A Kinematic Comparison of Dive Fins

[jester1226]

I don't need studies. I definetly do not need the resident philosophers opinion on how things should be conducted and or done. I have something that far exceeds all of this. I have used many other fins, and have yet to find any close to giving the benefits my Force Fins give me. Being able to cover more ground when necessary with so much less effort than with those other anchors is all I need. I am able to relax when diving, no cramping, and my air consumption is down due to all of this. Beat that with any study you can come up with! Apparently the only time you have a positive post is when it is about yourself. GO back to your Tuna!

 

[Thalassamania]

Thank you for that positive remark, it sounds like you already know all there is to know, or at least all that you want to know ... so ... Welcome to my ignore list! Here you'll find several others, very much like you, you have a lot in common, they also know everything that there is to know, but their conclusions are ... I'm sorry to say, somewhat different than each others and from yours, but their manners are much the same as yours, so I'm sure you will all get along swimmingly. I do so hope that you'll all play nice and have big fun together.

 

[simmonsjr]

Thal,

Going back to the original issue, how would you go about designing a follow-up experiment to validate the results of the original from a biomechanics standpoint? Obviously there is the issue of increasing the sample size as you posited, but would you change any of the procedures utilized?

Jason

 

[Thalassamania]

One of my problems with the original study has to do with the claim of perfect form, that needs to be defined and then justified. I frankly find the study interesting, but not particularly persuasive of anything. It indicates to me that if I can replicate this chap's form I would have some likelihood of relieving cramps by switching fins ... cramps that I, frankly, am not suffering from (at least at present).

The problem with these sorts of studies (and which my quick outline suffered from) is the insidious way that new variables keep creeping in, and while an ANOVA is an excellent way to handle each on its own and to pull it out as a block effect that can be separately tested an evaluated you usually wind up with interactive block effect terms that are significant, and all you can do is shrug your shoulders and toss them into the error term, and so it grows, and grows, and grows, and swamps your data.

I guess the real question is what do you want to know. Cramps would be easy to study, swim people at ever increasing intensity (speed or distance) till them cramp and do that with multiple subjects and multiple fin types ... the proof is in the cramping, who cares about form or biomechanics (which are of interest in designing a better fin, but that's it). The more complex one is the one that I was addressing which is concerned with how far can a fin take you with repect to using up your gas supply.

 

[meesier42]

I too wish I still had the data, I believe that I did a fairly good job of normalizing the data, probably not as good as I could today, but not too bad. It was a limited data set, N=40, is a good start but not complete study, really I need 40 runs for each fin to really normalize the data.

Even though I strongly believe in Bob's Fins, it doesn't mean that I stop looking at new fins and trying to learn something new every time I get into it. For example, when the Slingshot came out, I gave them a serious test, and inside of 10 minutes I knew what was wrong with them and spent another 2-3 hours proving it, they're just a cheap rip off of the ForceFin patent with a crappy footpocket and those bands simply react too slowly to be effective.

For me, right now there is no better fin out there than the Force Fins. But even then, I own like 8 pairs of them. This allows me to pick and choose the best pair for each dive. For example, on a liesurely dive in a 3mm wetsuit I'll go with my Pro's or more recently I have been leaving the Pro's behind in favor of my new Flying Force. If I am pulling the drysuit out, then maybe the Excellerators or the SD-1 Military's. When it comes down to it, I haven't found anything that can provide the comfort and the shear thrust that they can.

To comment on cramping, I will tell you that I literally never cramped up in a Force Fin. and I have done endurance swims at max speed and my lungs blow out long before my legs cramp. I have even had a buddy (not wearing FF at the time) who was cramping really badly mid dive, since I didn't want to drag him home, or worse end up rescuing him in the ripping current, I just swapped fins with him mid dive, even though he was already enduring a series of knee binding cramps, once he had my FF Pros he immediately was able to begin swimming again. When we hit the beach he told me they actually stretched and relieved his cramp the longer he swam, here I am 8 years later and I still haven't gotten those fins back from him.

 

[simmonsjr]

All that was meant by "near perfect form" was that as a competitive swimmer the subject knows how to kick efficiently. He was included as the initial subject simply to reduce the number of variables in the experiment, but someone with horrible form could just as easily have been the subject. I think that you may be making a mountain our of a mole-hill with this one. There is no reason that the fin itself would behave any differently on someone who had abysmal form. The behavior of the fin during the kick cycle is the only factor being studied.

The issue, as far as I see it, is simply which fin design works better with the human body's biomechanics: a flat fin with an enclosed toe or a curved fin with an open toe box? The winner in this study was obviously the Force Fin.

The reduction of cramps as a symptom flows out of a design that is more aptly fitted to the design of the human body. Not everyone is prone to cramping; therefore, that is not an issue for everyone.

I find the basic design of this experiment to be valid: subject is recorded swimming different fin types, footage is graphed and analyzed via computer. I agree with you that there should be a followup study conducted which utilizes more divers of varying skill levels, more varieties of fins, and more runs conducted over a greater period of time, but I still have to conclude that the initial results are valid: the Force Fins do in fact work as advertised.

 

[Thalassamania]

All that was meant by "near perfect form" was that as a competitive swimmer the subject knows how to kick efficiently. He was included as the initial subject simply to reduce the number of variables in the experiment, but someone with horrible form could just as easily have been the subject. I think that you may be making a mountain our of a mole-hill with this one. There is no reason that the fin itself would behave any differently on someone who had abysmal form. The behavior of the fin during the kick cycle is the only factor being studied.

Could well be, I was aware of that. One thing that I thought might be interesting to look at, using the perfect form as an exemplar is the idea that one fin design "forces" a naive user into a better form than another does.

The issue, as far as I see it, is simply which fin design works better with the human body's biomechanics: a flat fin with an enclosed toe or a curved fin with an open toe box? The winner in this study was obviously the Force Fin.

This assumes that the best way to fin resembles the best way to swim kick. It may say nothing about the actual best way to obtain human powered underwater propulsion, consider the frog kick or the Aquellon (sp?).

The reduction of cramps as a symptom flows out of a design that is more aptly fitted to the design of the human body. Not everyone is prone to cramping; therefore, that is not an issue for everyone.

I find the basic design of this experiment to be valid: subject is recorded swimming different fin types, footage is graphed and analyzed via computer. I agree with you that there should be a followup study conducted which utilizes more divers of varying skill levels, more varieties of fins, and more runs conducted over a greater period of time, but I still have to conclude that the initial results are valid: the Force Fins do in fact work as advertised.

Force Fins are one of those things that people either love or hate. I do not care for them, but I have too many friends who love them, and who are levelheaded reasonable types, for to dismiss Force Fins as some sort of strange cult item, clearly they work ... for some people, perhaps the study gives us some indication of why, I was never a strong freestyler ... I swam butterfly and breast stroke. Perhaps it's my lousy flutter kick (I do tend to frog or dolphin) that keeps me from realizing Force Fins full potential.

 

[meesier42]

Thal- you brought up an interesting point that took me a couple of days to think about it to respond. What is perfect form and the follow on question to it being, is perfect form the same with and without fins?

I will start with my background, I was a competitive swimmer for 15 years, been hobby freediving and spearfishing since I was about 12, scuba diving for about 12 years now. Education, while I am not an expert in bio-mechanics, I am a systems engineer, have lots of experience in nuclear, chemical, and power plant engineering, and a couple of years now working in operations analysis/research and finishing up a masters that focuses in OR/OA and statistical quality control. So I think I have a reasonable background to draw intelligent conclusions about what I observe.

So here goes.

What is perfect form? Well perfect form (independent of fins or experience) is one that produces that maximum thrust for the maximum time. As part of this definition, it includes concepts like, max thrust per unit energy output, or max thrust per unit O2 consumed, or max thrust per unit lactic acid produced, or lots of other measures of effectiveness, but the main point is, the maximum sustainable thrust output.

So what does it look like- well lets look at high level swimmers, they use a tight, high cadence, low amplitude flutter kick. Key points to observe with this kick, the power of the kick is initiated from the abdominals (core muscle groups), the abs drive the hips, the hips then drive the quads (which amplify the power), the quads drive the calves which of course lead and drive the foot. The entire kick stays within the slipstream of the body and the knees only barely flex. If you carefully study the kick without fins you will find that significant thrust is actually coming from the legs and the foot is only the termination of the legs, not the power source on it own. In this way the foot is analagous to what is seen with fish, their tail does not provide the thrust, sure it adds to it, but really it act more to direct the thrust.

Given the relative high power that this kick develops and the duration that these swimmers can sustain this kick it would follow that this is the ideal biomechanical method form for the production of thrust.

So what happens with you place a fin on the foot. Well, the most obvious is that the fin acts to lengthen leg, making the effective lever arm longer which should of course increase the thrust that can be developed. Does this mean that the form should change? Well, initial thought is, yes form should change to match the new longer lever arm. But on further thought you should realize that its the same muscle groups moving the legs, so should it change... not unless you suddenly developed new muscles to drive the fins.

Now there are critical aspects to a properly designed fin that will permit the muscles to work as they did prior to putting a fin on.

First- the main pivot point of the fin. the pivot point of the fin should be under the instep. This makes the fin act as a natural extension of the ankle.

Second- the fin needs must work with the muscles of the leg, this means that fin should not be balanced in design. The power stroke should be stronger than the recovery stroke. This is due to the natural arrangement of the muscles in the leg, the quads and calves are contract to kick the leg down, the much smaller and weaker ACL and hamstrings pull the leg up.

Third- the footpocket must be comfortable, must support the heel and minimize contact with the toes to prevent hyper-extension of the ankle.

Most fins fail on all 3 points, the pivot is well beyond the toes, the toes are entrapped and the heel support is abysmal. Even on full foot fins, the heel support is so flexible that it might as not be there. And of course they are all either equal power on both sides of the kick or... like Mares they are actually stiffer on the recovery than on the power stroke.

With Force Fins- all 3 points are in the design. Stiff support under the heel, toes are open and the pivot is under the instep of your foot. Lastly- the fin expands to maximum projected surface area on the power stroke and nearly collapses on the recovery.

The problem that most people have with the fins. They have become accustomed to how "other" fins work and they have developed a kick that works well for that style of fin, is that kick the biologically efficient, no, does it work well with that type of fin, yes. Do these "other" fins meet the "Good Enough" standard for most scuba diving applications, sure. Afterall, nearly all scuba diving has a ridiculously low power and thrust requirement.

When people are new to Force Fins, they come in with preconceived notions that they have learned from other fins and when the fins fails to meet these ideas the believe the fin doesn't work. But as they continue to use the fin, they stop thicking about what they think a fin should work and just start making the fin on their foot work. Once they unlearn the habits and expectations they come to realize how good the Force Fin line is. What I have found is that swimmers take to the fin immediately, most other people need 4-5 dives without interruption of another fin to relearn how to kick. After those dives, if people do switch back to another fin they often instantly realize what was wrong with the fins from the very beginning and toss the old fins in the trash. One of the weirdest things that people have to get over is the feeling of the blade flexing away from their toes, they think this is loss of power, even though its actually one of the get power producing features of the fin.

 

[Thalassamania]

What is perfect form? Well perfect form (independent of fins or experience) is one that produces that maximum thrust for the maximum time. As part of this definition, it includes concepts like, max thrust per unit energy output, or max thrust per unit O2 consumed, or max thrust per unit lactic acid produced, or lots of other measures of effectiveness, but the main point is, the maximum sustainable thrust output.

Or it might be peak speed or peak towing power ... it could be a lot of things so it must be clearly defined. I tend to think in terms of minimum oxygen consumption for a defined trial ...but that's just my bias showing.

So what does it look like- well lets look at high level swimmers, they use a tight, high cadence, low amplitude flutter kick. Key points to observe with this kick, the power of the kick is initiated from the abdominals (core muscle groups), the abs drive the hips, the hips then drive the quads (which amplify the power), the quads drive the calves which of course lead and drive the foot. The entire kick stays within the slipstream of the body and the knees only barely flex. If you carefully study the kick without fins you will find that significant thrust is actually coming from the legs and the foot is only the termination of the legs, not the power source on it own. In this way the foot is analagous to what is seen with fish, their tail does not provide the thrust, sure it adds to it, but really it act more to direct the thrust.

So far, so good.

Given the relative high power that this kick develops and the duration that these swimmers can sustain this kick it would follow that this is the ideal biomechanical method form for the production of thrust.

For a swimmer using a flutter kick.

So what happens with you place a fin on the foot. Well, the most obvious is that the fin acts to lengthen leg, making the effective lever arm longer which should of course increase the thrust that can be developed. Does this mean that the form should change? Well, initial thought is, yes form should change to match the new longer lever arm. But on further thought you should realize that its the same muscle groups moving the legs, so should it change... not unless you suddenly developed new muscles to drive the fins.

It is a different airfoil, the power source is not what's important, how to best drive the new airfoil with a different Reynolds Number and all is more to the point. When you consider that man did not evolve to swim or to use fins, the muscles are organized and optimized for land based tasks, locomotion is the water with or without fins is an evolutionary juryrig at best.

Now there are critical aspects to a properly designed fin that will permit the muscles to work as they did prior to putting a fin on.

Now, to my perception, you're off the beam, your making an untested, and IMHO an unwarranted assumption that the way in which a swimmer used the muscles bears any relation to the use of fins. It may well be a wholly different question.

First- the main pivot point of the fin. the pivot point of the fin should be under the instep. This makes the fin act as a natural extension of the ankle.

Perhaps, but whose to say that a fin that extended out from the knee to the ankle would be superior? And your making an assumption that negates the dolphin kick in which the fin's primary function is to extend the the length of the airfoil that is the entire body from outstretched finger tips to the tip of the fin itself thus raising the Reynolds Number as well as providing propulsion. I just don't think that it is all as simple as your model would have it.

Second- the fin needs must work with the muscles of the leg, this means that fin should not be balanced in design. The power stroke should be stronger than the recovery stroke. This is due to the natural arrangement of the muscles in the leg, the quads and calves are contract to kick the leg down, the much smaller and weaker ACL and hamstrings pull the leg up.

That's an artifact of our evolution as terrestrial locomotors, not an imperative for optimum aquatic performance except as measured by freestyle swimming speeds. Move to other strokes like the trudgen, trudgen crawl, double trudgen, double trudgen crawl, and dolphin crawl and I suspect that all the answers (including the question of how to messure efficiency) shifts with each new choice.

Third- the footpocket must be comfortable, must support the heel and minimize contact with the toes to prevent hyper-extension of the ankle.

Actually it would likely be better ignore the ankle joint entirely an transfer all the force directly to the lower leg as Osterhaut tried (and failed) to do with the FaraFin II.
Most fins fail on all 3 points, the pivot is well beyond the toes, the toes are entrapped and the heel support is abysmal. Even on full foot fins, the heel support is so flexible that it might as not be there. And of course they are all either equal power on both sides of the kick or... like Mares they are actually stiffer on the recovery than on the power stroke.

With Force Fins- all 3 points are in the design. Stiff support under the heel, toes are open and the pivot is under the instep of your foot. Lastly- the fin expands to maximum projected surface area on the power stroke and nearly collapses on the recovery.

The problem that most people have with the fins. They have become accustomed to how "other" fins work and they have developed a kick that works well for that style of fin, is that kick the biologically efficient, no, does it work well with that type of fin, yes. Do these "other" fins meet the "Good Enough" standard for most scuba diving applications, sure. Afterall, nearly all scuba diving has a ridiculously low power and thrust requirement.

There's my biggest problem with your argument: "is that kick the biologically efficient, no ..." No where has that claim been substantiated. All that has been, arguably, demonstrated is that a Force Fin in more like the kick of freestyle swimmer than one other fin is. If you shift the paradigm (as I do most of the time when I dive) into one in which the ideal is a dolphin kick rather than a flutter kick, I suspect that while some constants such as not loading the toe remain, most all of the rest of what you have so carefull constructed gets knocked into the proverbial top hat,

When people are new to Force Fins, they come in with preconceived notions that they have learned from other fins and when the fins fails to meet these ideas the believe the fin doesn't work. But as they continue to use the fin, they stop thicking about what they think a fin should work and just start making the fin on their foot work. Once they unlearn the habits and expectations they come to realize how good the Force Fin line is. What I have found is that swimmers take to the fin immediately, most other people need 4-5 dives without interruption of another fin to relearn how to kick. After those dives, if people do switch back to another fin they often instantly realize what was wrong with the fins from the very beginning and toss the old fins in the trash. One of the weirdest things that people have to get over is the feeling of the blade flexing away from their toes, they think this is loss of power, even though its actually one of the get power producing features of the fin.

I don't disagree with you about having to learn how to use a particular fin, but I'd also say that is true of virtually every fin that has ever been. While my comrades with their Force Fins are moving along with their legs thrashing at a rather high beat, I'm lazily sending waves of propulsion down my body in a very relaxed an relaxing dolphin kink. Which is more efficient? Define efficient and then we can test it.

 

[meesier42]

It is a different airfoil, the power source is not what's important, how to best drive the new airfoil with a different Reynolds Number and all is more to the point.

Perhaps, but whose to say that a fin that extended out from the knee to the ankle would be superior? And your making an assumption that negates the dolphin kick in which the fin's primary function is to extend the the length of the airfoil that is the entire body from outstretched finger tips to the tip of the fin itself thus raising the Reynolds Number as well as providing propulsion. I just don't think that it is all as simple as your model would have it.

the point is that no matter what fins you wear or no fins at all, the muscles that you have to drive the fins remain unchanged, exception being if you switch to a frog kick, but I don't anyone is going to argue that a frog kick can create the sustained thrust that a flutter/scissor kick can. The "Trudgen" style uses scissor kick, which is nothing but a wider flutter kick, still the same muscles.

Now, to my perception, you're off the beam, your making an untested, and IMHO an unwarranted assumption that the way in which a swimmer used the muscles bears any relation to the use of fins. It may well be a wholly different question.

That's an artifact of our evolution as terrestrial locomotors, not an imperative for optimum aquatic performance except as measured by freestyle swimming speeds. Move to other strokes like the trudgen, trudgen crawl, double trudgen, double trudgen crawl, and dolphin crawl and I suspect that all the answers (including the question of how to messure efficiency) shifts with each new choice.

Yes, there are some assumptions here, I don't believe they are unwarranted, this is afterall an internet forum not a review panel with subject matter experts, so I am taking certain liberties in argument development, although I am trying to form a whole discussion. The argument that I am making is that no matter what fins you wear, your legs have muscle groups that only do certain exercises. It is the muscles that define how to best move the legs and the only way to justify that the leg movements should change by placing fins on, is saying that the flutter style kick is faulted and needs to change. And yes, a dolphin kick uses the same muscle groups in the same fashion as well, although it does utilize a larger planning area, it doesn't change the muscles. in scuba applications, the planning area is limited by the rig to only allow the legs to drive.