Critical Speed

Critical Speed (CS) is the pace at which a runner can just maintain their metabolism. It is the maximum speed at which oxygen uptake, energy consumption, and metabolic byproducts remain in balance. If you run faster than CS, there is still enough oxygen available, but the energy demand exceeds the capacity of the oxidative system. Glycolysis has to kick in, which results in a more unstable, less sustainable form of energy production. That is precisely why one thing is crucial in this training zone: intensity control.

Why training "just below the limit" is so beneficial

Training around CS taps into the most efficient system the body knows: oxidative energy production via slow glycolysis. Here, carbohydrate combustion is still largely controlled in the mitochondria. As long as you stay within this zone, the body learns to deliver a lot of power with relatively little disruption.

However, as soon as you train just above CS, you enter the world of fast glycolysis. Here, carbohydrates are broken down very quickly, but with many by-products: acidification, high lactate levels, increased breathing rate. This fast route is less sustainable and causes the body to become increasingly unbalanced. You are then training something other than what you think: no longer capacity in steady state, but mainly your tolerance for chaos.

Reps in Reserve (RIR): the gold standard for intensity control

RIR is a subjective measure that describes how many repetitions or minutes you think you can still perform at the same intensity. In CS training, RIR helps you determine whether you are still within the limits of steady state.

A well-executed CS block ends with 1-2 RIR: you feel that you have something left. Are you just surviving at the end? Then you were above CS. RIR is therefore essential for maintaining the right intensity, especially since heart rate and pace can be misleading when you are tired, hot, or stressed.

Examples of well-controlled CS training

1. 5x6 min at CS / p=2min

Goal: Strengthen steady state at block level

RIR: 1.5-2 on the last rep (you could do 1 or 2 more)

Mistake: First blocks too fast = accumulation = last blocks above CS = no longer steady state

2. 10x3 min at CS / p=60 sec

Goal: Many 'entries' into steady state, without cumulative stress

RIR: 1-2; repetition 7-8 feels heavy, but manageable

Monitoring: Breathing accelerates, but remains controllable; no urge to stop

3. Climax run 75 min: 60 min AeT, then faster every 5 min with last 5 min at CS

Goal: Address CS under fatigue without exceeding the threshold

Check question: "How much longer could I have continued at this pace?" - that is your Time in Reserve. Because most of this session remains below AeT, you should be able to continue for at least 15-20 minutes after the 5 minutes at CS. If you only have 1-2 minutes left or are close to collapse: the pace was too high.

Why intensity control is more valuable than daily success

1. You train the right energy system

With proper CS training, you improve your oxidative capacity. You develop a more efficient muscle metabolism and improve the pace at which you remain in balance. Training above CS works on a different system: your anaerobic capacity and buffer capacity. Both are valuable, but require a different training goal.

2. Steady state = consistency = progression

Properly executed CS training does not feel heroic. It feels controlled, repeatable, sometimes even "too easy." But that is exactly what leads to structural progress.

3. Too much stress = less adaptation

Every time you run too hard, you produce fewer mitochondria and more cortisol. Not only are you training inefficiently, but you may also be breaking down what you previously built up.

4. Efficiency allows for volume

By performing your CS training in a tightly controlled manner, your overall metabolic stress remains limited. You don't waste energy on excessive intensity, which allows you to handle more training volume during the week. And ultimately, training volume is the most powerful stimulus for adaptation.

5. Reps in Reserve makes you self-directed

With RIR, you learn to feel where your limit is, instead of relying on heart rate or GPS. This makes you less susceptible to conditions (wind, heat, fatigue) and helps you stay in control of your training goal.

Finally: train with discipline, not ego

Critical Speed training is an investment in physiological purity. The best workouts may not feel impressive, but they ensure the greatest progress. Use RIR, be honest with yourself, and give your body exactly the stimulus it needs.

As Steve Magness aptly said: "Training is to improve your fitness, not to prove your fitness."

Train for repeatability, not for impression.