Kinematic Comparison of Swimming with Two Different Designs of Dive Fins

The purpose of this study was to determine the difference, if any, in the mechanics of legs only swimming while utilizing the two different designs of dive fins.


In scuba diving there are two kinds of dives, the dives you remember and the dives you remember enjoying. The dives you remember are the dives in which you fell behind your dive group all day, your legs were tired and cramped, and a half day dive trip felt like it lasted two weeks. Where as the dives you remember enjoying are the dives in which you remember seeing the vibrant colors of a Princess Parrot fish, the clarity and almost bath like temperature of the water, and the fact that you were down all day, yet it seemed to last only minutes. There is one essential piece of dive equipment that will mean the difference between the dive you remember and the dive you remember enjoying, that’s the dive fin!

There are many different dive fins out there, but most of them are just variations on the same theme. There is however a dive fin out there that has broken away from that theme and set itself apart in the diving industry, the Force Fin . Most of the fins on the market today are considered to be conventional in design, in that they utilize a flat blade shaped surface to provide propulsion, as well as an enclosed toe foot hold with a heel strap as a means of securing the foot. Due to enclosing the toes in the foot hold, the foot is forced into extreme plantar flexion on the downward power stroke phase of the swim kick. This forced, extreme plantar flexion places an intense ballistic strain on the tibialis anterior, extensor digitorum longus, and extensor hallucis longus muscles. This also isolates the muscles of the lower leg into providing the primary power necessary for propulsion.

The Force Fin does not utilize a conventional fin design. But rather incorporates a blade surface that is shaped like the tail on a whale, as well as a open toe foot hold that rests across the instep of the foot and is secured with a heel strap. By leaving the toes exposed and securing the foot hold across the instep, it does not force the foot into extreme plantar flexion. This also isolates the rectus femoris, vastus lateralis, vastis intermedius, and the vastis medialis muscles into providing the primary power for propulsion in the downward power stroke phase. The fin is designed so that while in the prone swimming position, the tips of the fin are pointed down. The fin is also designed to flex upward on the downward power stroke of the swim phase and then snap back to its original position during the upward recoil phase of the kick.

This design provides propulsion in both phases of the kick, instead of just in the downward power stroke phase. Due to gaining propulsion in both phases of the kick, the diver needs only to imply a simple flutter kick to sufficiently propel him/her self through the water. The purpose of this study was to determine the difference, if any, in the mechanics of legs only swimming while utilizing the two different designs of dive fins. Both intermediate and pro models of the conventional and Force Fins were utilized in this study.

Design and Sample

Using a single subject, two category, two variable design, the study was conducted in a closed environment pool at San Diego State University. The subject swam, with legs only, from left to right in front of a viewing window in the side of the pool, where he was video taped by a camera set up in the viewing room. He was given a rest between each trial and a rest period in which he changed into another pair of fins. The subject first swam in the intermediate Force Fins , past the viewing window for a total of four trials. He then changed into the intermediate conventional fins and completed the same number of trials. The subject then swam in the pro model Force Fins for a total four trials. Lastly the subject swam in the pro model conventional fins for a total of three trials.


A biomechanical analysis software program called Peak5 was utilized to digitize the captured video footage. Once the video footage had been digitized, again using the Peak5 system, it was analyzed for range of motion, acceleration, and velocity of the, right and left, joints of the hip, knee, and ankle. The lower body was also broken down into separate points. Those points were the right hip, right knee, right ankle, right heel, right toe, left hip, left knee, left ankle, left heel, and left toe. Numerical and graphical data were both obtained.


A direct comparative analysis was run on the data collected. The data was separated into two groups and then analyzed. The groups were intermediate fins and pro model fins.


The results of the analysis show that there is a definite change in mechanics when comparing the conventional fin design to that of the Force Fin . By isolating both the minimal and maximal points in the range of motion (ROM) of the ankle, I was able to show that at the intermediate level there was an overall ROM of 59.32 in the left ankle and 70.71 in the right ankle while using the conventional fins. This corresponds to 32.96% of the total ROM of the eft ankle joint and 39.28% of the total ROM of the right ankle joint. However, when using the Force Fins there was an overall ROM of only 34.80 in the left ankle and 39.17 in the right ankle. Resulting in 19.33% of the overall ROM of the left ankle joint and 21.76% of the overall ROM of the right ankle joint.

At the pro model level there was an overall ROM of 47.92 in the left ankle and 55.82 in the right ankle using the conventional fins. Equating to 26.62% of the ROM of the left ankle joint and 24.23% of the ROM of the right ankle joint. Although, when using the Force Fins there was an overall ROM of only 39.18 in the left ankle and 43.61 in the right ankle. This corresponds to 21.77% of the overall ROM of the left ankle joint and 24.33% of the overall ROM of the right ankle joint.

When comparing the maximal end range of motion of the conventional and Force Fins at the intermediate level, there is a difference of 11.62% in the left ankle and 13.57% in the right ankle. The end ranges of motion for the conventional fins were 7.05% beyond total plantar flexion in the left ankle and 10.76% beyond total plantar flexion in the right ankle. At no point in the study did the Force Fin exceed the 180 mark of total plantar flexion.

At the pro model level, the difference in the end range of motion was 8.05% in the left ankle and 8.72% in the right ankle. The end ranges of motion for the conventional fins were 5.69% beyond total plantar flexion in the left ankle and 6.65% beyond total plantar flexion in the right ankle. Again, at no point in the study did the Force Fin exceed the 180 mark of total plantar flexion.


From conducting this study I have concluded that when I compared the mechanics of swimming with legs only in conventional fins to the mechanics of swimming with legs only in Force Fins , there is a definite difference. This difference results in a definite change in mechanics. I have also concluded that the conventional fin, at both the intermediate and pro level, force the joint of the ankle beyond the natural end range of motion, resulting in a ballistic strain of the tibialis anterior, extensor digitorum longus, and the extensor hallucis longus.


This study has produced far more insight into the mechanics of legs only swimming than has been reported in this paper. It has laid a path for the examination of velocity and acceleration of the joints involved with legs only swimming. It has also laid a path for the examination of the range of motion of the kicking phase and how large or how narrow it should be for maximal efficiency. These are a small sample of the possible avenues of study that can be undertaken using the data that has been generated from conducting this study.

Ryan Lindsey, M.A.
Department of Exercise and Nutritional Science
San Diego State University

US Navy Study Efficiency of Best Fins

Navy SEAL fins, military fins, force fins

SEAL Team members are pictured wearing Pro Model Force Fins. Their wing tips are secured in an upward position for entry.

Summary of US Navy Sponsored Research on Fins

Following are actual results of US Navy sponsored studies on efficiency of divers wearing Force Fins as compared to two popular name brand fins which had been shown in speed tests to be the most efficient of 15 fins. The tests were conducted in a doughnut shaped flume (current) tank to control for speed and simulate open water conditions. Efficiency was determined by the actual oxygen consumption of fin kickers. To date, we know of no other fin test conducted by any facility that has the resources and objective control exhibited in this 4-year US Navy sponsored study..

Note that 75% of the divers tested did not reach the VO2 max, or fatigue level, only when wearing Original Force Fins. Only those subjects that were S.E.A.L Team members and, therefore, in outstanding physical condition, were able to sustain a kick speed of 50 meters per minute for 6 to 8 minutes with any fin tested.

Throughout the test, divers could achieve a maximum sustainable speed of 40 meters per minute with any fin. However, they burned less air when wearing Force Fins. Interestingly, all 200 participants, when surveyed for their subjective opinion, believed that the longest, stiffest fin, that which they felt was most powerful, was also the most efficient, but their actual oxygen consumption levels showed the opposite to be true.

fin efficiency, navy research on fins

The upper plate shows the energy cost to swim with the four fins at speeds from 30.8 m/ min-1 to 40.6 m/min -1.

Reprinted with the permission of the US Department of Naval Research, The US Navy endorses no products.

Text of letter from:
WASHINGTON DC 20372-5300

To:   Ms. Susanne E. Chess, Vice President, Force Fin

Dear Ms. Chess:

This responds to your Freedom of Information Act request of March 25, 1993 addressed to the Chief of Naval Research in which you request copies of reports of fin studies conducted by the University of Buffalo. Your request was received in this office May 6, 1993 for a determination on whether the documents you seek are releasable.

Your request (censored) if disclosed, is likely to cause substantial harm to Force Fins competitors and were determined exempt from disclosure under title 5, United States Code, Section 552(b)(4).

The documents:

Part l

The purpose of this series of experiments was to determine the metabolic cost of underwater swimming with “force” fins as compared to fins that previously had been shown to be the most economical out of 15 fins tested. Two versions of Force fins, amateur and professional, were compared to (censored) and (censored) fins. The latter two fins have previously been shown to require the least energy to swim at several speeds. Our previous work has shown that the swimming speed influences the selection of the “optimum” fin.

The data from these experiments are presented in Figure 1. The upper plate shows the energy cost to swim with the four fins at speeds from 30.8 m/ min-1 to 40.6 m/min -1. Although eight subjects swam at 30.8 m/min-1 to 40.6 m/min-1, only two subjects could sustain 50.4 m/min-1 for the six to eight minutes required for the test. The two subjects who could sustain the effort at 50.4 m/min-1 had VO2 maxs of about 2.5 l/min-1 , while the other subjects were about 2.0l./min. -1 .

There were no statistically significant differences between the energy cost of the four fins at any of the investigated speeds. The energy cost was about 74 l./km- 1 , 74 l/ km-1 and 79 l/ km -1 for speeds up to 40.6 m/min-1 . The values for the better swimmers were about 65.5 l/km -1. Over the entire range of speeds, kick frequencies increased from about 35 k/min -1 to 45 k/min -1 However, the subjects kicked had a 10 k/min -1 higher kick frequency with the Force fins (amateur and professional) than with the standard fins.

A further conclusion is that the amateur [Original] Force Fin, which is more flexible, required less energy for all speeds than the professional fin, which is more rigid. The observation that a more flexible fin may require less energy than a more rigid fin confirms what we observed with other fin manufacturers.

Progress Report for contract number (censored): Diver’s swimming efficiency as a function of buoyancy, swimming attitude, protective garments, breathing apparatus, swimming technique and fin type September 1991 to January 1992

During this period, we have continued our study of the effectiveness of selected fin types on the swimming performance of divers. During the previous period, we reported on the energy cost of swimming with these various fins.

In review of the fin combinations tested, the small, flexible non-vented fins had the lowest energy cost (200 kcal/km-1 ).

The larger, stiffer fins required about 25% more energy, while vents did not effect the energy requirement (Fig 1). Although this difference is small, when extended to 10 km the difference is 250 kcal or 50 l 02, requiring an extra ventilation of about 1250 l.

It should be emphasized that the factor that effects the swimming cost the most was the swimmers skill. The highest values (300 kcal/km-1 ) were observed in novice and the lowest values (150 kcal/km -1) in elite swimmers (Fig 2).

Our recent data analysis has suggested that the cost of swimming cannot be determined from the kick frequency. This is somewhat paradoxical. If the frequency is low, the force per kick can be high; while, if the frequency, is high, the force per kick can be low. Therefore, one can observe energy requirements for any combination of kick frequency and kick force.

As far as fin selection is concerned, the differences between the fins were not remarkable underwater. It would appear that for short swims there are no differences; however, for longer swims the small differences become significant.

The most economical fins were the smaller, very to moderately flexible styles. The larger, less flexible fins were the least economical.

The presence of vents did not seem to be an advantage under any condition. It would appear that the smaller, more flexible fins did not compromise neither speed nor force underwater. At the surface, it would appear that the greatest force and least fatigue could be generated by the larger, less flexible fin.

*The United States Navy does not endorse any product.

The Innovative Designs of Bob Evans, Erik Buell and Burt Rutan


The Art of Design by Harry J. Wirth

Bob Evans was and still is an accomplished scuba diver. He initially became interested in the beauty of sea-life and thus became an underwater photographer. He is quite an accomplished man with the camera as his photographs have been published widely in books and magazines.

Having worked underwater extensively and after logging thousands of dives and expeditions, he became intimately involved with the many difficulties of swimming and maneuvering while attempting to photograph his submariner subjects. Evans literally grew tired of the constant pushing and pulling of water with the conventional fins as he watched the effortless gliding of the fish around him. He had observed and continues to this day his observation of water as a three-dimensional living substance. He realized the fin cannot be designed with the typical two-dimensional mentality.

fish tail fins, efficient finsAnother observation was the faster, more efficient fish had a split tail as a main thrusting feature. All of the fins on the market at the time were flat single fins. As Evans puts it, the flat type is very clumsy and inefficient for swimming and diving. The reinforcement ribs restrict water flow around the fin and increase unwanted drag. An astute and observant individual, Evans made intuitive connections between man and nature. He found that the highly evolved fish shapes could be adapted to the human form. Humans are “by nature” one of the most inefficient and awkward objects moving in the water. In essence, he wanted to make the human body more fish like, but in a simple manner with the application of an appropriately designed fin.

The original “Force Fin“, the first model, was physically conceived in 1980, however, the initial concept was realized in the early 1970’s. Evans painfully and diligently evolved the design through many developments of configuration, size and materials. His design methodology was the handling of the design as a dynamic object rather than that of a static one. There was an extensive undertaking of the study of the movements of the foot, ankle, and leg in order to get a good understanding of the dynamic movements of the parts together and separately. An interesting drawing Evans compiled from these leg-movement studies hangs in a prominent place in his studio as a work of art. The innovation was a result of observations of nature in the sea and the application of those observations.

Bob Evans, Fin Designer, Funk Zone, Santa Barbara, Force Fin, PrototypesQuickly made of crude materials, many designs were tested, rejected, and refined for quick evaluation. His many configurations were quickly prototyped with chicken-wire and newspaper. His feet would get cuts and bruises as he tirelessly tried concept after concept. Until he arrived at a shape that worked. Evans would make the molds himself in his shop, pull the product and run to the beach to try a new shape. To this day he still follows the same basic “hands-on” method of design development. Many new materials were tried, tested, and evaluated to seek out the best formula suited for his needs with the fin. He needed a stable compound with the necessary snap and flex that would augment and amplify the movements of the leg and foot.

The latest material being applied to the fins is a two-part liquid polyurethane with a good memory of shape. Evans found this material to be quite forgiving in his unique composite molds. Once the fin was formed and pulled from the mold, he discovered the polyurethane had a chain molecular bond arrangement that allowed t to be more durable, yet flexible on the flipper ends or as he calls them the deflecting foils. He found this product while perusing technical journals and saw that it wa good for mallet hammers and heavy duty wheels. It was resilient, yet very durable and flexible.

It is the split ends on the fin that are so special. Observing fish with split fins, Evans saw the tremendous maneuverability of these fish. Trying to do a roll underwater with conventional long stiff fins is quite difficult to accomplish. With the shorter Force Fin, and the flexible tips, which work independently of one-another, maneuverability underwater is smooth and easy. Divers who take their first swim with the Evans’ fins first remark that the fin isn’t working because they feel no resistance. Evans is quick to point out that this is the quality he was striving for, an efficiency that makes underwater kicking effortless.

Another unique feature of the Force Fin is its ability to flex and snap. This action-reaction of the polyurethane structure increases the divers thrust. As Evans states, “when in operation, it has power in one direction and then collapses while throwing water behind in the other direction so that it can get back to where you kick against it without strain.”

He saw that this action was the same action as what he had seen occur on the fins of a harbor seal on a slow motion video. The Force Fin is an elegant, organic, fluid-formed device that becomes a natural extension of the human leg. Looking rather strange in our dry conditions walking about on the pier, once in the water the beauty of the design is immediately apparent and functionally is better and more efficient than any other fin on the market. Other models followed as the design became more widely accepted. He now has the “Beaver”, the “Rip Force“, the “Tan Delta” and the most advanced the “Extra Force” with adjustable “winglets“.

The “Extra Force Fin” is an adjustable fin to suit the diving and the kicking preferences of the diver. Two, independent, small winglets are attached to either side of the fin. These are mounted with hex-head bolts to allow for adjustment in the field. A diver can increase thrust of the stroke by moving these winglets in or out dependent on the performance desired.

Navy SEAL fins, military fins, force finsRecently, the US Department of the Navy conducted exhaustive test of many diving fins. Bob Evans’ Force Fins came out on top in the tests. The tests concluded that divers using the Force Fin used less oxygen while active in diving than with any other design. Interestingly enough, the elite Navy Seals teams use the Force Fin as further proof of their superiority as a dive fin. “Eventually”, as Evans states, “all fins will be made this way.”

The Force Fin is continuing to catch on. The sales have increased to such a high level, Evans has to consider now the future of his small company and where he wants to go with it. His desire is to design newer fins and try new technologies, but at present the pressure of meeting the tremendous consumer demand for his superior product is taking most of his time.

— Harry J. Wirth for “The Art of Design: The Innovative Designs of Bob Evans, Erik Buell and Burt Rutan, Design ist okay Innovationstransfer, Herausgegeben von Heiko Bartels, Bauhaus-Universitat Weimarn, Universtatsverlag, 2000

Reprinted with the permission of
Harry J. Wirth © 2000

Force Fin Challenge

Force Fin, How to Kick Fins, How to Kick Force Fins, Fin rebound

Force Fins Work for You

The Force Fin Challenge is a test you can do for yourself and on your own. It will tell you how well your fins are working for you, or if you are working for them.

Force Fins work for you and harness the force of the water to maximize thrust with each kick. That means more speed with less energy expended by you. To prove our point, we offer this test and challenge:

Take the heel strForce Fin, kick finsaps off any other fins and kick; Do this test in a pool only as your terrestris, flat fins will fall off. That’s the drag of the fins working against you. Drag is the resistance you feel when kicking against these other fins.

No matter how secure it may feel. No matter how it may make you think you are moving. That feeling of resistance is working against you when in the water.

Try the same test with Force Fins; they will not kick off. With each kick, Force Fins drive you forward with the in-water freedom of an aquatic being.

If you want to drag your fins through the water, then your choice of terrestris fins, all other fins, is vast.

If you want the freedom of an aquatic being, with fins efficiently propelling you with each kick, then there is but one choice – Force Fin.

The Truth About Dive Fins

manatee, force fin

Are you confused by all the theories, claims and opinions about dive fins? If not, maybe you should be. Since kicking is your primary physical activity when diving, and because the efficiency of your fins and your comfort in the water affects your bottom time more than almost anything else, dive fins are one of your most important pieces of equipment. But over the years, a lot of myths and misinformation have built up about them.

We’ve been designing and making improvements to fins for over 35 years, with millions of satisfied customers, and many major product design awards under our belts, we can honestly say that we are the experts in fin design.

That’s all we do, and we think our innovative fins are without equal in terms of comfort, efficiency, durability, performance, versatility and sheer value for your money. We pioneered fin colors, extended heel sections, leading edges, water channeling and accelerating through a split-V shape, fin, adjustability, flexibility, and the use of snappy and high performance materials, such as polyurethane. Just about every change and development in the way divers think about moving through water today came from Bob Evans and his Force Fins.








best fins for underwater photographyIf you’re used to traveling, packing, and lugging around two huge fins, get ready for a surprise when you take along Force Fins. They weigh less than 2 pounds each, are easier to handle, take up less room, fit in carry on luggage, are easy to get on and off, and you can even walk forward in them!

How well you move through the water is based on many factors including your kicking style and individual characteristics like leg strength and ankle flexibility in addition to your choice of fins. In the underwater world, you need all the help you can get, and we know that you want fins that are comfortable, efficient, durable and reasonably priced. That’s why we invented Force Fins.

jump with fins, force fins, military fins, what fins do Navy SEALS wearAmong those divers in the know, Force Fins are a closely held secret. Dedicated Force Fin users are a special group of divers who were willing to try a revolutionary diving product that delivers on its promises. They will never go back to other dive fins again. Would you like to join this elite group? We could go on forever describing the unique benefits of using Force Fins, but there’s only one way to really find out why our fins are the best diving fins in the world: try a pair. You’ll be happy you did.