Discussing boot flex is as controversial as Trump and his "alternate facts". Every bootfitter has a slightly different take on what the right flex should be for a customer. I'm going to throw my 10 cents into the mix coming more from an anatomical and biomechanical perspective. And let me say from the get-go, there is no straight answer. I will also add that a lot of boot problems I see come from boots that are too soft in the ankle flexion for the client. Let's begin...
THE SKI BOOT'S JOB
The ski boot's job is to transfer your movements into the ski so you can control turns and your speed. To do this, you need to be standing in an athletic position - slightly flexed at the ankles, knees and hips. The boot needs to support you in this ski stance position. I'm sure you're already familiar with the forward lean of ski boots.
Boot flex in a ski boot is referring to how easily the boot can be bent forward at the ankle joint, and how much support the boot offers you while you ski.
Boot flexion selection is as personal as choosing how you have your coffee. The boot may fit the shape of your foot perfectly, but if the flex is too soft or too hard, then you won't be able to control your ski. A boot that's too stiff will push you back and you'll watch the front of your skis chatter around from your dunny-seat-driver position. Finding it hard to initiate your turns? This could be from a flex that is too stiff for you. But it could also be that it's too soft.
A boot that is too soft may still have you riding in the back seat. If your weight/strength is too much for the boot then you might end up not being able to trust it to hold you, putting you into the back seat. The boot needs to resist you to a point or it won't translate your movements to the ski.
WHAT IS A FLEX RATING?
Ski boot manufacturers use a flex index rating. This refers to how much pressure it takes to bend the cuff of the boot at the ankle pivot point (where it's supposed to bend). The higher the number, the stiffer the flex. These numbers usually range from 50 to 140. But, this is not a useful indicator of stiffness as there is no industry standard about how this is measured between boot brands. These numbers typically offer only a relative measure between boots of the same brand. Flex numbers can also be used in marketing for boot categories (all-mountain, beginner, race etc.). So ignore the numbers and go by feel.
Typically a softer flex boot is aimed at beginner skiers or freestyle skiers. The theory goes that a beginner is still learning to perfect their movements and technique, so less force to control the ski is considered a good thing. The trade off is that there is less rebound force and it is therefore less responsive. Freestyle skiers don't want a really stiff boot as they need to absorb the constant bumps and jumps in their runs without developing shin problems or stress fractures.
If you're a heavier-set beginner skier, then a soft flex beginner boot will not do you any favours. You will still need a stiff enough boot that will resist you and hold you in your ski position. You don't want a boot that just collapses under you every time you lean forward.
WOMEN AND MEN ARE DIFFERENT
Painfully stating the obvious? Yes indeed. True - women do tend to need a slightly softer flex but it's not because they are the fairer sex and not aggressive, it's because they tend to be smaller and lighter. But this is not a given, so it's better to say that someone with a small frame and less aggression in their skiing will likely need a softer flex than someone who is heavier set and likes to ski hard and fast. But (in boot flex there's always an exception to the rule) I have also seen a light framed, 16 year old girl bend a 130 flex boot like it was made from rubber. This was because the boot pivot lined up exactly where her ankle joint was, she had good ski technique, and she had a very flexible ankle.
ANKLE FLEXIBILITY (DORSIFLEXION)
This plays a much larger part in boot stiffness than it's given credit for, and this is where many people get wrong advice in the shop. If you have a really flexible ankle, you will probably need a stiffer boot. This is true even if you are a beginner. But on the other hand if you have a really inflexible ankle, then you will also need a stiffer boot. Confused yet? Stick with me as we keep moving through the quagmire that is boot flex...
An inflexible ankle has a smaller range of movement to utilise and place pressure onto the front of the ski to control turns. So you need to have a boot that offers enough resistance to translate your minimal amount of available motion to the front of your ski. A soft boot for you will leave you feeling out of control, cause heel lift, and possibly calf cramps and forefoot pain.
A good boot fitter will make a flex recommendation based on a number of factors such as the skier’s ability, weight, height, and personal preference.
SO HOW DO I MAKE SURE MY BOOT HAS THE RIGHT FLEX FOR ME?
Ignore the marketing on boots, try it on and see how it feels. A rough guide is that in the shop, where the plastics are warm and more flexible than out on the slopes, you should be able to comfortably forward bend the boot when all your weight is on one leg.
Listen to the advice of a good boot fitter or ski podiatrist. Experience and knowledge goes along way in specialist equipment. You can read a little more about our boot assessment here.
QUICK RUN DOWN
Flex rating refers to how hard it is to bend a boot forwards at the cuff
No standard of flex rating between boot brands
Ignore the marketing and go for how it feels
Ability, height, weight and personal preference is all factored into getting the right flex for you
Find a good ski podiatrist or boot fitter and listen to their advice