In simple terms
A friendly intro before the formal notes — no formulas yet.
Break Down to Build Up Stronger
Exercise causes temporary fatigue, which is a necessary signal for your body to recover. During recovery, your body doesn't just repair to its previous state; it overshoots and becomes slightly stronger, a process called supercompensation.
Imagine you're building a brick wall. A tough workout is like pushing against that wall, causing a few bricks to loosen (fatigue). This is a signal that the wall isn't strong enough. During recovery, you don't just put the loose bricks back; you use stronger mortar and add reinforcing rods (adaptation). The next time you push, the wall is stronger and can withstand more force. This cycle of stress, recovery, and improvement is how we get fitter.
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Begin at your baseline fitness level, a state of homeostasis where your body is balanced and at rest.
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Apply a training stimulus, like a challenging run. This depletes energy and causes a temporary drop in performance capacity, known as fatigue.
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Cease exercise and begin recovery. The body works to repair muscle damage, replenish energy stores, and clear metabolic by-products. It overshoots the original baseline, entering a state of supercompensation where you are fitter than before.
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Apply the next training stimulus during this supercompensation peak to progressively increase fitness. If you wait too long, the benefits fade (involution); if you train too soon, you risk overtraining.
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Full topic notes
Formal explanation with the rigour you need for the exam.
Understanding Fatigue
Fatigue is a complex and multifaceted phenomenon, broadly defined as a decrease in force production or performance capacity resulting from exercise. It is a protective mechanism that prevents cellular damage. We can categorise fatigue into two main types: central and peripheral. Central fatigue relates to the central nervous system (CNS) and describes processes that reduce the neural drive from the brain to the muscles. Peripheral fatigue occurs at the level of the muscle itself, involving factors that disrupt the process of contraction from the neuromuscular junction down to the actin-myosin cross-bridges.
Central Fatigue: Involves the CNS. Reduced motor unit recruitment and firing frequency. May be influenced by neurotransmitters like serotonin and dopamine, affecting motivation and perceived exertion.
Peripheral Fatigue: Occurs within the muscle. Can be caused by energy substrate depletion (ATP, PC, glycogen), accumulation of metabolic by-products (H+, Pi), or failure of the excitation-contraction coupling mechanism (e.g., impaired Ca²+ release/uptake).
Metabolic Causes of Peripheral Fatigue
During exercise, the body's ability to match energy supply with demand is critical. In high-intensity, short-duration activities (e.g., sprinting), the rapid breakdown of ATP and PC, followed by anaerobic glycolysis, leads to an accumulation of metabolites. While lactate was once blamed, it is now understood that the accumulation of hydrogen ions (H+), which are co-produced, is the primary cause of the drop in intramuscular pH. This acidosis inhibits key glycolytic enzymes and interferes with calcium's ability to bind to troponin, directly impairing muscle contraction. For longer-duration, sub-maximal events (e.g., marathons), the main metabolic limiter is the depletion of muscle and liver glycogen stores, forcing the body to rely on slower fat metabolism.
In exams, be precise with your terminology. Avoid simply stating 'lactic acid causes fatigue'. Instead, explain that high rates of anaerobic glycolysis lead to the accumulation of lactate and hydrogen ions (H+). It is the increase in H+ concentration that lowers muscle pH (metabolic acidosis), which in turn inhibits enzyme activity and muscle contraction.
The Recovery Process and EPOC
Recovery is not a passive process but an active series of physiological events designed to restore homeostasis. A key concept is Excess Post-exercise Oxygen Consumption (EPOC), often termed the 'oxygen debt'. This is the elevated rate of oxygen intake following strenuous activity. The extra oxygen is used to restore the body to its resting state, and its magnitude and duration are proportional to the intensity and duration of the exercise.
Fast (Alactacid) Component: Completed within minutes. Primarily involves using oxygen to resynthesise ATP and phosphocreatine (PC) stores and to replenish oxygen levels in myoglobin and haemoglobin.
Slow (Lactacid) Component: Can last for several hours. Involves using oxygen for several processes: removal of lactate via the Cori cycle (conversion to glucose in the liver) or oxidation in other tissues; supporting the elevated metabolic rate due to increased body temperature; and providing energy for tissue repair and glycogen resynthesis.
Supercompensation, Overreaching and Overtraining
The ultimate goal of training is adaptation. This is governed by the supercompensation principle. A training stimulus induces fatigue, causing a temporary decrease in performance. Following this, a period of recovery allows the body not only to return to baseline but to 'overshoot' it, resulting in an improved physiological state. If the next training session is timed correctly—during this supercompensation phase—a gradual improvement in fitness occurs. However, an imbalance between training stress and recovery can lead to negative outcomes. Functional overreaching is a planned period of intense training that leads to a temporary performance dip, followed by supercompensation after a short rest. In contrast, non-functional overreaching (NFOR) and overtraining syndrome (OTS) represent states of severe fatigue where performance is suppressed for weeks, months, or even longer, due to inadequate recovery.
Worked examples
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An athlete has a resting oxygen consumption () of 0.3 L·min⁻¹. After a high-intensity interval session, their post-exercise is measured for 10 minutes. The total oxygen consumed in this 10-minute period is 15.5 L. Calculate the volume of the EPOC in this timeframe. [2 marks]
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Calculate the expected resting oxygen consumption over 10 minutes:
A 17-year-old swimmer increases her training volume by 30% and adds two extra morning sessions per week. After three weeks, her coach notes the following: her 100m freestyle time has increased by 2 seconds, her resting heart rate has increased by 8 bpm, she reports persistent muscle soreness and poor sleep, and she feels irritable. Using the principles of training adaptation, identify and justify her likely condition. [3 marks]
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Identification: The swimmer is likely experiencing Non-Functional Overreaching (NFOR). [1 mark]
How it all connects
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Glossary
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Quick check
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Revision flashcards
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What is fatigue in a sports science context?
A reversible, exercise-induced reduction in the ability of a muscle to produce force or power. It can be classified as central (involving the CNS) or peripheral (at the muscle site).
Key takeaways
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Central Fatigue: Involves the CNS. Reduced motor unit recruitment and firing frequency. May be influenced by neurotransmitters like serotonin and dopamine, affecting motivation and perceived exertion.
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Peripheral Fatigue: Occurs within the muscle. Can be caused by energy substrate depletion (ATP, PC, glycogen), accumulation of metabolic by-products (H+, Pi), or failure of the excitation-contraction coupling mechanism (e.g., impaired Ca²+ release/uptake).
Practice — then mark it
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Test Your Knowledge on Fatigue, Recovery and Adaptation
Test Your Knowledge on Fatigue, Recovery and Adaptation
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