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In this episode, Dr. Iñigo San Millán, Assistant Professor at the University of Colorado School of Medicine, explains the crucial role of mitochondrial function in everything from metabolic health to elite exercise performance. Iñigo provides a masterclass into the many different energy system pathways, the various fuel sources (including the misunderstood lactate), the six zones of exercise training, and the parameters he uses to measure metabolic health. Additionally, he highlights the power of zone 2 training in its ability to act as a powerful diagnostic tool, and perhaps more importantly as a treatment for mitochondrial and metabolic dysfunction.

aphorist wrote:I'm going to modify my own exercise regimen around this Zone 2 conditioning framework. The explanations of how lactate works in the body (and its role in cancer) were superb. The discussion really connected a lot of metabolic pathways and concepts for me.

xactly wrote:I'm trying to confirm that San Millan's definition of Zone 2 is 60 - 70% of your max heart rate. Peter Attia talks about keeping track of his Zone 2 workouts by using watts and FTP, not heart rate. And San Millan talks about a Zone 6, which I have never seen before in conjunction with heart rate monitoring.

Skeletal muscle is composed of 2 kinds of muscle fibers- Type I, also known as slow twitch, and Type II, or fast twitch. Fast twitch fibers are also divided in two subgroups called Type IIa and IIb. Muscle fiber contraction obeys a sequential recruitment pattern where Type I muscle fibers are the first ones to be recruited. As exercise intensity increases muscle contractile demands increase and Type I muscle fibers cannot sustain the necessary demand. Type IIa muscle fibers kick in and eventually as intensity keeps increasing Type IIb will finally be recruited. Simply put, slow twitch fibers are used at slower speeds and fast twitch at faster speeds. Each muscle fiber has different biochemical properties and thus different behaviors during exercise and competition. Type I muscle fibers have the highest mitochondrial density and capacity and therefore are very efficient at utilizing fat for energy purposes. Type IIa fibers have a lower mitochondrial density and a higher capacity to utilize glucose. Type IIb muscle fibers have a little mitochondrial density and a very high capacity to use glucose as well as ATP stored in these fibers for instant anaerobic energy. Therefore, each exercise intensity implies different metabolic responses and muscle fiber recruitment patterns which also corresponds to different training zones [see above]

BrianR wrote: I did find this article Training Center: Why heart rate shouldn't be ignored which suggests that, once you've developed a lactate utilization profile and correlated it with heart rates, you can use heart rate as a proxy to define your zone. And you might also be able to use disciplined time trial type tests to develop approximate zones.