with reference to the annexed drawings , fig1 illustrates a shoe designated comprehensively by the reference numeral 10 , using the improved sole in accordance with the present invention . the shoe 10 includes a vamp 12 secured to a bottom 14 to form a foot receiving enclosure 16 . the present invention is concerned with the construction of the bottom 14 that determines in large part the comfort potential of the shoe 10 and its stability . the bottom 14 is a layered structure comprising an outer sole 18 made of carbon rubber . if desired , the surface of the outer sole that contacts the ground may be sculptured to create a tread pattern . the outer sole 18 is bonded in a face - to - face relationship with a midsole 20 made of expanded polymer , such as ethylene - vinyl acetate ( eva ) copolymer . the midsole has a thickness of about 5 mm and it is comparatively hard ( a hardness in excess of a40 is preferred ). the upper layer of the shoe bottom 14 is constituted by an insole 22 that is bonded to the upper surface of the midsole 20 . the insole is made from a material selected to provide a resiliency index in the range from about 0 . 05 to about 0 . 5 , preferably from about 0 . 1 to about 0 . 35 and most preferably from about 0 . 1 to about 0 . 2 . the resiliency index is a custom parameter designed to quantify the rate of recovery of a material with relation to time following a compressive deformation . the measurement procedure is a modification of the standard test astm f36 - 88 designed for assessing compressibility and recovery of gasket materials . fig3 illustrates schematically the testing set - up 100 which comprises a frame 102 having a flat base portion 104 supporting the test specimen 106 . a horizontally extending arm 108 is pivotally mounted at 110 to the base portion 104 of the frame . a main load 112 of 3 . 17 kg is suspended from the free extremity of the arm 108 . the distance between the point 110 and the site on the arm 108 at which the main load is attached is 30 . 48 cm . an anvil 114 made of hardened metallic material is provided for locally compressing the test sample 106 . the anvil 114 is in the form of a cylinder having a diameter of 31 . 8 mm . the horizontal arm 108 applies pressure to the anvil 114 by the intermediary of a ball made of hard metallic material having a diameter of 15 . 9 mm . the rate of recovery upon removal of the sample is measured by a linear variable displacement transducer 118 having a maximum range of 2 . 54 cm ( a penny / giles transducer available from durham instruments , ontario , canada has been found satisfactory ). the testing procedure consists of positioning a test specimen having an area of 20 square centimeters ( cm 2 ) and a thickness ( uncompressed ) of 50 mm . the test specimen may entails a single ply or a number of superimposed plies sufficient to give the desired nominal thickness ( if there are several plies they should not be bonded or otherwise attached to one another ). the assembly formed by the anvil 114 , ball 116 and arm 108 ( free of the main load 112 ) is first deposited on the test specimen for a duration of 15 seconds ( sec ) to create a preload condition of 0 . 9 kg . this value is the weight that the surface of the specimen “ sees ” before the application of the main load . the individual weight of the various components , such as the arm 108 , ball 116 , anvil 114 , etc ., contributes to this preload condition , thus the materials of these components and their dimensions should be selected to achieve a combined weight creating the desired preload value . the main load 112 of 3 . 17 kg is then applied to the anvil for a period of 1 minute ( min ). the main load 112 is instantly removed and the recovery of the test sample is recorded for a period of at least 2 sec . the resiliency index is expressed by the following formula : r - m p - m r : peak recovered thickness observed within the one second time frame immediately following the removal of main load ; values r , m and p can be expressed in centimeters ( the same units of measure are used for each factor ). it is important to note that the resiliency index of the sole 22 is assessed with the sole removed from the shoe 10 , otherwise the results may be corrupted . for composite materials having a variable resiliency index over their surface , the measurement is performed in the region receiving the ball of the foot or the region receiving the heel . indeed , structures that exhibit different resiliency indices at the ball region and at the heel region are similar to two soles placed side by side . in other words , the ball region is considered to form one sole while the heel region forms another sole . this definition is consistent with the meaning given to “ sole ” earlier in this specification . more specifically a layer of material does not need to extend under the entire plantar surface to form a sole ; the layer may extend only under the ball region or under the heel region to be considered a sole . the benefits of the invention are realized best when the insole 22 has a hardness within a predetermined range selected to enhance stability . the hardness should be in the range from about shore a 2 to about shore a 40 , preferably from about shore a 2 to about shore a 14 . hardness is measured according to the standard test method for determining rubber property - durometer hardness ( annual book of astm standards , phila ., astm , 1988 , 09 . 01 , pp 596 - 600 ). as in the case of the resiliency index measurement , hardness is determined on a sample separated from the shoe . the sole 22 preferably has a thickness in the range from about 2 millimeters to about 50 millimeters , more preferably from about 5 millimeters to about 25 millimeters and most preferably from about 12 millimeters to about 20 millimeters . expanded polymer , available from pandel inc ., atlanta ga ., usa under the designation “ tennis embedded floor matting ” is the material of choice for manufacturing the sole 22 . this material is a pvc aerated polymer foam of shore a5 hardness . fig4 illustrates the low resiliency properties of this material . the resiliency index is assessed with the formula ( r − m )/( p − m ). the parameters p and m have values of 0 . 92 cm and 0 . 28 cm , respectively . r is the peak recovery value within the 2 and 3 second marks on the time axis . the peak is found at the 3 second mark and has a value of 0 . 38 . this produces a resiliency index of 0 . 156 . fig5 illustrates for comparison purposes the recovery pattern of eva polymer aerated into expanded foam that is commonly used in athletic footwear . this material is considered unsuitable for use in an article of footwear , from the perspective of the present invention , because it is too resilient . the resiliency index of the eva polymer is about 0 . 714 . a variant of the invention is illustrated in fig2 . here , the improved sole is made part of a sock 24 . more particularly , the sole is sewn or otherwise attached to the material enclosing the foot , so it faces the plantar surface of the foot . the above description of preferred embodiments should not be interpreted in any limiting manner since variations and refinements are possible without departing from the spirit of the invention . the scope of the invention is limited by the terms of the following claims and their equivalents .
a resilient sole for use in an article of footwear . the sole has controlled stiffness and manifests a slow shape recovery following compression by the plantar surface of the foot . a footwear construction using a sole with such characteristics improves stability of users of all age groups , while enhancing comfort .