Hamstrings are group of three muscles which help bend the knee and extend the hip. Hamstring tightness is very common and can cause running back injury, low back pain, knee pain, running injuries and impair performance. Here’s a cool and effective hamstring stretch exercise using Proprioceptive Neuromuscular Facilitation or PNF technique. This is a physical therapy technique which uses the anatomy and physiology of the neuromuscular system t effectively stretch the muscles. Standing hamstring stretch is a contract relax technique is used here to inhibit the hamstrings so they can be stretched more effectively. 10 secs of the contract phase and 30 seconds of the stretch phase for 2-3 minutes.