Exercise Physiology — all lessons
Muscle Physiology
Muscle physiology: excitation–contraction and the sliding filament modelConcentric, eccentric and isometric muscle actionsMotor units, recruitment and rate codingNeuromuscular adaptations to trainingEnergy Systems
Energy systems overview: how the body makes ATPThe ATP-PC (phosphagen) systemAnaerobic glycolysis and lactateThe aerobic oxidative systemCardiovascular Physiology
Heart anatomy and the cardiac cycleStroke volume, heart rate and cardiac outputBlood components and oxygen transportAerobic Fitness and Intensity
Aerobic fitness concepts and adaptationsVO₂ maxLactate thresholdHeart rate zones and RPE-based intensity controlAnaerobic glycolysis and lactate
How glycolysis rapidly produces ATP from carbohydrate, why lactate forms, what lactate actually does, and what causes fatigue in hard efforts lasting tens of seconds to minutes.
Learning objectives
- Outline glycolysis and its ATP yield from glucose and glycogen
- Explain why lactate is produced and how it is cleared and used
- Describe current understanding of metabolic acidosis and fatigue
- Apply glycolytic concepts to interval training and carbohydrate fuelling
Before this lesson
Why this matters
Efforts of roughly 10 seconds to 2 minutes — a 400 m race, a long rally, a hard set of 15 — rely heavily on glycolysis. Misconceptions about “lactic acid” are among the most persistent in fitness.
The science
Glycolysis is a sequence of ten enzyme-catalysed reactions in the cytoplasm, splitting glucose (6 carbons) into two pyruvate (3 carbons) molecules.
| Starting fuel | Net ATP | Notes |
|---|---|---|
| Blood glucose | 2 | One ATP used by hexokinase, one by PFK |
| Muscle glycogen | 3 | Glycogen phosphorylase bypasses hexokinase |
Phosphofructokinase (PFK) is a key rate-limiting enzyme, stimulated by rising ADP and AMP during intense exercise [hargreaves2020].
Why lactate forms
Glycolysis needs NAD⁺. When glycolytic flux is high, pyruvate is converted to lactate by lactate dehydrogenase, regenerating NAD⁺ so glycolysis can continue. Lactate is produced even at rest and in well-oxygenated muscle; production rises with intensity.
Where lactate goes — the lactate shuttle
Lactate moves between fibres and organs: oxidative fibres and the heart oxidise it as fuel, and the liver converts it to glucose (the Cori cycle). It also acts as a signalling molecule [brooks2018].
Acidosis and fatigue
Strenuous exercise lowers muscle pH. Robergs and colleagues argued the protons come mainly from ATP hydrolysis, while lactate production actually consumes a proton — so “lactic acid” is a misnomer [robergs2004]. Fatigue in these efforts is multifactorial: accumulation of inorganic phosphate, ionic disturbances, PCr depletion and central factors all contribute.
Practical application
Common mistakes and misconceptions
Case study
A CrossFit athlete eating very low carbohydrate reports poor performance in 2-minute workouts. After discussing with a sports dietitian, carbohydrate is increased around hard sessions; workout times improve, illustrating glycolysis’ dependence on carbohydrate.
Further learning: video lessons
Video links are only marked “verified” after a human has watched them. Unverified entries open a YouTube search for that channel and topic rather than a guessed link.
Glycolysis overview
Khan Academy
You will understand: the stages and products of glycolysis
Why: Clear biochemistry foundation.
Lactate: myths and facts
university exercise physiology lectures (search “lactate shuttle Brooks”)
You will understand: lactate as a fuel and signal
Why: Researcher-led perspective; verify specific video.
Knowledge check
Knowledge check · 0/5
1. Net ATP yield from one glucose molecule via glycolysis is…
2. The main reason lactate forms during rapid glycolysis is to…
3. The liver converting lactate back to glucose is the…
4. Which statement is best supported?
5. Glycolytic capacity is limited during high-intensity work when…
Short-answer & application questions
- Explain why an athlete on a very low-carbohydrate diet may struggle with interval sessions.
- Describe the lactate shuttle in your own words.
- Design a glycolytic-capacity session and describe how athletes will feel.
Key takeaways
- Glycolysis converts glucose or glycogen to pyruvate in the cytoplasm, yielding 2 (glucose) or 3 (glycogen) ATP net.
- Lactate production regenerates NAD⁺ so glycolysis can continue rapidly.
- Lactate is a fuel and signalling molecule shuttled between tissues, not a waste product.
- Acidosis during intense exercise arises mainly from ATP hydrolysis; fatigue is multifactorial.
Connected topics
Lactate threshold
physiologyThe aerobic oxidative system
sports nutritionPre-, intra- and post-workout nutrition strategies
hybrid trainingZone 2, threshold, VO₂ max and alactic work
References
- Brooks GA (2018). The science and translation of lactate shuttle theory. Cell Metabolism, 27(4):757–785. doi:10.1016/j.cmet.2018.03.008 Narrative review check pending
- Robergs RA, Ghiasvand F, Parker D (2004). Biochemistry of exercise-induced metabolic acidosis. American Journal of Physiology — Regulatory, Integrative and Comparative Physiology, 287(3):R502–R516. doi:10.1152/ajpregu.00114.2004 Narrative review check pending
- Hargreaves M, Spriet LL (2020). Skeletal muscle energy metabolism during exercise. Nature Metabolism, 2(9):817–828. doi:10.1038/s42255-020-0251-4 · PubMed 32747792 Narrative review verified
Educational content only — not medical advice. Consult a qualified clinician for diagnosis or treatment.