A pocketed spring, such as that used in a mattress, comprises: a compression spring having an upper end convolution and a lower end convolution opposite the upper end convolution, and a plurality of helical intermediate convolutions between the upper end convolution and the lower end convolution; a flexible enclosure including a top wall positioned adjacent to the upper end convolution of the compression spring, a bottom wall positioned adjacent to the lower end convolution of the compression spring, and a side wall that extends from the top wall to the bottom wall; and a tension member connected to the flexible enclosure. The tension member acts in opposition to the compression spring until the pocketed spring is compressed to a point at winch the tension member no longer applies any force. Thus, the pocketed spring exhibits a non-linear response when compressed.