The Brooks team have looked to try to help people run a touch more midfoot and not to heel strike as heavily with their “ideal heel” principles taken from their “pure project” product which I am a big fan of.
In principle this is great, I love it personally as it’s how I run and it means that I can now run in a shoe which has a great deal of cushion underfoot, amazing fit but works with my midfoot strike really well – winner!
What I don’t like is that for those runners who ever so mildly over pronate and a lot of this occurring at the rear foot, where the Glycerin 11 holds them really well with the wider base of support. The Glycerin 12 doesn’t as it’s a narrower base of support around the heel and allows the runner to continue to over pronate. Brooks have seen this coming and filled in the medial midfoot more and added some mild guide rails to try and compensate for this but by this time, it’s actually too late and we’re seeing those runners who have been in the 11 and had it function really well, not receive the support that they used to, it’s not by much but for some people this change can be enough to make them unstable in the 12 where they used to be very stable in the 11.