Workout Wednesday: The Dangers of Intervals

the danger of intervals

As a Dad, with a full-time day job, in addition to blogging, 60 minutes is usually the absolute max I can spare for a workout and still make a dent in my daily to-do list. In order to get the most fitness return for my time, I lean heavily on VO2 max intervals across biking, cycling and swimming to get me as ready as possible for my races. And this strategy can work really well in boosting strength and fitness, but you need to be careful as there is a dark side to leaning too heavily on interval training.

On paper, yesterday’s workout called for 18 x 30 second hard (RPE 9) intervals with 30 seconds recovery. This was a day after an interval session on the bike and my legs already felt heavy just lacing up my shoes. Should I push through it and hope my mind could kick my body into gear or should I call an audible and modify the workout? It’s a delicate line. When do you trust in the plan and the process you’ve chosen and when do listen to your body and adjust?

With my limited time, intervals are a lynchpin in my training to getting the necessary fitness to complete in medium to longer endurance events on a low volume of training. Why is it so crucial? Vo2 max is the highest rate at which your body can transport oxygen to your muscles. Oxygen is the fuel by which your muscles can then produce energy aerobically. VO2 max is mostly determined by genetics, but training your body to use up that potential is the key. Vo2 max has been shown in scientific studies as an excellent predictor of endurance performance, along with other factors such as lactate threshold and threshold power.

What is the best way to get to your V02 max potential? Interval training. 

So what’s the issue? Interval training is tough and puts a lot of strain on the body. Spreading the intervals across 3 different sports can help and I certainly see an uptick in my fitness, but I also am seeing a pattern of accumulating fatigue throughout the week that can carry into the weekend and end up negatively affecting the longer bike and run days.

Yes, training while fatigued is necessary and can have benefits, but it leaves a thin margin for error. When fatigue sets in, form can get sloppy, which can lead to injuries. The purpose of session can also get lost. If you do a fatigued workout and miss your power or pace by 20%, you’re not doing the workout as intended and not getting the benefits, either.

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So how can you train smartly while fatigued and still get those critical V02 max benefits on low volume? Do a workout that incorporates intervals, but is not solely about intervals. Doing this workout on tired legs or at the end of your session will simulate the mental and physical challenge of running fast late in a race.


Here is what I did yesterday:

  • After a thorough dynamic warm-up (especially important in the cold, damp Northeast this time of year), do a 60-70 minute run at an easy pace, a few ticks quicker than your long run pace, but nowhere near tempo pace.
  • Toward the end of your run, find a short hill (200-400 yards).
  • Do 3 intervals up the hill at a hard pace (RPE 9), recover back down the hill.
  • After the first set of intervals, jog very easy for 5 minutes to a new hill or return to the same hill
  • Repeat for 3 more intervals.
  • Cool down to finish.

Here is a snap of the laps from my Strava file:

intervals at the end of a run


Focusing on form during the intervals is paramount in this workout. Running faster, especially uphill, will naturally force your biomechanics to self-correct. It’s hard to run fast and sloppy uphill. Just remember, to lean forward slightly, take short, quick strides pushing off with the whole leg and trying to land your foot under your hip. Try to keep your arms, shoulders and face relaxed. Don’t waste energy on things that aren’t helping you go fast!

Intervals can be huge and important part of training on low volume, but you need to be smart about how much and how fast you do them. They are short, but brutal on the body. You need to ensure as part of your training you are not skimping on pre-hab or recovery routines. Trying to push through fatigue or small injuries in an interval session (or any session really) will only lead to more pain and bigger injuries.