Muscular fatigue and adaptation of muscle to training
Fatigue is a fall in force or power from low fuel, acid build-up or reduced nerve drive. Training makes muscle adapt: more mitochondria for endurance, bigger fibres for strength.
Causes of muscular fatigue
| Cause | Typical event | What happens |
|---|---|---|
| Phosphocreatine (PCr) used up | 100 m sprint, repeated sprints in hockey | ATP cannot be rebuilt fast enough |
| Hydrogen ion build-up (acidosis) | 400 m, 800 m, repeated kabaddi raids | Low pH slows glycolysis enzymes and calcium binding to troponin |
| Inorganic phosphate build-up | Any very hard effort | Weakens the cross-bridge stroke and calcium release |
| Muscle glycogen depletion | Marathon, long cycling, end of a football match | 'Hitting the wall': pace falls sharply |
| Low blood glucose | Long events without carbohydrate | Brain fuel falls: central fatigue, poor decisions |
| Dehydration and heat | Cricket or athletics in Indian summer | Higher heart rate and core temperature; less drive |
| Electrolyte imbalance | Heavy, long sweating | Potassium shifts and sodium loss; cramps and weak contraction |
| Central fatigue | Any long or stressful effort | Brain lowers the drive to muscles (brain chemicals, temperature, motivation) |
Lactic acid: myth and fact
Next-day soreness is DOMS (delayed onset muscle soreness), which peaks 24-72 hours after unfamiliar eccentric work and comes from small tears in the muscle.
Adaptations of skeletal muscle to training
| Adaptation | Endurance training | Resistance training |
|---|---|---|
| Mitochondria | More and larger | Little change |
| Capillaries | More around each fibre | Little change |
| Myoglobin | Increases | Little change |
| Oxidative enzymes | Increase | Little change |
| Glycogen and fat stores in muscle | Increase | Glycogen increases |
| Fat oxidation | Higher, so glycogen is spared | Little change |
| Lactate threshold | Rises: harder pace before lactate climbs | Little change |
| Fibre size | Little change | Hypertrophy, mainly in Type II fibres |
| Fibre type | IIx shifts towards IIa | IIx shifts towards IIa |
| Nervous system | Better economy of movement | More motor units recruited, faster firing (first 6-8 weeks) |
| Tendons and bone | Stronger | Stronger; higher bone density |
Whole-body changes and the training cycle
- Endurance training also enlarges the heart's stroke volume, lowers resting heart rate, raises blood and plasma volume and lifts VO2max.
- Adaptation follows stress → fatigue → recovery → supercompensation: the body rebuilds slightly stronger if it gets enough rest and food.
- Too much training with too little recovery leads to overtraining: falling performance, poor sleep, frequent illness.
- Reversibility: gains fade within weeks when training stops.
Food can delay fatigue: carbohydrate loading and drinks for long events, fluids with sodium in hot weather, and enough iron for oxygen delivery. Adaptation happens during recovery, so the athlete needs carbohydrate and about 0.3 g/kg protein after training, plus sleep.
2 marks: “What is muscular fatigue?” A fall in the muscle's ability to produce force or power during or after exercise. It may be central (brain) or peripheral (muscle).
5 marks: “Explain the causes of muscular fatigue.” PCr depletion, hydrogen ion and phosphate build-up, glycogen depletion, low blood glucose, dehydration and heat, electrolyte loss and central fatigue, each with an event.
10 marks: “Discuss the adaptations of skeletal muscle to endurance and resistance training.” Training cycle, a two-column table of muscle adaptations, whole-body changes, overtraining and reversibility, and a nutrition note on recovery.
Fatigue causes: Please Hold Good Water Bottles: PCr gone, H+ up, Glycogen gone, Water lost, Brain tired.