The most important aspect of training and exercise science is how do we produce useable energy (ATP) for working muscles. The many assumptions associated with ATP production and consumption are often the driving factors in performance diagnostic, training planning, and training guidance. I believe, as is standard belief, that the transportation and utilization of oxygen is the determining factor in ATP production. This recognition led to the famous differentiation between a so called aerobic and anaerobic energy pathway by Archibald Hill and Otto Meyerhoff (earning them a Nobel prize). This discovery is one of the founding principles of performance diagnostic and training planning as seen by the development of exercise and training science over the last 80 years. The anaerobic or lactate threshold theory is based on the the difference between aerobic and anaerobic energy pathways and identifying where this shift apparently takes place. The assumption was that lactate production and accumulation identified a change from aerobic to anaerobic energy pathways due to a compromised oxygen delivery or utilisation. In other words lactate accumulation and production identified a hypoxic situation in the muscle tissue that forced the organism to produce the required ATP independently from oxygen. Following this shift from aerobic to anaerobic energy production it was then considered only a matter of time before metabolite accumulation, at the time considered to be lactic acid - now assumptions involve accumulation of hydrogen ions or inorganic phosphate - would compromise muscle contractility leading to cessation of activity. This recognition made it important for trainers and coaches to differentiate between anaerobic and aerobic energy production; the only logical step considering the assumptions made was that if lactate accumulation was a result of hypoxia which causes a metabolic shift from aerobic to anaerobic, lactate measurement could be used to identify a threshold between aerobic and anaerobic energy production. In the past 20 years various scientists have made very significant discoveries that question the above mentioned rational. One very significant realization is that the idea of muscle anaerobiosis is false. Numerous studies (Richardson et al., 1998 among others) clearly shows that even in highly active muscles oxygen supply is never compromised, or for clearer definition of this statement partial pressure of oxygen remains constant during graded incremental exercise in humans between 50% to 100% of VO2max. If oxygen supply is never compromised during exercise why is there a shift from an aerobic energy pathway to an anaerobic as Hill described? Or is there a shift at all? A theory postulated by Connett and colleagues (1990) tries to explain why energy would be produced oxygen independently, when it is clear that sufficient oxygen supply is still available. The idea is that a critical oxygen levels exist which enusure oxygen availability for energy production for life vital functions, and therefore these oxygen reserves are protected. Therefore if during strenuous exercise oxygen levels decreases towards this critical range the body starts to produce ATP independently in an attempt to maintain ATP production for the desired activity without further decreasing oxygen levels. Therefore even though oxygen is present, and ATP is continuously being produced oxidatively (even at a higher rate during intense exercise as seen by ever increasing VO2 values to max effort), oxygen independent ATP production supplement the ATP supply, which will overtime lead to debilitating metabolite accumulation and exercise cessation. This explanation leads to the simple conclusion that ¨it remains true that tissue hypoxia does indeed lead to increased [lactic acid] concentrations. However, the induction that elevated [lactic acid] production and accumulation necessarily indicates the presence of hypoxia is not correct¨ (Gladden, 2001). This acknowledgement, that blood lactate measurements are unreliable in nature, begs for further development in the field of exercise science to lend coaches and athletes tools for diagnostic and planning. The most prominent of such developments is VO2max, testing which as lactate threshold testing attempted to do, tries to analyse oxygen consumption and utilisation, as well as changes in metabolism. Regardless of the technique used, the brief history above clearly demonstrates that oxygen in a general term and oxygen measurements remain a key factor for performance diagnostic and training guidance.