I have often thought that before you can measure and quantify flex you need to be able to measure the variable of how a surfer swings their weight.
Therefore you need to measure how much pressure a surfer applies to sensors on the deck under each foot.
You hear descriptions that one surfer has a backfooted style, meaning that when they swing their weight when doing a bottom turn, they put 70% of their weight on their back foot.
At the opposite end you have front footed surfers who put only about 40% of their weight on their back foot when doing a bottom turn (notice their stiff front leg style).
Surely a board is going to flex differently if their is a 30% difference in back foot pressure.
Then there is the question of how a surfer transfers/swings their weight as they flow through the turn.
If sensors on the deck were capable of recording how weight is transfered in the 1 - 2 seconds it takes to do a bottom turn, this would be usefull information.
If I were to try and describe how I swing my weight as I flow through a turn, I would use as an example a wave I caught at the Superbank last thursday.
The wave was 5 ft and peeled from the middle of Rainbow Bay into Greenmount for 250 metres before closing out, the waves were pumping courtesy of Cyclone Sandra.
If I were to describe my style it would be carving roller coaster rhythm, meaning I spend 50% of my time on a wave face surfing above the fall line and 50% below the fall line, riding a wide tailed board that sits high in the water and does not decellerate through a top turn as much as all other boards and therefore has a faster all round planing speed.
I was riding a 7’ 4’’ by 22’’ by 3 1/4’’ wooden nugget (62 litres volume)(my weight 100kgs/220lbs), I paddled into the wave and dropped straight down the wave face, neither rail dragging in the wave face gives minimum drag and maximum accelleration, then as I start to do a bottom turn as late as possible, I apply about 80% of my weight onto my back foot, then as I rise up the face and bank off the very top of the wave which is at about 10 degrees before vertical, my weight is balanced on each foot at about 50/50, so between the start of my bottom turn and the top of the wave the weight on my back foot has changed from 80% to 50% as I flow through a bottom turn.
I tend to think that surfers who surf more vertical than me don’t put as much weight on their back foot (more like 70%) but transfer alot more of their weight onto their front foot as they flow through a bottom turn burying more of the rail.
Also I think that verticle surfers on low volume reactive boards (which therefore have a much slower planing speed than a high volume nugget), that you need to be a fast reacting surfer to surf, tend to surf more above the fall line (60%) than below the fall line(40%).