fast or slow makes you tick

I did not understand your question from the subject line.

I think it also depends a lot on the type of muscles you have.

You're right, in general making big amplitude kicks indeed isn't efficient.
I also know that the pressure perceived on the fin isn't equal to the real propulsion. In other words kicking with lot's of force isn't efficient. There is a 'sweet-spot' for each fin.

Having stiffer fins is nice at depth, where you need to start up lifting all that negative buoyancy. Having soft fins there can be surprisingly daunting.

I hope these remarks are helpful.

Love, Courage and Water,

Kars

Answer???
This is a very good question. I know that a soft fin and very straight legs can be efficient at low power levels and when you kick wider you increase thrust, power and drag. This (faster, smaller kick) works well for dynamic or long days of spearfishing but you pay for it leaving the surface and the bottom when you need more thrust. The 'sweet-spot' that Kars refers to gets wider as the fins get longer.
Thirty-five years ago I met a brilliant engineer who invented a device that was the fore runner of mono fins and he figured a way to measure fin efficiency. Air consumption from a scuba tank is a rough measurement of output power. Running a measured course at a given speed should allow you to compare styles and/or equipment.
Constant balance however is a game of so many variables that it defies description, let alone analysis.

that will only lead to more questions!

On the other hand, if you use smaller and faster kicks, you change the direction of movement of your legs more often than with slower larger kicks. Change in direction of movement costs quite a lot of energy and is not efficient. Also, if you go faster, you'll have to spend more energy to overcome drag. The energy you need because of the drag increases proportional with the square of the velocity. (I hope I said this correctly. My technical English isn't very good). This is applicable to both you as a person, as to the speed that your legs are moving at. In other words, moving faster costs a lot more energy than moving slowly.

There probably is an optimum, but it will be different for everybody, depending on style, equipment, your build, types of muscle.......

As for stiff fins. The would absolutely not work for me. I get cramps in my calf only by thinking about it.

I went down to 40m in the Kreiddesee in Hemmoor Germany. I was wearing a Waterway Model 1 Long distance fin (big soft fin). When I turned and made my first stroke, I noticed the fin was almost like a towel. It was scary, but refocussing on applying a more gradual push helped to overcome my relative negative weight at that depth.
I was actually not that heavy, like those bigger guys can be, but still it's scary to apply force and feel a collapsing, folding fin.

From my experience (65m CW) Being negative at depth requires more like a medium to a sprint fin (if you have the muscle to for the sprint fin) because there is a lot of weight to push up.

Now I 'm a happy owner of a Hyperfin style fin, bending it actually feels pretty soft, though underwater it's a medium fin. For my CW dives I wear a 3mm High Density Heiwa suit, and just 1,5 KG of weight. I take down about 6L of air, so at 60m I've lost about 5KG of buoyancy. At that depth I'm actually not that negative, I guess only about 3-4 KG. But after a gentle turn I do consider drag and technique important on the way up.
Improving this gave me a big speed boost and I went from a slow 0,6 -0,9 M/s to a 0,9 - 1,2 M/s.

With a stiff fin you can accelerate fast, even at great depths.
With a medium fin you can accelerate, but not very fast at greater depths (50+ needs a good swimming technique)
With a soft fin you hardly can accelerate because the fin collapses under the load a greater depths.

Almost like a nice big engine, having no traction is not efficient, though good throttle and driving technique help, they can help you only a limited amount. Going a little to deep with a soft fin is scary and dangerous too!

Kars