(Left) A single fiber is easy to stretch when it is crimped or wavy. However, once the fiber is straightened or recruited, the fiber becomes much stiffer, requiring more force to elongate the fiber. (Right) When many crimped fibers with different amount of slack are stretched together, the gradual straightening of the fibers creates a gradual, nonlinear stiffening, where more and more force is required to continue stretching the tissue. A fiber that is straightened is referred to as recruited. Conversely, a fiber that is not recruited, but would at higher stretch, is referred to as in reserve. The proportion of fibers recruited to those in reserve is directly related to the rate of recruitment. Adapted from (Jan and Sigal, 2018).