The Glycolytic System: Fast Energy From Carbohydrate
Quick Intro
The glycolytic system makes ATP by breaking down glucose, a form of carbohydrate, in the fluid of muscle cells. When the process runs at high speed it does not need oxygen, and it produces lactate. It supplies a large share of the energy for hard efforts lasting from about 10 seconds to 2 minutes, such as a 400-meter run or a hard set of 15 reps.
The Idea in Plain Terms
Glycolysis is a chain of chemical steps that converts one molecule of glucose into two molecules of a compound called pyruvate, yielding a small amount of ATP. What happens next depends on the rate:
- At a moderate pace. Pyruvate enters the mitochondria, where oxygen is used to make much more ATP.
- At a high pace. Pyruvate is produced faster than the mitochondria can use it, and the excess is converted to lactate. This keeps glycolysis running at a high rate.
For many years lactate, and “lactic acid”, were blamed for muscle fatigue and next-day soreness. The modern view is different. Lactate is a fuel that muscle, heart and brain tissue can use, and it moves between cells. The burning feeling during hard effort is linked to rising hydrogen ions and other changes inside the muscle, and lactate is a marker of high-intensity work and not the cause of soreness. Lactate returns to normal within an hour or so after exercise, long before soreness appears.
The glycolytic system is fast, with a higher power output than the oxidative system, though it can only run at full rate for a short time before fatigue builds.
A Worked Example
A session targeting the system, as an illustration:
| Variable | Setting |
|---|---|
| Effort | 6 repeats of 200 to 400 meters |
| Intensity | Hard, near race effort |
| Rest | 2 to 4 minutes of easy walking or jogging |
| Aim | Keep each repeat at the same pace |
The burning feeling and heavy breathing are expected, and technique should hold.
What the Research Shows
Work by George Brooks and colleagues on the lactate shuttle, summarized in a 2018 review in Cell Metabolism, shows that lactate is produced all the time, even at rest, and is an important fuel and signaling molecule. This challenges older textbook claims. Studies of muscle during intense exercise show that acidity and changes in ions are more closely tied to the sensation and loss of force than lactate itself.
Training this system with intervals of 30 seconds to a few minutes at high intensity improves tolerance to the by-products of intense effort and increases the activity of enzymes in glycolysis. The same intervals also improve oxidative fitness, so the gains overlap.
Estimates of the fraction of energy from each system during a given effort vary across studies and methods.
Who It Suits
Training aimed at this system suits:
- Middle-distance runners, swimmers and rowers.
- Combat sport and team sport athletes.
- Anyone doing circuit-style workouts with hard efforts.
How to Use It
- Use hard efforts of 30 seconds to 2 minutes.
- Rest 1 to 4 minutes, often at a work-to-rest ratio of 1 to 1 or 1 to 2.
- Start with fewer repeats, and add them over weeks.
- Warm up fully before hard efforts.
- Allow 48 hours before repeating such a session.
- Mix with easy aerobic training.
Common Misunderstandings
It is not “lactic acid” that causes soreness.
It is not purely anaerobic. Oxygen is used throughout.
It is not a system to train every day.
FAQ
What causes the burn? Changes inside the muscle, including hydrogen ions, and not lactate alone.
Is lactate waste? No. It is a fuel.
Is it good to train to exhaustion? Hard efforts are needed, and going to complete failure each time is not.
How often? One or two sessions a week for most people.
Where This Comes From
Brooks GA, 2018, the science and translation of lactate shuttle theory (Cell Metabolism, 27, 757 to 785). Gastin PB, 2001 (Sports Medicine, 31, 725 to 741). The general description follows exercise physiology textbooks.
Related to: energy systems overview, the ATP-PCr system, the oxidative system, sprint interval training and Tabata. Exercises include Hill Sprints.