VIBRATION DAMPENING HANDLEBAR AND METHOD FOR PRODUCING SAME FIELD OF THE INVENTION The present invention relates to vibration dampening of handlebars for the control of vibrating equipment and, more particularly, to the dampening of harmful vibrations experienced in the forearms of cycle riders, and especially motocross bike riders. Although the invention will be particularly described hereinafter with reference to motocross bikes, it is to be understood that the invention is not limited thereto and has wider application to all kinds of vibrating equipment, whether stationary or mobile. For instance, the invention has possible application to hand operated construction, repair and maintenance machinery, as well as to motorbikes in the form of two or three wheeled mopeds, motor scooters, motor tricycles and motorbikes with sidecars, although off road applications are the most likely. DESCRIPTION OF THE BACKGROUND ART Most, if not all, riders who participate in the sport of motocross will have experienced Chronic Exertional Compartment Syndrome of the Forearm (CECSF), commonly known as "arm pump". The condition is associated with rapid and vigorous exercising of the musculature of the forearms resulting in an increase of oxygen rich blood engorging the muscles and causing painful hardening of the forearms. Many remedies and supposed treatments have been tried ranging from special exercise regimes to dietary supplements and even surgery. Other attempts at solutions to the arm pump problem have sought to improve the suspension systems of bikes and/or modify the mounting of the handlebars, all with little or limited effect. None offer a simple and straightforward amelioration of the problem. In certain circumstances, the more debilitating condition of Acute Compartment Syndrome may occur. It is an object of the present invention to overcome or at least substantially ameliorate some of the above disadvantages of the prior art. SUMMARY OF THE INVENTION Accordingly, there is provided in a first preferred embodiment of the invention, a method for the reduction of adverse muscle reaction in the forearms of a rider of a cycle caused by vibration of a handlebar gripped by the rider, the handlebar having a tubular cavity, said method including introducing an elastomeric compound into the tubular cavity of the handlebar of the cycle until the tubular cavity is filled. According to another preferred embodiment of the invention, there is provided a method for the reduction of adverse muscle reaction in the forearms of a rider of a cycle caused by vibration of a handlebar gripped by the rider, said method including: (a) removing handgrips and control elements from the handlebar, (b) arranging a closure element within one end of the handlebar, (c) arranging the handlebar in a vertical or sufficiently near-vertical position, with the closure element being lowermost, and (d) pouring or injecting an elastomeric compound into the handlebar so as to substantially fill the handlebar. In a preferred form, the elastomeric compound is allowed to cure (i.e. set and harden) within the handlebar. Alternatively, the elastomeric compound is not allowed to cure. Preferably, the cycle includes motocross bikes. The adverse muscle reaction may be Chronic Exertional Compartment Syndrome of the Forearm or "arm pump". It is preferred that the elastomeric compound is a one part polyurethane adhesive, such as that sold commercially by Sika Corporation under the name SIKAFLEX 252. Alternatively, the elastomeric compound is an adhesive cement, such as that sold commercially by Minnesota Mining and Manufacturing Corporation (3M) under the name Super Weatherstrip Adhesive 08001. Further, the elastomeric compound may be a two-part urethane adhesive, such as that sold commercially by Minnesota Mining and Manufacturing Corporation (3M) under the name DURAMIX NVH Damping Material 04374. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described with reference to the accompanying drawings wherein: Fig. 1 is a front view of a rider gripping the handlebar of a motocross bike, the handlebar having been filled with an elastomeric compound in accordance with a preferred method of the present invention to dampen vibration thereof, Fig. 2 is a disassembled view of a handlebar of a motocross bike showing various components thereof, Fig. 3 is a view of the filling of the handlebar of Fig. 2 with an elastomeric compound in accordance with a preferred method of the present invention, Fig. 4 is a sectional side view of an end portion of the handlebar of Fig. 3 which has been substantially filled with an elastomeric compound and then covered with a handgrip, Fig. 5 is a sectional end view of an end portion of a handlebar filled with an elastomeric compound being gripped by a rider's hand, and showing a point at which various directional measurements of vibration acceleration were taken, and Fig. 6 is a graphic comparison of average vibration acceleration values taken when testing a motocross bike with a conventional handlebar against the same motocross bike with a handlebar modified by filling it with an elastomeric compound. MODES FOR CARRYING OUT THE INVENTION The present invention encompasses empirically derived improvements in the dampening of vibration in the handlebars of motocross bikes by the addition of an elastomeric compound to the interior of the handlebars, thereby alleviating or at least reducing adverse muscle reactions in the forearms of riders. Handlebars of motorbikes are generally of unitary tubular construction, at least partly closed off at each of their opposite ends by handgrips or other means. In the case of a motocross bike 12, as shown in Figs. 1 and 2, the handgrips 14, 16 on a handlebar 18 may be operably connected to the braking levers as well as other controls of the bike; typically a rotary motion of the right hand handgrip controlling acceleration and the left hand handgrip controlling the clutch. In order to carry out the method of the present invention as partly shown in Fig. 3 at an after market stage (or post manufacturing stage), it is necessary to close one end of the handlebar 18 of a motorbike while opening the other end to allow the introduction of the elastomeric compound. Typically, the handgrips fitted to motorbikes are not completely closed, and so it is required to remove the handgrip at one end and introduce into that end a suitable plug (such as plug 20 shown in Fig. 2), and to remove the handgrip at the other end and other controls of the motorbike from the handlebar. The handlebar 18 is then arranged with the use of a clamping means 22 in a near vertical orientation, as shown in Fig. 3, to allow a flow of an elastomeric compound 24 into the handlebar 18 with the use of a trigger applicator 26 and by simple force of gravity. In some instances it may be advisable to demount the handlebar 18 from the steering column of the motorbike. Fig. 4 shows the elastomeric compound 24 substantially filling the handlebar 18, the handlebar being covered by a handgrip 14. The method of the present invention may alternatively be readily carried out at the handlebar manufacturing stage prior to assembling the controls and handgrips on the handlebar, or even prior to bending the handlebar into shape. A number of elastomeric compounds may be used to fill the tubular cavity of the handlebar to provide significant vibration dampening thereof. Within the group of elastomeric compounds that may be suitable are included those commonly regarded as silicone based rubber-like compositions and mastic filler and sealant compositions, and other compositions having similar properties. One particular formulation of a one part polyurethane adhesive manufactured by Sika Corporation ( known by the trade mark SIKAFLEX 252), with a density of approximately 1.2kg/litre has been found particularly effective. The composition of SIKAFLEX 252 is as follows:- Component Proportion % wt Polyurethane Prepolymers 20-40% Plasticiser 15-25% Xylene 1 -3% White Spirits <3% Calcuim Oxide <1% Curing Agent & Adhesion Promoter <2.5% Pigment <3% IPDI/MDI <0.5% PVC Filler to 100%

Another suitable product is manufactured by Minnesota Mining and Manufacturing Corporation (3M) and is an adhesive cement (known by the trade mark Super Weatherstrip Adhesive 08001).

A first composition of Super Weatherstrip Adhesive 08001 is as follows:- Component Proportion % wt Methyl Ethyl Ketone 20-25% Polychloroprene 15-25% Magnesium Resinate 15-25% Toluene 7-13% Naptha Solvent Refined Lisht 20-25% Hexane 7-13% Cyclohexane <0.5%

A second composition of Super Weatherstrip Adhesive 08001 is as follows:- Component Proportion % wt Methyl Ethyl Ketone 20-25% Polychloroprene 10-20% Toluene 1 -10% Naphtha Solvent Refined Lisht 15-30% Hexane 5-15% Cyclohexane <2% Phenolic Polymer 10-25% Magnesium Oxide 1 -10% 2-Chloro-1 ,3-butadiene polymers and copolymers 1 -10% Benzene <0.05%

Yet another suitable product is known by the trade mark DURAMIX NVH DAMPING MATERIAL 04374, also available from 3M, which is a two-part urethane adhesive. The composition of Part A of this two-part urethane adhesive is as follows:- Component % by Wt Polyurethane prepolymer 40-70 4,4'-diphenylmethane diisocyanate 10-30 Benzene, 1 ,1 '-methylenebis[isocyanato-, homopolymer 7-13 lsocyanic acid, 3-(triethoxysilyl)propyl ester 5-10 1 , 1 '-methylenebis(isocyanatobenzene) 1 -5 Polyurethane prepolymer 1 -5

The composition of Part B of this two-part urethane adhesive is as follows:- Component % by Wt Polypropylene glycol 40-70 Polyether polyol 40-70 bis(methylthio)toluenediamine 7-13 lsophorone diamine 1 -5 Amorphous silica 1 -5

Still another suitable product is known by the trade mark RTV102 Silicone Rubber Sealant manufactured by Honeywell IAC, the composition of which is as follows:- Component Proportion %wt Methyltriacetoxysilane 1 -5 Polydimethylsiloxane silanol/STPD 60-80 Octamethylcyclotetra-siloxane 1 -5 Silanol/STPD Siloxane with methyl silsesquioxanes (hydroxy-terminated) 5-10 Treated fumed silica 10-30 Dimethylpolysiloxane 1 -5

Yet another suitable product is known by the trade mark SILATECH 100% Silicone Sealant White manufactured by Loctite Corp., the composition of which is as follows:- Component Proportion % wt Subst. Poly(dimethylsiloxane) 70-90 Silica, Amorphous 5-10 Methyltriacetoxysilane 1 -5 Ethyltriacetoxysilane 1 -5 Poly(dimethyl Siloxane) 1-5 Acetic Acid * * Acetic Acid is formed upon contact with moisture

The elastomeric compound should be injected or allowed to flow into the handlebar so that, ideally, the entire tubular cavity is filled. Vibration tests were calculated on a 250cc two stroke motocross bike over three consecutive runs on a test track. The tests compared a motocross bike with an unmodified handlebar against the same motocross bike with a modified handlebar, the modification being in the filling of the handlebar with SIKAFLEX 252 in accordance with the method of the invention. Vibration acceleration was measured by a triaxial accelerometer at point P (corresponding to near the knuckle in the rider's gripping hand), as shown in Fig. 5. The X and Z values were of vibration acceleration measured (in m/s2) in the directions as shown, and the Y value was of vibration acceleration measured in the direction perpendicular to both the X and Z directions. The vibration acceleration averages were based on the root mean squared amplitude. The results of the tests are shown in the following Tables:- TABLE 1 Unmodified Y X Z

RUN 2 28.0 68.0 31.6 RUN 3 28.5 67.1 31.0 AVERAGE 27.3 65.4 30.3

Modified Y X Z RUN 1 17.8 53.0 17.2 RUN 2 21.2 59.5 20.4 RUN 3 21.2 54.8 21.3 AVERAGE 20.1 55.7 19.6

Comparison Y X Z Unmodified 27.3 65.4 30.3 Modified 20.1 55.7 19.6 Average % reduction 26.6 14.8 35.2 25.5

A comparison of the vibration acceleration averages in each of the directions Y, X and Z is shown in Fig. 6. It will be apparent from the results of the tests that significant vibration dampening occurred when the handlebar was filled with an elastomeric compound in accordance with the method of the invention. It was found that the elastomeric compound used in the above tests had not cured, but retained its paste-like consistency. As SIKAFLEX 252 cures by reaction with moisture in the air to a tough, elastic rubber-like material, it is believed that the absence of the necessary amount of atmospheric moisture interacting with the compound when it is filling the handlebar prevents curing from taking place. However, other elastomeric compounds may cure from a paste or viscous form under suitable conditions when filling the handlebar, and provide significant vibration dampening thereof. It is important that, whichever elastomeric compound is used, upon curing from a paste or viscous form, a high degree of resilience of the compound is retained. Experiment has shown that for lighter motocross bikes, the lower end of the density range (i.e. a softer compound) is to be preferred, with the higher end of the density range more suited to heavier bikes. Although the present invention has been described primarily with reference to motorbikes, and especially motocross bikes, it will be appreciated that similar benefits may be derived from its application to any handles of vibrating equipment where an operator is required to exert force through the forearms for extended periods. The above describes only some embodiments of the present invention and modifications, obvious to those skilled in the art, can be made thereto without department from the scope and spirit of the present invention.