The boots are universally acclaimed as the most important piece of equipment, with the skis coming in second... but the proper ways to use and setup the boots are generally not trained nor well understood... especially the ways the boots interact with one's skiing: it all tends to be a very expensive dark art...
This is perhaps because the ski boots are the interface between our body and everything else and the effects of their setup are hard to distinguish from our movements and skill as well as individual style/technique... but make no mistake: poorly setup boots can significantly impact one's skiing. Further, the separation between coach (on the hill) and boot fitter (in the shop) as well as putting parents in the middle, adds to the issues...
Several aspects of boots that affect their usage are:
Each of these is important and must match the individual foot, leg and body of the skier as well as the technique. At the higher levels they are also often customized to suit snow conditions, for this or that race (canting, stiffness etc).
Not only that, but the skis can influence skier behavior - for instance the bindings mounting point can influence the fore/aft movements of the skier, skewing a possible boot fore/aft alignment or flex adjustments.
So, for instance, fore/aft movements are impacted by:
This is why it is extremely important to find a good boot fitter and a knowledgeable coach - as the boots are intimately tied to the performance on-snow. A good fitter can get the skier in the ballpark, but the final adjustments must be made by on-snow observation and testing, with an understanding of how the boot setup can influence technique.
The easiest to diagnose are usually canting and alignment problems - but is it that simple?
Take this improvement over a few weeks:
Some of the elements of that change were physical and psychological: recovery from injury (favoring one knee), strength at the beginning of season, as well as quite some technical work, but also a boot change: cuff aligned, size reduced one full size !
However, the following improvement, another few weeks later:
... required more high-level technique work, but also a very precise boot canting and cuff alignment (involving extensive on-hill evaluation and testing), boots increased in flex significantly, better footbeds.
The skis setup never changed throughout... and now, with the equipment problems out of the way, the fine-tuning of his skiing can begin - all those flaws visible in the last snapshot can be addressed one-by-one, without interference from the boots!
This is a topic to further explain proper boot usage Using the boots.
Flex is the easiest boot attribute to pick on, because it's a simple number, but some think that athletes are more commonly put in boots too soft, to compensate for other boots alignment problems or technique rather than boots too stiff.
However, of course we also do see athletes in boots too stiff (wishful thinking?) but judging the correct flex for each athlete is not easy - especially not while in the shop. Correct judgement must be done by the coach on the hill, based on the skier's skill and technique.
It is important to note that we don't need to bend the boots to get forward enough, if the boots are properly setup fore/aft (cuff angle, cuff height, heel height, delta angle, ramp angle). Note that boot flex is not part of fore/aft setup while it interacts with fore/aft setup, it is a separate adjustment/criteria!
The general process of buying boots should start by consulting with the coach, to determine overall attributes that you should look for (flex, cuff height, fit).
In the shop, several models are usually tested to find a good fit, but for performance skiing, a race fit is critical and many consider that a boot that can be skied straight from the shop is a bad fit - a proper race boot should take some fitting to be skied properly... wink-wink.
It is very hard and often impossible to retrofit a boot that's too large or too soft - but it's always possible to customize a boot that's a little tight or a little too stiff.
If you want to read more about boot setup, read:
Here's a quick overview from the CSCF, on fitting race boots for young athletes:
The most common parent mistake is buying boots too large and impairing technique development.
Related sessions and articles:
Bottom line - when you set out to create high-level and high-performance skiers, you need to start by quite literally laying out a proper foundation by choosing and setting up the boots!
Great boot alignment and setup details can be found in: