Figure 1.

Schematic of two alternative pathways from glucose to the lactate pyruvate pool. The direct glycolytic pathway produces two lactates and two ATP, whereas the “glycogen shunt” produces only one ATP. We propose that during contractions, glycogenolysis makes a major contribution to the energy needs and that glycogen is replenished between contractions. Because the lactate production is determined by the power requirements during the millisecond contractions, it does not match the average energy requirements that are supplied efficiently by oxidation of lactate/pyruvate. Excess lactate is generated to buffer short- and long-term energy requirements.