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Kravitz: Top 10 Flexibility FAQs

A primary function of muscles is to create tension and produce force for movement of the body’s skeletal system. The intrinsic capacity of muscles to go through a full or optimal range of motion is called flexibility. It is developed through the use of various streching procedures.

What type of stretching should you do with your clients? How long should they hold a stretch? What about PNF stretching? Here are some answers to frequently asked questions about different aspects of stretching.

1. How long should you hold a stretch for flexibility improvement?
The American College of Sports Medicine (ACSM 2006) recommends holding a stretch from 15 to 30 seconds.

2. What is the optimal number of times to repeat a stretch?
According to ACSM (2006), 2-4 repetitions is optimal, as further repetitions do not elicit additional benefits.

3. How many days per week should someone stretch?

Each person differs, but ACSM (2006) suggests 2-3 days per week as a minimum, although 5-7 days per week of some type of stretching routine would be ideal for most persons.

4. What is hypermobility syndrome?
Hypermobility syndrome is a congenital (present at birth but not necessarily hereditary) laxity of some ligaments and joints. It occurs most frequently in the knees, elbows, wrists, hands and ankles (Adib et al. 2005).

5. What are proprioceptors?
They are specialized nerves that communicate information about the musculoskeletal system to the central nervous system. Proprioceptors (also called mechanoreceptors) are the source of all proprioception, which is the perception of one’s own body position and movement. Found in all nerve endings of the joints, muscles and tendons, the proprioceptors related to stretching are located in the tendons (Golgi tendon organs) and in the muscle fibers (muscle spindles).

6. What is the best flexibility method?
In a review of 27 peer-reviewed studies on range of motion (ROM) techniques, Thacker et al. (2004) noted that all methods have been shown to be very effective in improving ROM, with no clear best method. Several studies have found proprioceptive neuromuscular facilitation (PNF) superior to static and dynamic stretching, whereas other studies have shown a number of stretching methods to be equally effective (Haff 2006). Sharman, Cresswell and Riek (2006) contend that since PNF stretching improves passive and active ROM, it may provide additional functional benefits.

7. When doing PNF stretches, do you maximally contract the target muscle?
Conventionally, maximal contractions have been recommended because it was felt that the Golgi tendon organs (receptors in the tendons of muscle-tendon units) respond only to high forces. In fact, the Golgi tendon organs are sensitive to very low forces, and a contraction of as little as 20%-70% of maximal contraction will suffice (Sharman, Cresswell & Riek 2006). The lower intensity of contractions will help reduce the risk of any type of injury from the PNF stretching.

8. What is the best application of PNF stretching?

Evidence-based research provides the following recommendations (Sharman, Cresswell & Riek 2006):

  • Duration of static contraction of the target muscle is 3-15 seconds.
  • Contraction intensity of the target muscle is 20%-70% (see #7). The researchers note that there is evidence that progressive increases in intensity (within the 20%-70% range) may provide greater gains in ROM.
  • No studies have investigated opposing muscle contraction intensity for PNF stretches in this phase.
  • One complete repetition seems to be sufficient (i.e., one contract-relax or one contract-relax agonist-contract stretch).

9. Will using heat packs before stretching enhance ROM?

Knight and colleagues (2001) compared static stretching of the plantar-flexor muscles preceded by no warm-up, active exercise, hot packs (superficial heat before stretching) and ultrasound (deep heat before stretching) in 97 subjects (59 women, 38 men) who had limited dorsiflexion ROM. All experimental groups increased active and passive ROM, with the deep-heat intervention being the most effective.

10. What is the stretch reflex?
As a muscle is stretched, so is the muscle spindle (which runs parallel to muscle fibers). The muscle spindle records the change in length (and speed of length change) and transmits this signal to the spinal cord. This triggers the stretch reflex (also called the myotatic stretch reflex), which initially attempts to oppose the change in muscle length by causing the stretched muscle to contract. The more sudden the change in muscle length, the stronger the contraction is. Thus the muscle spindle attempts to protect the muscle from injury. One of the reasons for holding a stretch for a sustained period of time (15-30 seconds) is that the muscle spindle gradually becomes accustomed to the new length and reduces its opposing signaling, thus allowing for greater muscle lengthening.

For the very latest research on stretching, with a complete reference list, please see the full article, “Stretching–A Research Retrospective,” in the online IDEA Library or in the November-December 2009 issue of IDEA Fitness Journal.


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Len Kravitz, PhD

Len Kravitz, PhD is a professor and program coordinator of exercise science at the University of New Mexico where he recently received the Presidential Award of Distinction and the Outstanding Teacher of the Year award. In addition to being a 2016 inductee into the National Fitness Hall of Fame, Dr. Kravitz was awarded the Fitness Educator of the Year by the American Council on Exercise. Just recently, ACSM honored him with writing the 'Paper of the Year' for the ACSM Health and Fitness Journal.

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