In order to understand correct stretching techniques it is beneficial to know a little about the physiological properties of the soft tissues that are being stretched.
Muscles have a neural reflex arc that prevents excessive tension developing on the muscle. Sensory receptors within the muscle are sensitive to changes in the length and tension on the muscle. When a muscle is stretched, the muscle spindles a sensory receptor sends a message to the central nervous system to cause a reflex contraction of the muscle in order to prevent stretch damage. The force of contraction is directly proportionate to the velocity of the stretch. A striking example of this is when the head rolls & collapses to the front when one falls asleep in the seated position (Like in a train). The collapse of the head causes the muscle spindles in the muscles of the cervical region to contract & the head is whipped back by the person in a sudden jerk.
However, if the stretch is maintained for more than 10 seconds the tension becomes great enough to risk rupture. Hence the contraction abruptly stops & the muscle relaxes. This relaxation is called as autogenic inhibition & is chiefly dependant on another sensory organ called as the Golgi Tendon Organ (GTO). It is located at the insertion of skeletal muscle fibres into the tendons of skeletal muscles. The new added length of the muscle achieved through the stretch is recorded in kinesthetic memory of the CNS.
A muscle can be stretched only if the muscle is completely relaxed. Since the ability of a muscle to increase its elasticity is based on the recall of the length that it was stretched during the stretching regime, stretching must be done every day. The total duration of the stretch should be about 12-15 seconds. As a muscle can be stretched only when it is relaxed, stretching every day does not pose a problem to recovery. Stretching should be done slowly with a gradual increase in the range of movement.
All soft tissues are more extensible when they are warm. This is because they are thermoelastic. This is like a blacksmith working with a horseshoe; when the horseshoe is hot it can be stretched and manipulated, but when it is cooled it is rigid and inflexible. Stretching is ineffective if it is performed when the body is cold because when we try a stiff muscle to stretch it can cause micro tears in muscle itself and can lead to injuries. So should therefore be preceded by a series of warm-up exercises to increase tissue temperature.
Stretching might increase performance in sports that require lots of flexibility but could temporarily decrease muscle strength if done between sets as it momentarily reduces contractile ability of the muscles and can impair performance in the following set. This should not be misunderstood to think that stretching reduces strength. Stretching, in the long term, can only help increase strength. Flexibility allows the body to maintain the right posture while working out and hence also increases the performance in lifting weights.