Nine years ago I was in the United States, and was talking with a very good Lydiard-based coach who was amazed at the improvement in his athletes’ general immune health and training ability when he replaced their anaerobic threshold tempo run that was formerly done weekly in the aerobic base, with a strong aerobic sub-threshold tempo run.

Training at or near the anaerobic threshold tends to burn up limited glycogen stores, or blood glucose. Training at higher aerobic steady states well below anaerobic threshold tends to burn stored fats, which even the thinnest of us have ample stores of. Enough to ensure we never run out of endurance fuel at aerobic paces, once we become fat-adapted.

Fat-burning can theoretically power us for hours aerobically, with an athlete who is prepared to train at a slow enough pace and low enough heart rate for long enough to initiate a fat-burning adaptation, building an aerobic fat-burning engine. Patience is key.

What exactly is a sub-threshold run, and what should it feel like?

For a start, it is a run designed to subtly push up the anaerobic threshold from efforts and intensities well below threshold, while immersed in a base period of long aerobic training volumes. Lydiard was firmly opposed to any extended training that was not strictly aerobic when building an endurance base.

We are aiming at a comfortably fast, mildly exhilarating run that one finishes feeling energized, and which one could possibly repeat again if need be without undue fatigue. There will be no bending over huffing and puffing to catch one’s breath afterwards: (a sure sign of anaerobic exertion), and the next day the athlete will feel fully recovered.

One of the purposes of an aerobic endurance base is to teach the body to run off its fat stores, as opposed to burning up glucose and glycogen. If we train at anaerobic threshold in an aerobic base period, we are training exactly the wrong muscle fibre types and energy systems to become aerobically efficient. We are effectively de-training the aerobic physiology we have built so carefully every time we dip into anaerobic metabolism for too long. So don’t do it if you want to race at your best when it absolutely counts.

Once or twice a week in an endurance base or aerobic build-up, Lydiard’s athletes would run a ‘3/4 effort’ high-aerobic steady run for up to an hour on a set, measured circuit of up to ten miles on an undulating course with firm footing. The idea was to start a long endurance base phase with these ¾ effort runs at a controlled strong aerobic pace well below anaerobic threshold, and to week by week aim to run the course a few seconds faster each time, in a strong and controlled manner, till after 8 to 12 weeks the circuit was being covered several minutes faster, aerobically and comfortably, at high-aerobic speeds that would approach those previously achieved at near-threshold efforts.

The great New Zealand marathoner Barry Magee could start his aerobic buildup at a rate of about 6 minutes a mile (3:43 per km) for ten miles (16 km), and steadily whittle the time down over the course of up to 12 weeks from a strong but relaxed 60 minutes to a strong but relaxed 54 minutes. Years later, Rod Dixon and Dick Quax did similar high-aerobic effort runs in their aerobic bases as well, but often got their circuit times down to about 51 or 52 minutes before moving on to hill exercises or specific anaerobic training. This was a way to increase aerobic adaptation more rapidly than by simply running at lower aerobic steady states, which also could increase aerobic capacity and aerobic running speeds year by year.

The simplest way to see if an athlete is approaching the anaerobic threshold at which he or she starts to burn up glucose rapidly is the “talk test”. When an athlete has difficulty holding an extended conversation, and is huffing and puffing quite a bit, he or she is probably approaching anaerobic threshold. The huffing and puffing starts at the point where the lungs blow off more carbon dioxide than oxygen breathed in, and where glucose goes from being burned with oxygen (aerobically) to without oxygen (anaerobically).

If one uses a heart rate monitor to measure intensity, then the sub-threshold sweet spot occurs at a heart rate that is about 70% of the maximal heart rate reserve with a seasoned endurance athlete, well below the 80-85% of maximal heart rate reserve that represents the onset of anaerobic threshold for the majority of seasoned endurance athletes.

I asked Rod Dixon a month or so ago how long his aerobic base periods were before his very long European track tours in the 1970s. He said usually about sixteen weeks to make sure he’d get through months of high-quality racing without his form dropping off.

He and John Walker maintained their aerobic adaptations on tour by lots of easy running at recovery heart rates, in between hard races, and mainly used regular racing as specific anaerobic work. Light fast stride-outs over sprint distances were sometimes run in between races, with plenty of easy recovery between efforts.

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