Because this spring is intended to control longitudinal stretching, we avoid adding unnecessary constraints by allowing the core to compress. We set the spring coefficient to zero during compression, determined from the spring length .We blend from zero to its full value upon extension to avoid a large force introduction. We empirically found that using a cubic Hermite blend function from [0; 0:5] is sufficient because the segments of our hair lengths are on the order of 1 unit of length.