Fitness varying a lot?

[anna7711]

I think what has surprised me the most from running regularly is how much my fitness can vary and how random it is when it does. Some days I can Comfortably run at 9 mins per mile for 5k and some days I can barley run 1.5 miles at 10 mins per mile. I usually run at the same time, similar distance (usually 5k), similar moods, have the same amount of sleep, stable diet and run every other day. This also happened to me when I used to cycle to work - some days I went up the hill in 6th gear sometimes in gear 1 lol.

Has anyone else experienced really random/large variations?

Does anyone know the science behind this or have any studies to link aside from the obvious like not resting enough/latent fatigue/mood and sleep changes?

[John_W Graduate Ambassador]

You're confusing fitness with other things. Your fitness will increase a little with every run you do.

What also changes is your fatigue and that is very dependent on stress, diet and sleep mainly. For women, studies have shown that menstrual cycle can have a significant impact also. You can decrease your fatigue level by resting a few days - your fatigue level will decrease much quicker than your fitness level.

Your 'form' is a function of your fitness (the benefit from the training) minus the fatigue induced from the training.

The science behind 'recovery', i.e. what you're doing when not training, isn't exact but it's pretty well understood that how you recover/rest is just as important as the training.

So a well-recovered (rested) athlete will have good fitness, small amounts of fatigue and therefore have (or be in) good form and ready to perform to a good level.

For the importance of sleep (generally), have a look at Dr Michael Mosely's latest book (him of the 5:2 diet, HIIT etc)