Stretching Anatomy-2nd Edition by Arnold Nelson, Jouko Kokkonen Page B

the legs and can cause muscles to tighten more. When the muscles are tight, they are also short, and short knee flexors increase the strain on the lower-trunk extensor muscles, especially when you bend forward at the waist. This added strain then injures the lower-trunk extensor muscles and is one of the most common causes of a sore lower back. Also, a lack of flexibility in the knee flexors makes these muscles more injury prone when a person suddenly increases movement speed or experiences greater workloads.
There are many reasons why an inactive person may have short knee flexor muscles. First, you can be born with naturally short hamstrings. Second, the hamstrings can become short if you sit for long periods. Regardless of the reason, your hamstrings can become longer if you perform regular stretching exercises.
Stretching the knee flexors one leg at a time reduces the stress on the legs and back. The knee flexor stretch can be performed on a soft couch or other soft surfaces and can be done at any time—while sitting on the couch watching TV or just relaxing after a long day’s work. Doing this stretching exercise from a sitting position with one leg up on the couch surface and the other leg hanging down allows you to concentrate solely on stretching these muscles and allowing the other muscles in the body to relax. If you are not flexible or are a beginner to a stretching program, it would perhaps be better to start this stretch with your right knee slightly bent and then gradually work on straightening the knee as your flexibility improves. If you want to maximize the stretch of these muscles, start working with a straight knee position. While executing this stretch, try to avoid tilting the pelvis forward or curving the back. It is also beneficial to bend the trunk forward as a single unit, keeping it centered above or next to the side of the right thigh.

Intermediate Standing Knee Flexor Stretch

Execution
     
Stand upright with the right heel a comfortable distance ahead of the left toes.
Keeping the right knee straight and the left knee slightly bent, bend the trunk over toward the right knee.
Reach the hands toward the right foot.
Repeat this stretch for the opposite leg.
Muscles Stretched
     
Most-stretched muscles: Right semitendinosus, right semimembranosus, right biceps femoris, right gluteus maximus, right gastrocnemius, lower right erector spinae
Less-stretched muscles: Right soleus, right plantaris, right popliteus, right flexor digitorum longus, right flexor hallucis longus, right tibialis posterior
Stretch Notes
When you start participating in a sport and do not stretch properly, you are more likely to have your hamstrings tighten up. Tight hamstrings are common among both distance runners and sprinters who have significantly increased their speed, the distance run, or the amount of uphill climb. Tightness in the muscles can ease away during exercise as the muscles get warmer, but after stopping it can return. Also, tightness is often an indicator of minor or major muscle strains, a common occurrence mainly felt postexercise. In addition, muscle strength imbalances, in which the knee extensors are stronger or the gluteal muscles are weaker than the hamstrings, will also cause tightness. Thus, it is especially important to stretch properly after exercise because this is when the muscles are warm and more receptive to stretching.
This is the most commonly used stretch for the hamstring muscles. It can easily be done at any time whenever you feel the need to stretch your hamstrings. After any type of fitness activity, minor aches and tightness in the hamstring muscles are possible. It is not unusual to have such discomfort after almost any exercise session. This is the optimal time to do some light stretches for these muscles. In most cases, this stretch will relieve those uncomfortable symptoms, and you will be able to continue on to your other daily routines without any concern about your muscle