(Flexibility: NSCA gives a nice overview ~ NASM gets into depth)
There are a variety of factors that affect flexibility. Some we can control, some we cannot. Joint structure is an uncontrollable factor. The shoulder is the most flexible joint and is a ball and socket joint. The other ball and socket joint is at your hip. Hinge joints perform extension and flexion, like your knee and elbow joint. Other factors to consider are age and gender. As we get older, we tend to become more fibrous in our joints limiting our range of motion (ROM). The connective tissue is naturally elastic and plastic. It stretches and returns to its original shape and we call this elasticity. Something that is plastic will stretch and maintain the “new” shape. Weight training will increase your flexibility if you are inflexible. If you are flexible already, then resistance training will decrease your flexibility. The growth of muscle can decrease your flexibility. However, your activity level will increase your flexibility and delay a loss of flexibility.
Knowing how and when to stretch is important. Stretching before an activity will serve as a test of your ROM before your sport. Some say this will help prevent injury. Others say it may injury your “cold” body and may make you weaker. It is assumed that static stretching may cause a decrease in strength and power (not jumping as high as an example). It is also worth noting that “cold stretching” is necessary for “normal” activity. This is often the argument of martial artists who need to be flexible to use their advanced techniques. Often people will stretch during an activity (and after) to promote recovery. Gym goers may stretch between exercises to save time. However, stretching a fatigued muscle should be done carefully and gently.
Proprioceptors are special cells in the body that sense body position. They are involved in stretching. Golgi Tendon Organs (GTO) are located in the tendons (tendons attach muscle to bone). GTOs detect a load on a muscle. You experience this when you pick up something and are able to guess its weight. When the GTO is activated as a safety mechanism, the GTO shuts down/turns off the muscle. You experience this when you are performing an action and suddenly lose all strength. Another proprioceptor is a Muscle Spindle. These are located within the muscles and detect a stretch (speed and ROM) on the muscle. When you stretch to your limit (Yoga class) and start shaking a bit, this is the muscle spindle activating.
Autogenic inhibition is when the muscle is most flexible (relaxed) and is immediately after the same muscle contracts. (Massage therapists will use this to “turn off” a tight muscle.) Reciprocal Inhibition is contracting a muscle, forcing the antagonist muscle to relax. (Personal Trainers will use this to help you gain more ROM during a stretch.) GTOs, Autogenic and Reciprocal Inhibition are explained here: http://www.acefitness.org/blog/2868/what-s-the-difference-between-autogenic-and
Proprioceptive Neuromuscular Facilitation (PNF) : PNF stretches are usually performed with a partner in a “contract then relax” routine. PNF takes advantage of the aforementioned proprioceptors to increase the ROM. 🙂
Read more about the different types of stretching techniques here: http://www.acefitness.org/blog/2966/what-are-the-different-types-of-stretching/