The present invention relates to swim fins (also commonly known as flippers) and, more particularly, to swim fins which are used for swimming training.

Although the invention will be hereinafter described with reference to its application to swimming training, it will be apparent to persons skilled in the art that the invention is not limited thereto but has wider application, such as in scuba diving, snorkelling or skin diving, body surfing, and body board surfing. BACKGROUND ART

It is well understood that competitive swimming places a high demand on a swimmer. Elite swimmers aim to achieve a maximum work rate, in the course of which they train their arms, legs, and the remainder of their body to achieve their maximum forward speed through the water. Since a swimmer cannot maintain maximum swimming pace at all times during training, swim fins have been developed which function specifically as training fins to displace more water during the swimmer's kicking action than would otherwise be achieved without the fins, and to give the swimmer the sensation of swimming faster, or at least simulating race speeds. The energy of the swimmer is thereby conserved in the kicking action, and can be expended by other parts of the body, such as the arms, to achieve maximum speed.

Conventional swim fins have, over many years, been modelled on ducks feet or the like, whereby they are flared or splayed with edges extending forwardly and sidewardly from a foot enclosing portion or pocket. Webbing in the form of elastic or plastic material forms a blade portion by which the user is

propelled forward when the blade portion displaces water during the kicking action.

In most cases, the blade portion may be three to five times the surface area of the portion which encloses the foot. Such a large blade portion creates a significant resistance to the rapid kicking action necessary to swim at race speeds. Thus, while such a fin may increase propulsion per kick, it does not allow coordination of the feet to achieve a kicking action that simulates race conditions, such as the natural twisting motion of the feet through the water. The force imparted from the fins to the water during the kicking action is experienced as a load on the swimmer's feet. Having a large blade portion, most of which extends forwardly of the foot enclosing portion, places a considerable torque or load moment on the musculature, soft tissue, and bone structures surrounding the ankle, as well as the knees, in order to displace the volume of water necessary to at least simulate race speeds. Whilst this may be addressed somewhat by shortening the blade portion extending forwardly of the foot enclosing portion, thus resulting in less volume of water being displaced in the kicking action, the load, however, although reduced, may still be considerable and, over time, lead to tendon and other tissue injury of the legs. Indeed, the provision in conventional swim fins of having the widest part of the blade portion located most forwardly of the foot enclosing portion, such that the swim fin is forwardly flared, is not ergonomically suited to the natural kicking action of a swimmer, particularly an elite swimmer, through the water, and places the swimmer's body at risk of injury. SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a swim fin that provides at least the same level of water displacement as conventional

swim fins, but which places much less load on a user's feet, particularly around the ankle joint.

It is another object of the present invention to provide a swim fin that provides much less resistance to the natural twisting motion of the feet during a swimmer's kicking action through the water than conventional swim fins.

According to one aspect of the present invention, there is provided a swim fin, comprising:-

(a) a foot enclosing portion having an enclosure means for retaining a user's foot therein, (b) an inner side blade portion and an outer side blade portion extending from respective opposite sides of the foot enclosing portion,

(c) the widest extent of the swim fin being defined between the outermost points of the inner and outer side blade portions. BRIEF DESCRIPTION OF THE INVENTION

Fig 1 is a plan view of a swim fin according to a preferred embodiment of the invention.

Fig 2 is an underside or bottom view of the swim fin of Fig 1. Fig 3 is a plan view of the swim fin of Fig 1 identifying the inner and outer side blade portions by broken lines.

Fig 4 is an underside or bottom view of the swim fin of Fig 3 identifying the inner and outer side blade portions by broken lines. Fig 5 is a front elevation view of the swim fin of Fig 1. Fig 6 is a rear elevation view of the swim fin of Fig 1. Fig 7 is a front elevation view of the swim fin of Fig 1 identifying the inner and outer side blade portions by broken lines.

is a rear elevation view of the swim fin of Fig 1 identifying the inner and outer side blade portions by broken lines. is a perspective view in broken outline of a swim fin according to another preferred embodiment of the invention. is a cross sectional view of the swim fin of Fig 9. is a long sectional view of the swim fin of Fig 9. is a side elevation view of a foot inserted in a foot enclosing portion of a swim fin according to the embodiment of Fig 1 or Fig 9. is a plan view of the foot and swim fin shown in Fig 12. is an overlay of a preferred swim fin of the present invention and a conventional flared fin, the fins having the same vertical profile surface area. is a side view of a user of the conventional flared fin shown in Fig 14, showing the location of dimensions and other parameters used in a biomechanical study comparing the two fins shown in Fig 14. is a side view of a user of the preferred swim fin of the present invention shown in Fig 14, showing the location of dimensions and other parameters used in a biomechanical study comparing the two fins shown in Fig 14. is a diagrammatic representation of the parameters shown in Fig

15 for the purpose of assessing the torque effects of that fin. is a diagrammatic representation of the parameters shown in Fig 16 for the purpose of assessing the torque effects of that fin. is a graphical representation comparing the torque effects at the knee and the ankle for the two fins shown in Fig 14.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Typically, a swim fin is divided into key regions which relate to functionality of the fin. Conventional swim fins comprise a foot enclosing portion or pocket disposed in a central region of the fin. There is a retaining strap which defines an opening through which a user's foot is inserted. The retaining strap engages a user's heel to retain the foot in the foot pocket. The fin further comprises a distal or trailing end which forms the major area of the fin. The distal or trailing end is flared or splayed outwardly such that it is widest at a distal edge. The broad width at the trailing end enables the fin to displace a substantial amount of water when used by a swimmer.

The swim fin 1 shown in the Figures is preferably used for swimming training and comprises a fin body 2 which includes a foot enclosing portion 3 having an enclosure 4 for retaining a user's foot therein. Fin 1 further comprises an inner side blade portion 5 (corresponding to the inner side of the user's foot) and an outer side blade portion 6 (corresponding to the outer side of the user's foot) integrally attached to, and extending from, respective opposite sides of the foot enclosing portion 3.

Swim fin 1 , when worn on a foot during the kicking action of a swimming motion, produces load on the foot enclosing portion 3 and on the side blade portions 5 and 6 resulting in forward propulsive thrust.

The foot enclosing portion 3 extends to a distal region 7 of the swim fin 1 and defines an enclosure or foot receptacle 4 which envelops the foot up to the ankle. An opening 8 receives a foot of a user and retains the ankle. The opening 8 is asymmetrical (as shown in Figure 1), and a distal region 9 of the opening 8 engages the raised bridge of a user's foot at the highest point of the opening 8. The shape of the foot receptacle is asymmetrical and is highest along an elongated ridge 10 corresponding to the raised bridge of a user's

foot. The asymmetrical shape of the foot receptacle allows for a tighter and more comfortable fit as it is more compatible with human foot anatomy and imparts less tension across the raised bridge of the foot. There is an opening 11 to the foot enclosing portion 3 at the distal region 7 of the swim fin 1. The inner and outer side blade portions 5 and 6 are preferably of a varying width along their length, with either the same or varied thickness along their length. The widest extent of the swim fin is defined between the outermost points of the inner and outer side blade portions 5 and 6. The outermost points of the inner and outer side blade portions are, when the fin is viewed side on, at locations that fall within a range defined by the main arch of the user's foot (see line B in Figs. 1 and 3). The inner and outer side blade portions have a surface area of approximately between 0.01 and 0.49 times the surface area of the foot enclosing portion. The particular geometry of the swim fin 1 imparts aquadynamic load characteristics to the fin and, depending upon the combination of width and thickness of the blade portions, produces a particular water flow and resistance for the swim fin during the kicking action of a swimmer.

In this embodiment, the inner side blade portion 5 is concave up and produces a load and thrust during the forward kicking action of the leg. The outer side blade portion 6 is concave down and produces a load and thrust during the backward or rearward kicking action of the leg. Since the outer side blade portion 6 has an opposite geometry to that of inner side blade portion 5, the thrust and side vortices will be substantially balanced during the kicking action of both legs. The inner and outer side blade portions 5 and 6, and the asymmetrical streamline shape of each fin, produce water flow-on and resistance during the left and right rocking motion of the body and the legs during a freestyle

swimming action. This results in a kicking motion of not just up and down, but a fluid rotation to the left and right known in competitive swimming as a rocking motion. Furthermore, this natural motion of freestyle swimming enables excellent trail off of water during swimming, that is, water dispersal from the surface and trailing edges of the fins. The opening 11 to the foot enclosing portion 3 at the distal region 7 allows unrestricted water flow therethrough and around the foot. It also allows for various foot sizes and for foot growth. The inner sole of the foot enclosing portion 3 is ergonomically shaped to accommodate the human foot. Figures 3, 4, 7 and 8 highlight the inner and outer side blade portions 5 and 6 by the broken lines 12 and 13 respectively. The foot enclosing portion 3 is also highlighted in these Figures by broken lines 14.

The swim fin 20 shown in Figures 9, 10 and 11 has a foot enclosing portion 21 including treads or ribs 22 formed therein. The ribs 22 are, in this embodiment, shaped and aligned to suit the contour of the underside of a user's foot, and particularly the arch. The thickness, height pattern, and concentration of the ribs 22 allows them to distort and conform to the contour of the underside of a user's foot. This arrangement of ribs 22 aids in holding and stably positioning the foot in the foot enclosing portion, and also provides a cushioning effect to the underside of the user's foot. In addition, grooves 23 or channels are formed between the ribs 22 (see particularly Figures 9 and 10). The depths of the grooves 23 or channels may be the same or variable along their length.

The ribs 22 provide a layer of soft rubber of about 35 to 55 duromete _. r over a hard rubber layer of about 65 to 85 durometer forming the base or outer sole of the swim fin.

The outer sole has ridges 50, 52, 54 and 56 (see particularly Figures 2 and 4) which flex under the weight of a user's body when downward pressure is applied. This produces a cushioning effect between the ground surface and the underside of the foot. The outer sole ridges also create tension and grip on a surface upon which the user is standing or walking.

Figure 12 shows a foot 30 inserted in a foot enclosing portion 31 of a fin 32. A distance between an axis 33 corresponding to the ankle joint 34, and the toes 35, is demonstrated generally by arcuate broken lines 36 and 37. The fin 32 is shown extending slightly beyond line 37 but, in comparison to conventional fins, the small extension 38 beyond toes 35 of foot 30 makes the fin fit into the category of a training fin. When the fin 32 is used, it sweeps backwards and forwards (or, more correctly, up and down) in the water in the direction of arrows 39 and 40. Aquadynamic loads are distributed over the surface area of the fin 32, but a resultant load will be applied at a position commensurate with the centre of the distributed load. The resultant load will create a torque about the axis 33 through the ankle joint 34. The torque will be the product of the resultant load and the distance between that load and the axis 33 through ankle joint 34. Thus, the torque on ankle joint 34 about axis 33 will vary according to the location of the resultant load which, in turn, will vary according to the length and width of the fin 32.

As also shown in Figure 13, the fin 32 includes a distal end 43 and a proximal (or heel) end 44. Disposed laterally of the foot enclosing portion 31 are inner and outer side blade portions 45 and 46. A substantial part of the surface area of the blade portions 45 and 46 is disposed adjacent a mid line 47 through fin 32. Figure 13 includes a circle which defines a "sweet spot" within which a resultant load is preferably applied so it occurs over the foot, rather than forwardly of the foot as in conventional flared fins, thereby reducing

the torque on the ankle joint 34. A centre of resultant aquadynamic load will be applied at about the point indicated by numeral 49 which is over the foot and behind the toes. Reduction of the surface area and tapering of the fin forwardly of the foot, and increasing the surface area of the fin to the sides of the foot, places the resultant load closer to the ankle. Reducing the torque on the ankle increases wearer comfort and reduces the risk of injury, but will not compromise propulsion if the overall surface area of the fin remains the same or similar to that of conventional flared training fins.

Although the widest extent of the swim fin is no longer at the most forward part of the swim fin (as in conventional swim fins), the swim fin 32 is able to sweep through a larger arc, due to reduced torque on the ankle and knees, and so can displace at least the same volume of water during kicking action as conventional flared training fins.

The following is a description of a biomechanical study comparing a preferred swim fin according to the present invention with a conventional swim fin of the same vertical profile surface area. The description will be made with reference to Figures 14 to 19.

Figure 14 shows an overlay of the vertical profiles of a swim fin 60 according to the present invention and a conventional wedge-shaped or flared fin 62, the fins 60 and 62 having the same vertical profile surface area. A significant amount of the force imparted from a fin to the water is through the widest region of the fin. The factor of particular interest is the relationship of the pivot forces (torque) through the leg (and particularly through the ankle joint) as a result in the forces imparted, and hence load experienced, at the widest region of the fin.

Relative Dimensions with Respect to the Foot

Let: f = foot length h = standing height

Figures 15, 16, 17 and 18 identify the location of the dimensions and other parameters referred to hereinafter.

Deriving from Newtonian Anthropometry it can be generalized that: f = 0.15Oh (foot-length is to 15% of standing height) a = 0.243h (hip to knee distance is 24.3% of standing height) b = 0.235h (knee to ankle distance is 23.5% of standing height)

From measurement of the fin 60: c = 0.491 f (distance ankle pivot to widest region)

From measurement of the conventional fin 62: d = 1.02f (distance ankle pivot to widest region)

To simplify, all relevant dimensions can be expressed in relation to the foot size: a = 1.62f b = 1.57f c = 0.491f d = 1.02f

Hypothetical Situational Parameters

To gain an understanding of the influence of the forces at the widest region of a fin on the torque forces in the leg, let us consider a moment in time in a vertical plane along one leg as represented in Figures 17 and 18. Force (F) is applied to this widest point (point E for the conventional fin 62 in Figure 17, and point D for the fin 60 of the present invention in Figure 18) by rotation of the hip and resulting force of the water on the fin. What is helpful to consider is the amount of resulting Torque (T) that effects the points at the hip (A), knee (B) and ankle (C). For this purpose we will consider a horizontal leg position as shown in Figures 15 and 16 with force acting perpendicular to the direction to the hip at the points E and D respectively, θ = 180° (thigh and shin align resulting in the distance from A to C being a + b) β = 169° (at maximum foot extension) λ = 156° (at maximum foot extension)

Using trigonometry, the distance of A to E is: XAE = 4.143f

Let the torque at A be the constant: T _A The formula for Torque (T): T = Fχ (where χ is the distance of the force from the pivot point)

The force at E due to the Torque at A: FE = T _A / 4.143f

Again using trigonometry, the distance of B to E is:

XBE = 2.536f

To calculate the Torque at B due to the Force at E, only the vector of the Force that is perpendicular to the pivot is used. This is calculated using trigonometry:

The torque at B due to the Force at E:

= 2.531f x (T _A / 4.143f) = 0.611 T _A

Vector of Force at E that acts upon C: Fes = 0.950F _E

The torque at C due to the Force at E:

T _CE = 1.02f x 0.9501 FE

= 0.969f x (T _A / 4.143f) = 0.234 T _A

Using trigonometry, the distance of A to D is: XAD = 3.673f

The Force at D due to the Torque at A: FE = T _A / 3.673f

Again using trigonometry, the distance of B to D is: XBD = 2.054f

Vector of Force at D that acts upon B FBD = 1.000F _D

The torque at B due to the Force at D:

T _BD =2.054fx1.000F _D

= 2.054fx(T _A /3.673f) = 0.559T _A

Vector of Force at D that acts upon C: FcD = 0.986F _D

The Torque at C due to the Force at D:

T _CD =0.491fx0.986F _D = 0.484fx(T _A /3.673f)

= 0.132T _A

As apparent from the graph in Figure 19, the torque (T _C D = 0.132T _A ) at the ankle due to the resulting force from the water at the widest region of the fin 60 is almost half (56%) that of the conventional fin 62 (where TCE = 0.234T _A ).