Stretching after a hamstring injury is extremely important. Initially, stretching may have to be done passively within a pain free range of motion. To get a better idea of why stretching is so important, consider this. During the first two days to six weeks after an injury, collagen is formed and laid down to repair the injured area. If an injured person is performing the correct stretching exercises, the majority of the collagen will be laid down in the same direction as the tissue being repaired. This makes the repaired tissue stronger and more capable of performing its function. If the individual is not stretching, the tissue will be laid down in more random patterns, leading to the development of weaker tissue that is easily re-injured (scar tissue).