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Stretching is not the same as warming up. The purpose of a warm-up is to increase the temperature of the core and muscle tissue. An indication that the body temperature has increased is a slight sweat appearing on the skin. The time prior to class should not be used to increase flexibility. Warm muscles are more extensible and responsive, so it is far better to stretch immediately AFTER class or rehearsal when muscles have been exercised for 1-2 hours. Research has shown that applying a small amount of stretch force to warm connective tissues lengthened them more effectively than larger stretch forces (four times larger) applied to the same tissues at normal body temperature.²⁷ Long-term retention of tissue length lasted more than twice as long when the low-load stretch was applied to warm tissue. Additionally, stretching with higher tissue temperatures, as seen following class or rehearsal, resulted in fewer injuries.²⁷ Holding the stretch while cooling down allows even greater increases in tissue length to be achieved.²⁸

Even when warm, it is not advisable to do major stretching before a demanding class, performance or rehearsal. Stretching has been shown to impair subsequent performance parameters, such as strength,^29,30 power,³¹ endurance,³² balance,³³ grip strength,³⁴ sprint time,³⁵ jumping height,^36-38 reaction time and movement time.³³ Probably of most concern to dancers, the magnitude of the jumping deficit is 5-30%,³⁹ accompanied by a decrease in ankle plantar-flexion strength (25% decline straight after a 30 minute stretch).^30,39 The deficit in jump height does not differ amongst short stretch durations;⁴⁰ however, longer duration of stretch seems to cause progressively more impairment in jump height.³⁸ Jump height decrement was shown to be greatest at 28% straight after a 30 minute stretch. It significantly declined as stretches shortened, to a 4% decrement after a 30 second stretch.^37,40-42 However, even a 15-30 second stretch can cause a decline in muscle force.⁴⁰ These changes within the muscle during a stretch are thought to be both mechanical and neurological.³⁰ Mechanically, a stretched muscle may not transmit force as effectively to the bones.⁴³ Neurologically, a recently stretched muscle may exhibit less activation of the stretch reflex of the muscle’s contractile force.^44,45 Certain positions of stretch compromise nerves more than others, and subsequently interfere with muscle activation. A sign of nerve compromise is a feeling of pins and needles in the toes or fingers during a hamstring stretch or a shoulder stretch, respectively, and stretches that elicit these symptoms should be avoided, especially prior to performing demanding choreography where fine control is important.

Luckily, the detrimental effect of stretching on subsequent muscle performance dissipates over time. The neural activation recovers approximately 15 minutes after a static stretch, but the muscle properties may not recover for up to one hour after a they have been stretched for 30 minutes, according to some researchers.³⁰ Additional activity between stretch and performance has NOT been shown to prevent these stretch-induced problems,^46-48 but this question is still a topic of current research. The effects may also be less detrimental in trained athletes than in untrained recreational athletes.^49-51 There are no gender differences in these performance deficits.⁵¹

In summary, it is safe and effective to move through the body’s normal range of motion before class, rehearsals and before performances. Brief stretches of less than 15 seconds are unlikely to cause performance problems. Dynamic stretches, such as dance movements, are less detrimental to performance than static stretching.⁴⁵ If increasing flexibility is a goal, it should occur over time (at least six weeks) and should occur following class, rehearsal or performance when the muscles are already warm and more likely to make permanent changes to increase flexibility. Research in this area is likely to continue to evolve as we learn more about the effects of stretching and performance.

Stretching is useful for increasing the flexibility of muscles and their associated connective tissues (fascia). The connective tissues in muscle include the muscle cell membrane (sarcolemma), the covering around individual muscle fibers (endomysium), the covering around and between bundles of muscle fibers (perimysium) and the covering around the whole muscle (epimysium). Although some resistance to stretch comes from stable bonds between the muscle’s contractile filaments (actin and myosin),⁵² the majority of the resistance to stretch comes from these fascia layers.^53,54 In order to increase the length of the muscle, thus increasing the flexibility, these connective tissues need to be stretched.

Tendons connect the muscle to bone, producing a lever system within the body to cause movement. They are made up of connective tissue; however, they are bundles of dense connective tissue, which do not stretch as easily as the loose connective coverings surrounding the muscle. Therefore, most of the changes from stretching occur in the muscle itself and the fascia surrounding it, and not in the tendon.⁵⁵

Conversely, it is important NOT to "stretch” (elongate) ligaments and joint capsules. Ligaments and joint capsules are intended to provide stability to the joints. If ligaments and capsules are healthy (not injured), they are the correct length for this purpose. "Stretching” these tissues can permanently elongate them, making a joint hyper-mobile and less stable.⁵⁶ Less joint stability requires muscles to work harder to hold the joint in a stable position, thus making the joint’s movement properties less efficient. For example, the fibular (peroneal) muscles may need to work harder to stabilize an ankle that has become unstable after being over-stretched through repetitive sprains.⁵⁷

Every dancer’s body is different. Some dancers are inherently less flexible. Dancers with "tight” bodies are built for stability, with dense connective tissues. Their muscles are less extensible.⁶³ Conversely, some dancers are innately more flexible. The hyper-mobile physique has an increased risk of injury.^58-62 They have a larger joint range of motion, but they are also more vulnerable to serious ligament sprains.^63,64 Hence "loose” dancers should spend less time stretching than "tight” dancers. Loose dancers should spend that time on strengthening exercises instead. It is important to avoid comparing the flexibility of one dancer with that of other dancers. Instead, work on what each individual dancer needs – strengthening/stabilizing exercise for the loose dancer, stretching/flexibility for the tight dancer. When stretching, it is vital to not push the body too hard. Pushing stretches too hard may cause a muscle strain (tear), so is important to be cautious. Stretches should be held to a point of mild discomfort, not pain.

Holding a static stretch for 30 seconds is enough to maintain joint range of motion and current flexibility. Typically, each stretch is repeated 3 to 5 times in a single stretching bout. But, if increasing flexibility is the goal, then deformation of the connective tissue is necessary to produce permanent muscle length change. When trying to increase flexibility, it is important to employ stretching AFTER dancing or other activity when muscles and connective tissues are warm. Avoid stretching structures that shouldn’t be stretched.

The benefits gained from a single bout of stretching last for a brief period of time, approximately five minutes. In contrast to a single bout, a multi-week program of stretching creates benefits that last for several weeks after stretching is discontinued.^64-66 Stretching just once a week is sufficient to maintain current flexibility.⁶⁷ Other research showed that increasing the frequency of stretch from once per day to three times per day did not increase range of motion.⁶⁸ Additional benefits can be gained by doing up to four repetitions within a bout of stretching.¹³ There appears to be little benefit in doing more than four repetitions of stretch per bout.

In general, females are more flexible than males at the same age.⁶⁹ While it may not be desirable to make males as flexible as female dancers, male dancers need to develop enough flexibility to prevent injuries.

The most comprehensive study of childhood flexibility was performed on 4,500 Baltimore (USA) school children, ages 5-18 years.⁷⁰ Boys and girls performed two flexibility tests: sitting reach for toes and sitting head to knees. For each age, Kendall recorded the percentage of children who could successfully perform each test. On the sitting reach (sitting with knees straight and reaching fingers toward toes), 100% of 5-year-olds could do it. However, by age 12, only 30% of children could do so. The low point in flexibility occurred at age 12 for boys and age 13 for girls. Flexibility then increased again, with approximately 60% success on this test in 18-year-olds. Graphs of more detailed results, with each gender plotted separately at each year, are available in the article for the interested reader. (On the forehead to knees test, very few children of any age could perform the test, unless they had dance or gymnastics training.)

Why is there a loss of flexibility in children of both genders, with minimum flexibility being reached at 10-12 years?⁶⁹ This age coincides with the skeletal growth spurt, so muscle tissues are shorter relative to bone length until muscle growth catches up to bone growth. Dance teachers need to recognize that young dancers will go through a phase of apparent loss in flexibility. During this time there is also an increased chance of injury to muscles.⁷¹ The age at which this occurs in dancers may be different from non-dancers, since female dancers may have delayed onset of puberty. Additionally, age at puberty has also changed in the 60 years since Kendall’s study, tending to occur earlier now.

At the other end of the age spectrum, aged adults also become less flexible with the passing years. Allander et al. found that males lose 1.8 degrees of hip motion and 2.2 degrees in shoulder and wrist motion every five years.⁷¹ These age-related flexibility losses are likely to be because the connective tissue loses elasticity.⁵⁶ There is reduced maximum tensile strength (resistance to stretching before tearing), and the rate of adaptation to stress is lower with age.⁷²