formed by braided water repellent synthetic threads advantageously the elastic strands are paired or grouped two by two 1 u2032 1 u2033 2 u2032 2 u2033 3 u2032 3 u2033 4 u2032 4 u2033 and each pair of strands 1 2 3 and 4 comprises a circular elastic link or of an endless elastic link according to a characteristic arrangement of the invention the different elastic strands 1 2 3 and 4 are fastened through the intermediary of their opposite ends on the one hand in a common location 6 of the strap 5 and on the other hand in points 7 8 9 10 spaced from each other of said strap and differently spaced from their common point of connection 6 advantageously the fastening locations 6 7 8 9 and 10 of the strap 5 are formed by connectors said strap is equipped with over a portion of its length each circular elastic link 1 2 3 and 4 being engaged on the one hand in a common connector 6 and on the other hand individually in one of the connectors that are spaced from each other 7 8 9 10 and placed at different distances from said common connector 6 in other words the elastic.