The slap comes from transitioning from your initial heel contact directly to your forefoot, instead of smoothly pronating along the outside of your foot until your weight rests on the ball of your foot (at which point you start your takeoff). Of the shoes you named, the only one I have direct experience with is the Glycerin, but I would bet that the shoes you don’t slap in are shoes that give you lots of heel cushioning. A very cushioned heel can completely collapse as you land, which encourages you to roll through the cushioning along the outside of your foot (assuming you don’t overpronate) and gives you a smooth footstrike. There are lots of problems that come up from having too much heel cushioning, but slapping isn’t one of them. On the other hand, a firmer heel in a shoe that’s designed for a more midfoot strike will act as a brake if you’re a heel striker. Your heel will touch first and since the cushion doesn’t give as much, your heel will decellerate while the front of your foot maintains its velocity. The front of your foot can’t move any faster than the back of your foot, so the resulting rotation causes the the slapping that you feel.
Go to your local running store and try on some shoes that have a LOT of heel cushioning. Nike Pegasus or Air Max Motos come to mind. (That Air is good for some things!) Or if you need more stability, try the Brooks Trance or the 2007 Brooks Adrenaline (we’re getting them December 1). I’ll wager that those shoes with really plush heels are the ones that don’t make a slapping sound.
And once you find that pair of shoes that lets you avoid slapping, be very cautious about achilles tendonitis or plantar fascitis. Lots of cushioning lets your heel drop relative to your forefoot during the support phase, and you can get pretty messed up as a result.
Lee Silverman
JackRabbit Sports
New York City
Some interesting points here. Thanks.