| [quote=REDLAN;294185]
your muscles will predominantly burn fat whenever they can. purely aerobic exercise will primarily utilise fats for energy. As exercise intensity rises and you move into the lactate zone (typically above 65% MHR), your glucose utilisation increases. Lactate is the end product of the pyruvate pathway, and pyruvate comes from glucose. High intensity exercise uses fast twitch muscles fibres to provide power, and unlike slow twitch muscle fibres which rely on fat, these rely heavily on glucose for energy. Physiologically I believe the primary reason is that the strong contraction of muscle fibres impedes blood flow, and the body becomes unable to deliver enough oxygen to completely burn fat.
[quote]
During aerobic exercise your muscles will preferentially utilize glucose as a primary fuel.This will be converted to pyruvate in an anaerobic process called the Emden Meyerhof pathway. Provided the oxygen supply is sufficient, pyruvate is then further metabolized to carbon dioxide and water in the TCA (or Krebs) cycle in an aerobic process. This is more efficient than burning fat because less oxygen is required. When the supply of oxygen is inadequate to allow full utilization of all the pyruvate via the TCA cycle, pyruvate is shuttled into lactate (which can build up in the muscles). Anaerobic metabolism of glucose through the Emden Meyerhof pathway allows glucose to continue to be metabolized to produce energy under conditions of oxygen limitation.
Slow twitch muscle fibres are rich in mitochondria and are therefore adapted for metabolism through the TCA cycle. They are rich in myoglobin an oxygen carrying protein. In contrast slow twitch fibres have few mitochondria and little myoglobin. They are primarily adapted for anaerobic metabolism of glucose via the Emden Meyerehof pathway.  | 
