Heat-curing reactive composition based on natural and/or synthetic polymers containing olefinic double bonds, and vulcanizing agents, which composition contains a) at least one liquid cis-1,4-polyisoprene with a weight-average molecular weight of between 20000 and 70000, b) a vulcanization system, c) at least one polybutadiene liquid at 22° C., in which the liquid cis-1,4-polyisoprene a) constitutes a proportion of the entire composition in the range from 9 to 15 wt. %, the vulcanization system b) is selected from the group consisting of: b1) sulfur and one or more vulcanization accelerator(s), b2) peroxide or disulfide vulcanization systems, b3) quinones, quinone dioximes or dinitrosobenzene, and the polybutadiene c) liquid at 22° C. constitutes a proportion of the entire composition in the range from 16 to 29 wt %.