If you’ve been around the distance running scene for a while, you’ll probably have noted this phenomenon. It is often seen in longer races, or long slow runs, where a reasonably talented athlete is ‘there’ one moment, and ‘gone’ the next.

This phenomenon particularly applies to seemingly talented runners who have lungs like bellows and a heart like Phar Lap’s, and arteries like hose-pipes, as well as a good amount of the perfect muscle fibre type for both the speed and endurance requirements for middle distance; the Type IIA fast twitch muscle fibre, which as well as being very powerful has the capacity to acquire aerobic qualities with suitable training.

Despite all these magnificent advantages that make it all seem relatively easy for them to cope with sustained running, they’re “there one moment, gone the next!”, whether in longer races, or long slow runs.

Often, the hardened endurance runner will type-cast this talented athlete as being “mentally soft”, and that could be an accurate assessment for many runners who are not prepared to patiently put in the ‘hours’ of slower, steady running required to get the ‘truly aerobic’ fat-burning red Type I slow twitch muscle fibres ‘fit’ for their purpose. The genes responsible for stimulating enzyme pathways and fat-burning metabolism deep into the slow twitch muscle fibres need to be ‘switched on’ by many hours of very low-intensity running that preferentially recruit the Type I slow twitch muscle fibres, as well as enabling the all-purpose Type IIA Fast Twitch muscle fibres to switch on their aerobic gene expression.

Very often, the young ‘natural’ athlete who is ‘there’ one moment, and ‘gone’ the next, isn’t ‘soft’ at all. Ironically, the very best training volume and intensity which will enable that athlete to “harden up” physiologically is defined by the number of ‘soft, easy’ training hours done in training to switch on the ‘aerobic fat-burning engine’ that will eventually push up the aerobic capacity from ‘below’.

This is the direct opposite of what many athletes do in training; aiming to increase VO2 Max by training hard and fast, running up oxygen debt in a sea of rampant acidosis, which in turn leads to metabolic stress and a plethora of minor immune challenges.

However, recent research backs up experience with the fact that the sweet spot that is optimal to develop mitochondrial density is also exactly the same sweet spot where capillary density is most proven to occur; namely, that the biggest stimulus to both of those adaptations is the amount of time the athlete has spent training between 50% and 70% of heart rate reserve intensity.

So, to sum up:

To improve your aerobic fitness, and raise the anaerobic threshold as high as possible, you DON’T need to push hard and fast; just long and easy. That will get your oxygen uptake much higher, over several seasons, than just training at VO2 Max pace, which is only really useful when you’ve done several years of predominantly easier aerobic running.

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