The anaerobic threshold (AT) is a widely used tool for investigating aerobic performance characteristics in physiological of pathological conditions. The aim of the present paper is to show that, when the lactate concentration in blood [Lab] is constant in time, regardless of its absolute level, the whole body energy sources for muscular work are entirely aerobic. In fact, [Lab] can remain constant if, and only if, La production is equal to La removal. Since this last is an entirely aerobic process, it can be shown that the net anaerobic energy yield from La production is nil, even if some muscle fibres are indeed producing La at a non trivial rate. These conditions will be defined as "unevenly aerobic" to distinguish them from: (1) the traditional "evenly aerobic" ones, in which the net La production is also zero, but because neither La production nor La removal are significantly increased, and (2) "true anaerobic" conditions wherein La production exceeds La removal and therefore [Lab] increases continuously in time. Comparison of unevenly versus evenly aerobic conditions shows that in the former case the depletion of the glycogen stores is faster in the muscles (or muscle fibres) that are producing La than in those which remove it. Hence the La producing fibres may become crucial in setting the duration (or intensity) of performance. AT, irrespective of its precise mode of assessment, is presumably a measure of the exercise intensity corresponding (or close) to the transition between evenly and unevenly aerobic conditions, thus explaining why AT is a good estimate of the subjects' training status and/or performance capacity.