Aerobic threshold, anaerobic threshold, lactate threshold… The running community is mired in threshold-thinking. This mode of thinking is flawed, misleading, and distracting. Ultimately, it holds back many runners and keeps them from training well.
The terminology is flawed.
The intensity or pace labeled by these terms does not demarcate anything special with regard to physiology of training for endurance performance. There are thresholds in physiology, true thresholds, but the ones noted above here used commonly in the running world are not thresholds. Rather, they are vague ranges of intensity that have little pragmatic utility beyond a rudimentary estimation of intensities that correlate with graded effort levels. Indeed, there are underlying bioenergetics and neuromuscular recruitment patterns associated with these ranges of intensity, which are of interest to a scientist like me. But, they do more harm than good when used in the settings of training prescriptions.
The terminology is misleading.
For all running speeds, the active muscle cells are making lactate and consuming energy while using oxygen (aerobic). And, we’re always getting some energy through pathways that do not use oxygen (anaerobic). The implications associated with aerobic/anaerobic/lactate threshold terms is that there's a pivotal shift in the use of oxygen or the production of lactate. There is a tiny thread of truth to that implication but it is so nuanced and so far from meaningful utility for the athlete that these inaccurate terms tend to confuse the conversation more than clarify it.
The terminology is distracting.
Few athletes measure or monitor oxygen consumption, carbon dioxide production, or lactate in training. Moreover, few athletes really care about any of these, per se. Lactate has little or nothing to do with performance in an ultra-marathon. How our bodies use metabolic substrates (carbohydrates and fats primarily; protein to a small extent) in conjunction with oxygen is not our direct focus in training. Instead, we should focus on performance measures and train accordingly. Let millions of years of evolution do what they are supposed to do - adapt to the stresses we place. Put performance oriented stresses on your body and allow the bioenergetics to adapt as they will, as your evolutionary wisdom will do.
This terminology has a long history and has contributed to the development of ‘energy systems’ training. The ‘energy systems’ fad of training is just that, a fad or phase. It is natural human behavior to ride waves of conceptual novelty. The ‘energy systems’ fad is a wave moving through the running community. Like all such fads, there are important and accurate undercurrents, but a focus on ‘energy systems’ is superficial and overly narrow. There are myriad factors in the formula to calculate endurance performance. ‘Energy systems' are a small set of the variables. This does not mean that training focused on energy systems is necessarily insufficient. Rather, proper training design, no matter what it’s stated focus, necessarily incorporates many of the other myriad factors in the formula. Without stating them explicitly, well-designed training plans are multifactorial. It is the verbal emphasis on energy systems that is insufficient because it insufficiently describes the totality of the training.
I encourage all ultra-marathon runners to remove the ‘threshold’ labels from their vocabulary and stop using the words aerobic, anaerobic, and lactate/lactic acid, too. You’ll find that you are then forced to think deeply about what it is that you are really talking about. It will become apparent that the old terms did not capture your intent because they don’t accurately reflect either the underlying physiology or the relevant factors on which you are focused.