Exercise Biochemistry
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Direct answer
Muscle draws ATP from three overlapping systems: phosphocreatine (immediate, seconds), anaerobic glycolysis (the first one to two minutes of intense work) and oxidative phosphorylation (sustained effort fuelled by glycogen, then fat). Phosphocreatine buffers ATP through the near-equilibrium creatine kinase reaction rather than acting as stored ATP, which muscle can barely store at all. Lactate is not a poison but an exportable fuel — the Cori cycle returns it to the liver, which spends six high-energy phosphates rebuilding the glucose that gave muscle only two. Training shifts the lactate threshold upward, multiplies mitochondria through AMPK-PGC-1α signalling and expands glycogen stores: the molecular basis of endurance adaptation.
What you must remember
- Timelines: phosphocreatine covers roughly the first ten seconds of maximal effort; glycolysis sustains near-maximal work for about ninety seconds; beyond that, oxidative metabolism dominates — the logic of sprint versus marathon fuel economy.
- Creatine kinase shuttle: phosphocreatine plus ADP becomes creatine plus ATP at the myofibril, and phosphocreatine is resynthesised near the mitochondria — spatial buffering; the adenine nucleotide pool itself is tiny, so flux, not storage, does the work.
- Cori cycle arithmetic: the liver converts two lactate into one glucose at a cost of six high-energy phosphates (four ATP plus two GTP) to restore the two ATP muscle earned — a four-phosphate interest charge paid by the liver for postponing fatigue.
- Glycogen ceilings: muscle holds roughly 300-400 g of glycogen but lacks glucose-6-phosphatase, so it cannot share; the liver holds about 100 g and does export. Marathon "hitting the wall" near the two-hour mark in untrained runners is glycogen depletion, not weak will.
- Lactate threshold: the workload where lactate rises systematically — around 50-60 per cent of VO2max in the untrained, far higher after training; it predicts endurance performance better than VO2max itself.
- Adaptation signalling: energy stress raises AMP, activating AMPK, which drives PGC-1α coactivation of mitochondrial biogenesis — why endurance training shifts fibres toward the oxidative type I phenotype.
- Clinical corners: rhabdomyolysis (creatine kinase in the tens of thousands, myoglobinuric acute kidney injury); McArdle disease (muscle phosphorylase deficiency) — exercise intolerance with a flat venous lactate and a second-wind phenomenon; statin therapy adds myalgia and creatine kinase elevation.
From sprint to marathon: a fuel timeline
Put one runner through a 400-metre race and then a marathon. In the first five seconds ATP barely falls, because creatine kinase instantly rephosphorylates ADP. By thirty seconds, glycogen phosphorylase — switched on by calcium and adrenaline through phosphorylase kinase — is flooding glycolysis; hydrogen ions accumulate faster than buffers can absorb them, and the burning legs are intracellular acidosis, with lactate as fellow traveller rather than cause. Hours into the marathon, muscle glycogen is nearly spent: the trained runner has shifted to fatty acid oxidation, cortisol has risen, and "the wall" is the point where gluconeogenesis from lactate, glycerol and alanine can no longer keep blood glucose ahead of cerebral demand. After the finish line, oxygen consumption decays slowly rather than snapping back — the excess post-exercise oxygen consumption (formerly "oxygen debt") that restores ATP and phosphocreatine, reoxygenates myoglobin and clears lactate, mostly by oxidising it as fuel.
Where students slip
Two errors recur. First, "oxygen debt is just lactate removal" — most of the elevated post-exercise oxygen restores high-energy phosphates and restocks myoglobin, and lactate is largely oxidised rather than converted. Second, blaming lactate for delayed-onset muscle soreness: lactate clears within an hour, while DOMS tracks eccentric mechanical damage and inflammation a day or two later. The viva gem is McArdle disease: a patient whose venous lactate fails to rise on ischaemic exercise testing has a block at muscle phosphorylase, and the second wind arrives when hepatic glucose and fatty acids take over — physiology proving the pathway map.
Frequently asked questions
Which energy system dominates a 100-metre sprint?
Phosphocreatine via creatine kinase supplies most ATP for roughly the first ten seconds, with glycolysis covering the remainder.
Why is the Cori cycle energetically expensive?
The liver spends six high-energy phosphates rebuilding one glucose from two lactate that gave muscle only two ATP — a net four-phosphate cost borne by the liver.
What causes hitting the wall in a marathon?
Depletion of muscle and liver glycogen, after which output depends on slower fatty acid oxidation and limited gluconeogenesis.
How does endurance training change muscle biochemistry?
AMPK-mediated PGC-1α activation increases mitochondrial density, oxidative enzymes and fat oxidation at any given workload, raising the lactate threshold.
Why does lactate fail to rise during exercise in McArdle disease?
Muscle glycogen cannot be phosphorolysed for want of glycogen phosphorylase, so glycolytic flux — and therefore lactate production — is blocked, a diagnostic signature on exercise testing.