic energy system. An athlete who doesn’t have a well developed aerobic system will not be able to maximise the energy production power of the anaerobic lactic system, and they will not be able to recover between rounds of lactic intervals as quickly as those athletes who do have high levels of aerobic fitness.


The anaerobic and aerobic fitness levels of the athlete dictate how efficient the anaerobic lactic system works.  If lactate starts to accumulate in the muscle this indicates that the aerobic energy system can not recylcle the pyruvate as quick as the anaerobic lactic system is producting it.  The body can try to shuttle the lactate around to other working muscle in other areas that can then try to convert it and utilise it.  This lactic shuttling allows for greater metabolic flexibility and increases the overall athletic performance of the athlete.

Lactate can be transported (lactate shuttle) to:

  • other cells nearby where it’s produced (high twitch muscle fibres produce higher levels of lactate)

  • to the heart, brain, etc. (primary fuel for the heart and one of the best muscles to utilise lactate)

  • working muscles in other areas of the body

We can think about the lactate energy system as a bridge between anaerobic energy production and aerobic energy production that allows the body to produce higher rates of force and power than it could if only the ATP-CP and the aerobic system were used.  Remember from last week that the lactic energy system is used to recharge the ATP-CP stores.


The anaerobic lactic system can be developed through the correct training principles, as the total energy capacity is dependent on a host of factors like such as training background, genetics, and nutrition.  Training the lactic system must be aimed at increasing tolerance to lactate, the removal of lactate and improving the rate at which glycolysis produces ATP.  The aerobic system is slowly contributing an increasing percentage of ATP the longer the moderate intensity work period continues (more on this later).  This is why it becomes increasingly important to separate anaerobic lactic and aerobic energy system training if you want to increase anaerobic power and overall fitness levels.

When training the lactic energy system the work to rest ratios vary depending on the intended outcome. If you want the system to completely recover and clear the majority of accumulated lactate, so you can repeatedly condition anaerobic lactic energy,  you would use a ratio of 1:6 (6 seconds of rest for every second of work). This helps to condition the body to clear (get rid of) lactate. For example, a 30second high-intensity effort would require 3minutes of rest to allow the anaerobic lactic system to recover.

If the goal of the training session is to produce high levels of lactic a ratio of 1:3 can be used to carry fatigue into the next interval. This ratio causes a progressive accumulation of lactate as the very small rest interval doesn’t allow enough time for much of the lactate to be removed from the muscle.  This forces the athlete to continue to exercise with a lot of lactate present thus dramatically increasing their ability to tolerate lactic buildup.


Now might be a good time to repeat the fact that the anaerobic lactic system prefers glucose (carbohydrates) as its fuel, and without enough carbohydrates in your diet, your anaerobic lactate system will be severely limited in its ATP production.  If you know you are going to be performing anaerobic lactic intervals in your training session you would be smart to eat more carbohydrates in the days leading up to this training session.

NOTE: Glucose is converted into glycogen and stored in the muscles (~350–700g or 1400-2800cals; depending on training status, diet, muscle fibre type composition, gender and body weight) and the liver (~100g or 400cals).  The glycogen that is stored in the local muscle can only be used by that muscle, it can not be released back into the bloodstream and sent to other muscles/organs.  However, the glycogen store in the liver can be converted into glucose and released into the bloodstream to be taken up by the muscles/organs for further energy production.  Glucose consumed during the training session can also be usalised by the lactic energy system.

It should be a no-brainer here, the anaerobic lactic pathway prefers to run on carbohydrates as both fats and protein cannot be converted into energy without oxygen.  Sugars are the only foods that can be broken apart without the need for oxygen**. The aerobic system supports the anaerobic lactic system and oxidised proteins and fats can be used as fuel to support the ATP production, but this chemical process requires more steps and is slower than anaerobic glycolysis.

** Some crazy folk may read this and assume that glycolytic training is inferior to aerobic conditioning if fat loss is the goal.  This could not be further from the truth as the aerobic system plays an important role in anaerobic recovery.  The excess post-exercise oxygen consumption (EPOC) plays a primary role in producing greater fat loss in response to high-intensity interval training [1,2,3,4].  Higher intensity exercise requires more energy, which leads to the release of more growth hormone and the catecholamines, both of which drive fat burning in the body [5]

Should you rethink your training programme?

The current “fitness" system thrives on running people into the ground – only the athletes who possess superior genetic can rise to the top. However, research points to a much smarter sustainable training methods that can be used to improving an athletes fitness levels.

Far too often we see an overuse of high-intensity training which often produces undesirable ramifications. For example, the constant sympathetic nervous system activation associated with this style of training can impair an athlete’s recovery between intervals and stunt fitness improvements.  The relentless accumulation of high-intensity sessions will not improve longterm health or performance.  Short term gains are common, but long term impacts to the endocrine system will often stop progress after 6-18months of HIIT training (this depends each individual and the levels of chronic stress in their lifestyle).

The graphs below are taken from a 1999 study conducted by Parolin et al., which provides an accurate snapshot of what interval training actually looks like to our energy systems.


The athletes were asked to produce three rounds of 30second high-intensity intervals of cycling sprints followed by 4minute of recovery (1:8 ratio). The A graph above provides a snapshot of the energy systems used in the first 30second sprint while the B graph is a snapshot of the third and final interval.

If you compare the A and B you will notice the disappearance of the anaerobic lactic input from the first sprint to the third, even with significant rest between intervals.

The initial 6seconds in each sprint place a huge demand on the alactic system (ATP-PC), notice how the anaerobic lactic energy production (glycolysis) stay high in A for the first 15seconds before the aerobic (oxidative phosphorylation) increases from 15-30sec.  In B the anaerobic lactic energy production (glycolysis) is limited from the start and the aerobic component switches on much sooner and more significantly than it did in A.

Due to the accumulation of lactic by-products, the lactic metabolism ends up inhibiting the anaerobic lactic system and a greater demand in places on the aerobic energy system.  As previously mentioned, if an athlete doesn’t have an adequate aerobic capacity to clear away the by-products for lactic metabolism, and is faced with repeated intervals of high-intensity, the only option is to reduce power output.

Simply training and athlete to fatigue is not improving the anaerobic lactic system, in fact, it can actually lower athletic performance.  Far too many coaches fail to correctly train this system and hence many athletes fail to see ongoing improvements in their overall fitness levels.


Energy System: Anaerobic Lactic (glycolytic)
Power Output:  Moderate power
Duration of Activity: 30-90seconds
Time to Recovery: Depending on the intended outcome; 1:6 ratio to train the removal of lactate and improving the rate at which glycolysis produces ATP, 1:3  increasing tolerance to lactate
Prefered Source of Energy: Anaerobic (without oxygen) = glucose (carbohydrates).  Aerobic (with oxygen) = glucose, fats, and protein (in that preferred order)
Adaptability: Somewhat adaptable
Sports: Very few sports require high levels of lactic fitness and athletes often spend far too much time training the lactic system. 200m, 400m, 800m sprints, 400m hurdles, 4x400m relay, cycling sprints, gymnastics, winter sating events, swimming 50-100m events, wrestling, etc…


  1. Tremblay, A., Simoneau, J., Bouchard, C. Impact of exercise intensity on body fatness and skeletal muscle metabolism. (1994), Metabolism

  2. Hazell, T, Olver, T, Hamilton, D., Lemon, P. Two minutes of sprint-interval exercise elicits 24-hr oxygen consumption similar to that of 30 min of continuous endurance exercise. (2012). International Journal of Sport Nutrition and Exercise Metabolism

  3. Heydari, M., Freund, J., Boutcher, S. The effect of high-intensity intermittent exercise on body composition of overweight young males. (2012). Journal of Obesity

  4. Trapp, E., Chisholm, D., Freund, J.,Boutcher, S. The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women. (2008). International Journal of Obesity

  5. Higgins, S., Fedewa, M., Hathaway, E., Schmidt, M., Evans, E. Sprint interval and moderate-intensity continuous cycle training differentially affect adiposity and fitness in overweight women. (2016). Medicine and Science in Sports and Exercise