"So far, if you read the two previous articles regarding "The System", you know that high level endurance performance depends on 1) a high maximal oxygen consumption, or VO2 max, and 2) a high lactate threshold, or point of OBLA. Your VO2 max sets the upper limit for your sustainable work potential. For the elite endurance athlete, a high VO2 max is like the invitation to the big dance. Having an invitation to the dance does not ensure you will dance with the prettiest girl. But, not having one ensures you won't! The lactate threshold tells us something about how much of the cardiovascular capacity you can take advantage of in a sustained effort. It is determined by skeletal muscle characteristics and training adaptations. Multiplying VO2 max x LT (Oxygen Consumption at Lactate Threshold) gives us a measure of the effective size of your endurance engine. Now we come to efficiency. What does efficiency have to do with endurance performance? Victory goes to the person with the biggest endurance engine right? Well, let's use a race car analogy."
The rest is in the link, which is an extensive discussion of this factor in athletics.