Strength training – includes all-round and balanced core exercises for runners. When you run it is not only the leg muscles, but also the abdominals, back and shoulders muscles play an important role by providing core and spine stability during running.
Strength training program enhances running performance: improving coordination, flexibility, muscle and joint mobility, and improve endurance at long distances. Various types of strength exercises are known to us from school physical education lessons. It includes jumps, push-ups, lunges and twists. Such exercises are good as a warm up before a run to physically prepare for the training load and reduce the injury risks (Source: newrunners.ru).
Running is a repetitive contraction of certain muscles over a long period of time. It makes the muscle fibers shorter in length than normal, and misaligned (i.e. like hair that needs combing).
Physical therapists use stretching as a procedure to restore normal resting lengths of muscles and align the muscle fibers. Without these exercises, we are at risk for permanent muscle contractions (called adaptive muscle shortening) that change the function of the joints, where muscles are attached to. For example, if the hip flexors (actively involved into the running process) are contracted and shortened, they pull down the front of the pelvis and displace the lower back that may have lots of other implications as a chain reaction.
Thus, although we cannot state that stretching exercises reduce the injury risk or improve running performance, they certainly help to restore normal resting length of muscles after permanent contraction of the muscles during the time of running. Stretching helps runners to maintain range of motion in joints, preventing stiffness and lack of flexibility, or misalignment of muscle groups.