Intensity Control: Thoughts on Scientific Training for Endurance Athletes by Philip Skiba

While reading Philip Skiba’s Scientific Training for Endurance Athletes, I came across a point in his discussion of VO₂ max intervals that made me pause. His general recommendation is for most athletes to finish when they feel they could complete roughly one more repetition. He also acknowledges that experienced athletes may sometimes need to push further to get the response they’re looking for. I liked that distinction because it leaves room for judgement, and it made me think about how we evaluate our own training.

As athletes, it’s easy to finish a session with something left and wonder whether we should have done more. Another repetition, a little more power, a faster final kilometre. I understand the satisfaction of knowing you gave everything, but as a coach I also have to consider what that extra effort contributes and how it affects the training afterwards. In triathlon, that matters because today’s workout sits alongside swimming, cycling, running, and everything happening outside training.

What interested me was the context around his recommendation. Skiba explains how the intensity, duration, and recovery periods work together to shape an interval session. My takeaway was that we need to assess the work we completed and the cost of completing it. Feeling exhausted gives us only part of that picture. The examples below are how I think about applying that discussion in coaching, rather than specific workouts or rules prescribed by Skiba.

The first question this brings me back to is what we are trying to achieve with the session. That is something I want an athlete to understand before starting. A controlled threshold session, a VO₂ max session, and a race-specific long ride can all feel hard, but I’m looking for different things when reviewing them. In a threshold session, I want to see you accumulate the planned work at a controlled, repeatable effort. During VO₂ max intervals, I expect harder breathing and increasing difficulty as the set progresses. On a race-specific long ride, I might be assessing how well you maintain your output, position, and fuelling later in the session. Read the workout description before you start, and ask if the intended effort is unclear.

Another point I found useful in this section was Skiba’s explanation that adaptations overlap: exercise falls along a continuum, and a workout can influence several aspects of your physiology. A session labelled “VO₂ max” doesn’t exclusively train VO₂ max, just as threshold work doesn’t develop only one physiological quality. The label describes our main intention. Understanding that intention helps you execute the session and helps us decide whether increasing the difficulty would actually improve it.

To understand why the intensity matters so much, it helps to look at what happens as you increase the effort. At lower intensities, oxygen consumption can settle at a relatively stable level. At higher but still sustainable intensities, it takes longer to settle and the metabolic demands are greater. Above critical power on the bike, or critical speed when running, you cannot maintain that same stable metabolic response indefinitely. Even while your power or pace stays constant, the internal demands continue to rise. During sufficiently long efforts in this intensity range, oxygen consumption can reach its maximum before you have to stop or reduce the effort.

This helps explain why an interval can feel manageable early and become difficult without any increase in pace. It also explains why a small increase in intensity can shorten the amount of work you can sustain, particularly when you are already close to that boundary. Critical power, FTP, and the various measures people call “threshold” are related concepts, but they are not interchangeable. Your prescribed targets are practical estimates that we refine through testing, training, and feedback. Ten extra watts or a few seconds per kilometre may look minor on the screen, but the effect depends on where that effort sits relative to your capabilities.

A practical example I would use to explain this is a hypothetical bike session of five four-minute efforts. You start the first rep above target because your legs feel good, then push the second one as well. By the fourth, your power is falling, and you extend the recovery before attempting the last. You finish exhausted, but you’ve struggled to sustain the work we intended. A more controlled start may have allowed you to complete all five repetitions at the appropriate intensity. The first rep would look less impressive, but the full set may have served the purpose better.

When reviewing intervals, I’m interested in that pattern across the whole session. Did you maintain the output? Did perceived effort rise gradually? Could you hold your technique? Some variation is normal, and some workouts deliberately use changing intensities, but a large, unplanned drop-off deserves attention. It could reflect aggressive pacing, an unsuitable target, accumulated fatigue, or inadequate fuelling. We need context before deciding. Treat the opening rep as part of the full set and give yourself a chance to execute the later work well. If the prescribed effort repeatedly feels too easy, tell me so we can review it.

His discussion also made me think about how athletes approach the recovery between repetitions. Intensity, repetition length, recovery duration, recovery intensity, and total volume all affect the session. Short recoveries may help keep oxygen consumption elevated between efforts, while longer recoveries can allow you to reproduce a higher output. The appropriate choice depends on the goal. Cutting your recovery short or turning an easy recovery jog into another moderate effort changes the demands. You don’t earn extra credit for making the next rep harder to complete. If you need much longer recovery than prescribed, that is useful feedback rather than something to hide.

Coming back to Skiba’s suggestion, I find the idea of having one repetition available useful when assessing a session, especially when we want quality work that an athlete can recover from. I wouldn’t turn it into a rigid rule for every workout, and his acknowledgement that experienced athletes sometimes need to push further is important. The important detail is what that extra repetition would look like. Could you complete it at the intended output, with comparable technique and the prescribed recovery? Or could you only force it through by sprinting early, slowing dramatically, or changing how you move? At the end of a set, ask yourself whether you could repeat the effort once more at the same quality. Give your coach an honest answer. You don’t need to perform the extra rep to prove it.

For me, applying this to 70.3 and IRONMAN training also means looking at the next day. A session can go well in isolation and still cost more than intended. If an overly aggressive bike workout leaves you struggling to complete the following run and swim, we need to consider the effect across the week. Some residual fatigue is expected, and a demanding workout doesn’t become a mistake because your legs feel tired afterwards. I’m looking at whether your response matches what we planned and whether you recover well enough to continue. Soreness is also a poor scorecard for endurance training. It varies with the activity, your familiarity with it, and the type of muscular loading. You can complete a productive workout without being sore the next morning.

This is where your feedback becomes valuable. “Felt hard” gives me a starting point, but a few specific details give me much more to work with. For example, you might tell me that the first two reps felt controlled, breathing became hard by rep four, and you held the power throughout. You could add that the final rep felt like a nine out of ten, that you probably had one more available, and that your legs felt normal during the easy session the following morning. That tells me about execution, perceived effort, and recovery. If you underfuelled, slept poorly, or trained in unusually hot conditions, include that context too. The numbers alone won’t explain everything.

There will be sessions where I want you to explore your limits. Testing, racing, and selected demanding workouts all have a place. I want those efforts to be deliberate, with enough room around them to recover. For the rest of your training, assess the session against its purpose: whether you followed the intended intensity and recovery, maintained the quality of your work, and handled the fatigue well enough to continue training. Over time, we should see you producing more power, running faster at a comparable effort, or sustaining your output for longer.

That was my main takeaway from this section of the book: I want athletes to understand why they’re pushing, how much work we’re trying to accumulate, and when another rep would serve a purpose. There will be days to test your limits. There will also be days when finishing with a little left is a sign that you executed the session well. Reading Skiba’s discussion gave me a useful way to explain that judgement, and something worth keeping in mind the next time you finish a good session and wonder whether you should have done more.