Most age-group triathletes obsess over one number: their VO2 max. The bigger, the better. It is the metric every wearable estimates and every athlete wants to push up.
There is a better metric you are probably not tracking. It is the one the best long-course programs in the world care about more than VO2 max itself, and it is the metric that actually decides how fast you go on race day.
That metric is efficiency. And the way you train it is not what most athletes think.
VO2 Max Sets the Ceiling. Efficiency Decides What You Can Sustain.
VO2 max is your maximum rate of oxygen consumption. Your aerobic ceiling. It matters, and we have already written about why you should include VO2 max work in your base phase.
But VO2 max is only half of the story. The other half is how much oxygen you need to hold a given pace or power. Two athletes with identical VO2 max values can produce very different race times if one of them needs less oxygen to sustain their race pace. The more efficient athlete has more headroom below their ceiling. The less efficient athlete is burning through their tank faster just to stay on pace.
In the Norwegian triathlon system, efficiency is treated as the most undervalued metric in endurance sport. Oxygen cost at target intervals is measured in the lab and tracked across the season. When that oxygen cost drops for the same pace, the athlete is adapting. When it plateaus, the coaches know it is time for a new stimulus. That level of tracking is lab-dependent, but the principle is universal and you can train for it without any of the equipment.
A Bigger Engine Is Not Always a Faster One
The clearest illustration of this comes from the swimming world. At a purpose-built swim flume, two of the world's top long-course triathletes were tested against elite competitive swimmers. The triathletes actually had higher swimming VO2 max values than an Olympic gold medallist. Bigger engines on paper.
At the same swimming velocity, an Olympic bronze medallist used roughly 25 percent less oxygen. The swimmer did not have a bigger engine. The swimmer had a far more efficient one.
The same pattern shows up across every endurance discipline. Elite cyclists sit at a gross mechanical efficiency of around 20 percent, meaning only 20 percent of the metabolic energy they produce becomes propulsive work. The other 80 percent becomes heat. A rider holding 200 watts of mechanical output is producing close to 800 watts of waste heat. A small improvement in efficiency is a large reduction in metabolic cost for the same pace.
Why Efficiency Matters More for Long-Course Racing
Short-course racing is bounded by your ceiling. You are close to your maximum oxygen uptake for most of the effort, and raw aerobic capacity tends to win.
Long-course racing is different. Over four, six, or ten hours, you are not working anywhere near your VO2 max. You are sitting below your second threshold, defending a sustainable pace for hours on end. The athlete who can hold that pace at the lowest oxygen cost, the lowest heart rate, and the lowest thermal load is the athlete who has the smoothest race.
Because roughly 80 percent of metabolic energy becomes heat, an inefficient athlete is a hotter athlete. Heat is one of the major limiting factors in long-course racing, particularly in an Australian summer. A more efficient engine does not just save fuel. It cooks you more slowly.
None of this shows up on your wearable. None of it is captured by a VO2 max estimate or an FTP number. It is the invisible layer that separates athletes who fade at kilometre 25 of the marathon from athletes who still have something left at kilometre 35.
How to Actually Train Efficiency
Here is the part most athletes get wrong. Efficiency is not built by going harder. It is built by spending controlled, focused time at the intensities you want to become efficient at. For long-course triathletes, that is primarily Zone 3 in a five-zone model, the band between your first and second thresholds.
The mechanism is slow and cumulative. Mitochondrial density increases. Enzymatic activity improves. Substrate utilisation shifts so that you rely more on fat oxidation at sub-threshold intensities and spare carbohydrates for when you actually need them. Your slow-twitch fibres get better at recycling lactate as fuel through the lactate shuttle, turning what the old textbooks called a waste product into a performance asset. This is exactly why sub-threshold training is the engine of endurance development.
This is the adaptation that pyramidal distribution produces. The Tremayne Performance model spends roughly 70 percent of training time in Zones 1 and 2, 25 percent in Zone 3, and 5 percent in Zones 4 and 5. That Zone 3 block is where efficiency gets built. It is not junk mileage. It is the work that quietly raises the ceiling on how much of your engine you can actually use when it matters.
The polarised 80/20 model, by contrast, treats the Zone 3 band as a dead zone to be avoided. Athletes following it are told to stay either very easy or very hard. That structure can sharpen top-end capacity, but it does very little to improve how cheaply you can hold sub-threshold efforts. For a long-course athlete, that is the wrong trade.
A bigger engine is nice. A more efficient one is what wins.
Efficiency is what you cannot see on your watch. It is the slow, quiet adaptation that turns a capable athlete into a racer who never fades.