Overview & Facts
Overview: Flexibility is the ability to move a joint through a range of motion without pain.
- Range of motion is the movement of a joint or body segment from its starting position to its ending position.
- Stretching is the movement of a body segment in a way that lengthens the shortened soft tissues, thus increasing flexibility. Soft tissues include muscles, fat, and fibrous tissue.
- Performance improvements from stretching may include more force production, increased speed, and better jumping ability. In some cases, stretching may relieve pain.
- In a sports context, stretching refers to a set of exercises often used as part of the warm-up routine before practice or competition. Such stretching often involves moving body segments deliberately, often slowly, to the limit of their range of motion, holding them at that limit for a period of time, then returning to rest.
- In a clinical environment, stretching is used during physical therapy or occupational therapy to restore range of motion after an injury.
- One way to isolate the history of stretching from the history of sports is to examine sports that especially emphasize extending the individual’s range of motion, such as martial arts. A sports physician affiliated with the U.S. Department of Defense’s Military Health System pegged the origin of martial arts as far back as 5000 BC. Gymnastics began in 500-700 BC. Yoga has been practiced since about 3000 BC.
- Despite broad acknowledgement that lack of flexibility may be a risk factor for injury, literature reviews by several sports-medicine researchers found no association between stretching and injury prevention in athletes.
- Some research suggests that fitness in general is more important than stretching for reducing the risk of injury. (The U.S. Department of Health and Human Services defines physical fitness as a set of attributes that people have or achieve relating to their ability to perform physical activity.) Also, research suggests that stretching in order to increase the range of motion beyond what is needed to participate in one’s sport may not be beneficial and may actually cause injury and decrease performance.
The mechanics of stretching: The word stretch describes a physiological activity in the body.
- Nerve impulses: Movement occurs when nerve impulses from the brain are delivered to sensors called motor neurons, which are attached to special stretch-sensitive muscle spindles near tendons. (Tendons attach muscles to bones.)
- Muscle contraction/muscle stretching: In response to a stimulus, specialized nerve cells known as motor neurons cause the muscle to contract. The agonist muscle is primarily responsible for movement. As the agonist muscle contracts, the muscle that opposes it, the antagonist muscle, is stretched.
- Stretch reflex: Motor neurons sense this muscle stretch and send signals to the spinal cord, prompting a “stretch,” or myotatic, reflex. A stretch reflex causes the stretched muscle to contract.
- Even though the initial movement may have been a conscious movement, initiated by a signal from the brain, the stretch reflex is unconscious, delivered by the spinal cord. This “shortcut” allows the fastest possible reaction time. Fast reaction time is important in allowing speedy adjustments to changing conditions. Fast reaction time also helps prevent the antagonist muscles from being injured by excessive stretching. In addition to helping prevent injury, the stretch reflex aids in keeping muscles toned by prompting tension between the agonist muscles and the antagonist muscles. The stretch reflex is central to flexibility training because altering the stretch reflex increases the individual’s range of motion.
Types of flexibility and stretching:
- Flexibility: There are three principal types of flexibility: dynamic, static, and passive.
- Dynamic flexibility: Dynamic flexibility is the ability to move a body segment within a range of motion. Dynamic flexibility accounts for the increase in resistance as the body segment moves through its range of motion. Medical professionals measure dynamic flexibility in terms of stiffness, which is the ratio of force to elongation.
- Static flexibility: Static flexibility, sometimes called static-active flexibility, is the ability to stretch an antagonist muscle using only contraction or tension in the agonist muscle. Static flexibility is measured in linear or angular terms.
- Passive flexibility: Passive flexibility, sometimes called static-passive flexibility, is the ability to hold a stretch using body weight or some other external force.
- Stretching: There are several types of stretching.
- Dynamic stretching: Dynamic stretching consists of the controlled (usually slow) movements of a body segment to the limits of an individual’s range of motion. Over the course of a set of repetitions, the individual may gradually increase the speed of movement, reach, or both. Examples of dynamic stretching include slow, controlled leg swings or arm swings.
- Ballistic stretching: Characterized by fast actions and bouncing motions, ballistic stretching uses the momentum of the body in motion to stretch muscles beyond their resting points, as one might do by bouncing down repeatedly to touch one’s toes. The effectiveness of ballistic stretching is reduced, because a quick return to the starting position does not allow the muscles involved to stay in position and “relax” at the limit of movement. In addition, repeated activation of the stretch reflex may strain or rupture tissue.
- Static-active stretching: Static-active stretching (sometimes shortened to “active stretching”) involves moving a body segment to a particular position and holding that position using only the agonist muscles of the body segment itself. Static-active stretching increases active flexibility; strengthens agonist muscles, which are being contracted; and relaxes antagonist muscles, which are being stretched. Raising one’s leg high and holding it in position using only the leg muscles is an example of a static-active stretch.
- Static-passive stretching: Static-passive, or “static,” stretching is similar to static-active stretching, except that it incorporates an external force to hold the body segment at the end of its range of motion rather than using the agonist muscles alone. An example of static-passive stretching is bringing one’s leg up high and holding it in position with one’s hand.
- Isometric stretching: In isometric stretching, muscles are subjected to forces through an isometric contraction. Isometric contractions cause no appreciable change in the length of muscles. In other words, despite force being applied, there is no movement. An example of isometric stretching is pushing or pulling against an immovable object. In response to the additional force applied during isometric stretching, additional muscle fibers that otherwise would have been at rest are stretched. Isometric stretching increases an individual’s range of motion and helps develop strength in stretched positions.
- Proprioceptive neuromuscular facilitation (PNF): PNF is a postisometric relaxation stretching technique. (Proprioception is a sense of self-awareness regarding body position and limb movement.) During PNF, a muscle group is passively stretched while contracting against resistance. Then the muscle group is passively stretched again through the newly expanded range of motion.
- PNF relies on two neurophysiological mechanisms in addition to resistance and reflexes: irradiation and successive induction. Irradiation refers to the activation or use of nearby muscles or groups of muscles in synergy with the group that first responded to the trainer’s force. This expansion of muscle activity beyond the prime muscle or target muscle group is one of the main principles behind PNF. Successive induction refers to contraction of an agonist muscle group followed by activation of the corresponding antagonist muscle group.
- PNF requires the assistance of a trainer, therapist, or fellow athlete. After an individual moves a body segment into a stretched position, the trainer manipulates the body segment. When responding to the trainer’s force, the individual activates both muscles and reflexes.