Stretching exercises are most effective when you give them slack muscles. And there is such a jerk - at any crick responsible reduction. A vicious circle. Quit it can engage other reflexes.
 
Reflex 1

If, before stretching the muscle, for 5-10 seconds to load it isometrically, ie, without changing its length, then the next 5-10 seconds it significantly relax. To reflex triggered, the effort needed to do a little - somewhere in a third of the maximum possible.

Reflex 2

Tonus almost all the muscles increases on inspiration and decreases during the expiration.

Reflex 3

Tonus many muscles depends on the direction of gaze. For example, when you look left or right, further straining muscles, twisting his head and torso in the same direction, and further relax - in the opposite direction. A similar story happens to the muscles flexors and extensors of the trunk when you look up or down.

As an example, take the banner back of thigh muscles.

By the way, the shortening of the muscle groups is observed in almost all who are engaged in sedentary work, and often this is the trigger factor for back pain.
• heel slightly bent at the knee, put his feet on the rise: the crossbar of the Swedish wall, seat or chair. Emphasis should be tight.
• On the inhale, directing the mind down, try to bend the leg at the knee joint. The emphasis should not allow you to do it. Effort should be somewhere in a third of the maximum.
• On an exhalation, directing eyes upward, for 5-10 seconds, unbend his leg and stretch the muscles.
• For each leg exercise done 3-6 times.