Muscle cells during vigorous exercise do not get sufficient oxygen to support the electron transport chain. Under these conditions, pyruvate is converted to lactate.
Production of Lactate
When pyruvate is converted into lactate, this involves the transfer of hydrogen ions from the NADH produced in glycolysis to pyruvate in order to produce lactate.
This regenerates the NAD needed to maintain ATP production though glycolysis.
Muscle Fatigue and Oxygen Debt
As lactate accumulates muscle fatigue occurs.
The oxygen debt is repaid when exercise is complete. This allows respiration to provide the energy to convert lactate back to pyruvate and glucose in the liver.
Slow-twitch muscles
Slow-twitch muscles fibres contract relatively slowly but can sustain contractions for longer.
Slow-twitch muscle fibres rely on aerobic respiration to generate ATP and have many mitochondria, a large blood supply and a high concentration of the oxygen-storing proteinmyoglobin.
The major storage fuel of slow-twitch muscle fibres is fat.
Fast-twitch muscles
Fast-twitch muscle fibres contract relatively quickly, over short periods.
Fast-twitch muscle fibres can generate ATP through glycolysis only and have fewer mitochondria and a lower blood supply compared to slow twitch muscle fibres.
The major storage fuel of fast-twitch muscle fibres is glycogen.
Patterns of Muscle Tissue in Athletes
Most human tissue contains a mixture of both slow- and fast-twitch muscle fibres.
Athletes show distinct patterns of muscle fibres that reflect their sporting activities.