Can you see the problem in this runner’s gait ?

You should be able see that they are heel impacting heavy on the outside of the rear foot, and that they are doing so far laterally, more than what is considered normal.  
This is a video of someone with a rear foot varus deformity.
These folks typically have a high arched foot, typically more rigid than flexible, and they are often paired with a forefoot valgus.  
Q: Do you think it might be important as a shoe fitter to know this foot type ?
A: Yes
Q.Should they be put in a shoe with a soft lateral crash zone at the heel ? 
A: No, absolutely not. Why would you want to keep this person deeper and more entrenched on the lateral heel/foot ?!
This foot type has a difficult time progressing off of the lateral foot. The lateral strike pattern and the tendency for the varus rear-foot (inverted)  keeps this person on the lateral aspect of the foot long into midstance.  This eats up time when they should be gradually progressing over to the medial forefoot so that they can get to an effective and efficient medial (big toe) toe off.  This gait type is typically apropulsive, they are not big speed demons and short bursts of acceleration are difficult for these folks much of the time. Combine this person with some torsional issues in  the tibia or femur and you have problems to deal with, including probably challenges for the glutes and patellar tracking dysfunction. What to see some hard, tight IT Bands ?These folks are often the poster child for it. Good luck foam rolling with these clients, they will hate you for recommending it !
They are typically poor pronators so they do not accommodate to uneven terrain well.  Because they are more on the outside of the foot, they may have a greater incidence or risk for inversion sprains. You may choose to add the exercise we presented on Monday (link  here) to help them as best as possible train some improved strength, awareness and motor patterns into their system. In some cases, but only when appropriate, a rear foot post can be used to help them progress more efficiently and safely. 
These foot types typically have dysfunction of the peronei (amongst other things). A weak peroneus longus can lead to a more dorsiflexed first metatarsal compromising the medial foot tripod stability and efficiency during propulsion while also risking compromise to the first metatarsaophalangeal (1st MTP) joint and thus hallux complications.  Additionally, a weak peroneus brevis can enable the rear foot to remain more varus. This muscle helps to invert the rearfoot and subtalar joints. This weakness can play out at terminal swing because the rear foot will not be brought into a more neutral posture prior to the moment of heel/foot strike (it will be left more varus) and then it can also impair mid-to-late midstance when it fires to help evert the lateral column of the foot helping to force the foot roll through to the big toe propulsive phase of terminal stance.  (* children who have these peroneal issues left unaddressed into skeletal maturity are more likely to have these rearfoot varus problems develop into anatomic fixed issues…… form follows function.)
You can see in the video the failed attempt to become propulsive. The client speeds over to the medial foot/big toe at the very last minute but it is largely too late. Sudden and all out pronation at the last minute is also fraught with biomechanical complications.
One must know their foot types. If you do not know what it is you are seeing, AND know how to confirm it on examination you will not get your client in the right shoe or give them the right homework.
* caveat: the mention of Monday’s exercise for this foot type for everyone with Rearfoot varus is not a treatment recommendation for everyone with the foot type. For some people this is the WRONG exercise or it might need modifications. Every case is different. The biomechanics all the way up need to be considered. Medicine is not a compartmentalized art or science. 
Shawn and Ivo, The Gait Guys