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.

INTRODUCTION

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.

Measurement

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.

Analysis

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.

RESULTS

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.

CONCLUSION

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.

Summary

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:
DEPARTMENT OF THE NAVY BUREAU OF MEDICINE AND SURGERY
2300 E STREET NW
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.