When heel striking you have to actively generate more of the forces on toe off, but when you midfoot/forefoot strike you preload the calf and foot so that you get an additional passive force on toe off.  If you have transitioned but are still pushing off actively as strongly as you did when heel striking then the forces on toe off could be higher, and the associated stresses on the bones and soft tissue higher with it.

One part of Barefoot Bob’s advice to runners is that shouldn’t push off, instead lift their foot off the ground.  While I don’t believe that one should actively pull the foot off the ground, I think this advice is good cue that reminds one not to push off hard, and allow the toe off to be a more passive response.  If one habitually pushes off hard then this might take quite a bit too re-learn.

Another side effect of pushing off hard is on the overall gait and forces that it generates.  Pushing off hard will lead to more time in the air, and longer stride, but I don’t think the longer stride itself is the problem, it’s the ratio between time in the air and time on the ground that governs the ground forces that we have to generate. Not pushing off strongly will reduce the time in the air, and reduce ratio between time in the air and time on ground so reduces the ground forces we have to generate.

Another subtle aspect to that fact that reducing the time in the air is the crucial factor in reducing ground forces that we have to generate,  is that this time in the air is essentially how long we have to recover the leg from toe off and get it back into to landing position.  If you are slow at recovering the leg then you have to push off harder to give you time to get it back in place, so the two are linked.  I would then ask the question might an active push off hamper your ability to quickly recover the leg?  Would a strong push off be associated with a relatively straight leg that takes longer to fold up and pull forward?

The reason why I raise all this as one could, out of habit, get locked into pushing off too strongly.  Heavy shoes (that slow leg recovery) and heel strike will both make it more likely that one will rely more heavily on a strong and active push off.  Kicking this habitual activation of the muscles might be something that runners who are transitioning don’t think about enough – rather they will concentrate on the landing as being the key to a natural gait.

I also do wonder if increasing cadence/shorten the stride is over emphasised, as neither by themselves are the active ingredient in reducing ground forces – it’s the ratio of time in the air vs time on stance.  The one element that we should be working to minimize is the time in the air and to not compromise time on stance in achieving this.  Not pushing off hard will increase time on stance and reduce time in the air so is a win win.  Concentrating on quick recovery of the leg is the next element to focus on to make sure the leg is back in position to accomodate the shorter time in air.  

Another cue might be useful is to think about a balanced stride – to have perfect balance you land light and your launch light, like too weights on a balancing scale.

Pete,
I pulled a few article today to address the questions you have and will put together a response with some references to the current literature once I read through some of them.  I think you and your readers would be interested in some of the science behind the topics you regularly cover. 

Andy

As an avid wearer of FiveFingers (I still shudder calling them that.  They are toes after all, correct???) I can attest to the problems with fitment, specifically the toes.  I have taken to the Bikila and the Bikila LS, but with either pair I need to wear Injinji socks due to blistering in the arch area.  Without the socks I still have issues with the toes being a little constrictive, so you can imagine the problems with any extra layers on my feet!  I wouldn’t say I’ve had any fractures to report, but I have had episodes of “top of foot” soreness that I alleviated (er, eliminated) by cutting off the straps of the Bikila.  I bought a 42 and they fit snug across the foot, so they stay on even without the strap.  I think the soreness stems from tightening a strap with minimal cross sectional area, so the amount of stress placed on the metatarsals (and their relavent connective tissues) is amplified because the stress of securing the strap is not dispersed over a wider area.  But with the staps nonexistent, so too is the accompanying soreness!  Winning!!!
The toes do feel numb after about an hour or so, and I’ve been adding barefoot runs here and there (no more than 2-3.5 miles a few times a week) and I’ve noticed that my dress shoes do not fit anymore…it seems my feet have definitely widened and lengthened just a smidge!  It’s just a matter of course: Work muscles, they get stronger and larger.  Work bones, they get just a bit larger as they adapt (more slowly than muscles!) and increase their density.  Ergo, time to go shopping for larger dress shoes!!!
I also place some shoe goo on the outsole along the big toe/ball of foot of the FiveFingers (this is my high-wear area).  Perhaps this is contributing to my lack of fracturing (and potentially extending the life of the Bikilas as well???)? 

Thanks for sharing this. Your experience is interesting because my wife also
has a giant bunion, but only on the right side. In her case though, I think
it’s so extreme that the big toe does little in the way of support – I
filmed her barefoot and it kind of just flops up and down. Running in
Vibrams on the treadmill last winter helped her chronic right hip pain a
lot, and she is still running fairly well by mixing up the Vibrams, Merrell
Barefoot, and Newton Distance Racers. Luckily, no met issues in her case so
far. Personally, I think mixing up shoes to a certain extent is a good idea
as it varies the stress that gets applied. I like running in Vibrams, but
find that if I do it every day it can be hard on my feet. Doing shoe reviews
pretty much forces me to wear different things on a regular basis, but
Vibrams are always part of my rotation.

The key point about your story is that you have found a combo that seems to
be working, and unfortunately it takes a bit of experimentation, and
sometimes a few bumps in the road to get there.

Pete