I used to think that this was a ridiculous question. But I saw it the wrong way. My view was how on earth could wearing a pair of shoes that are, say, 300g lighter make any material difference?
But that was not the point.
Sure, if you wear a backpack with a 500g weight in it and then run 5k you will do it in a certain time. Sure, if you take out 300g of that weight you will, other things being equal, run just a tiny bit faster.
But, and here’s the rub. The 500g weight is NOT on your back and is not the analogous weight (fat) around your tummy. It is the weight of the shoes at the end of your leg. AND the end of your leg pivots, broadly speaking, from your hips. So you are swinging that 500g weight from the end of your metre long legs. I’m no mathematician or physicist but once I realised this it just becomes obvious that a heavier, swinging weight is hard to slow down and speed up than a lighter one. So quite a bit of energy is wasted in doing just that THOUSANDS of times over a 5k.
Hopefully, that explains why lighter running shoes will make you run faster.
IE IF you lose weight you will not run 5k that much faster (on the flat at least). But lose that weight from your shoes and you WILL. Honest! [Actually, as a broad rule of thumb, you will run approximately 1 second faster over 5k for every 100g you lose in terms of body weight/body fat].
This subject also links in with the technique whereby the 5k runner is encouraged to flick up his or her heels. This flicking makes the effective leg length LESS and so the pendulum effect described above is reduced. Probably there could be a similar argument with knee-lift, I’m not sure about that. (There is a debate about these two ‘techniques’, knee lift focus may well cause heel flick and not the other way around for example)
Also, lighter running shoes often have a thinner heel and that causes the runner to pivot forwards more than usual and often this causes more of a faster forefoot strike running than pseudo-flat-foot strike. That is a faster but more injury-prone running style. But that faster speed isn’t a result of the weight necessarily. And we start getting into barefoot running if we continue from here.
** I race up to 10k+5k with racing flats. (New Balance RC 130X). It knackers my calves, I have to use compression gear to recover and I can only go fast once a month with those shoes on! [And as Dan says below that’s probably because I should train in my flats]