Running after a hamstring injury will depend upon the severity of the injury. These strains are rated as first, second or third degree injuries. A first degree injury will result in some minor tears to the muscle that will heal well with rest and ice.
A second degree strain to the hamstring muscle experiences moderate tears with pain that generally covers a larger area of the muscle than a first degree tear. The muscle will be stiff and weak and may be black and blue. A third degree tear will result in wide-spread bruising and a balling up of the muscle may be felt or palpated with the hand. Third degree strains are less common than first or second degree.