Len Kravitz, Ph.D. and Vivian H. Heyward, Ph.D.
Interest in flexibility training has its roots in the early 1900's due to increased orthopedic cases resulting from World War I. Public attention was heightened with the 1950's publication by Kraus and colleagues that American children were unable to successfully execute some flexibility and muscular strength tasks (Kraus & Hirschland, 1954). Those who now proclaim the worth of proper flexibility training include coaches, personal trainers, fitness instructors, medical doctors, physical therapists, and health promotion specialists. The following review is designed to synthesize information, based on past and current flexibility research, for practitioners.
The Nature of Flexibility
Flexibility refers to the total range of motion of a joint or group of joints. Flexibility, which differs from person to person and from joint to joint, encompasses all components of the musculoskeletal system as well as specific neuromuscular pathways of the body. The structural characteristics of the joints and the mechanical properties of the connective tissues of the muscle-tendon structures largely affect the extent of movement around a given joint. The specificity of movement that a person performs in regular physical activities and stretching methods often define the development and improvement of the body's range of motion. The goal of all stretching programs is to optimize joint mobility while maintaining joint stability. Concern should always be focused on the systematic, safe and effective application of the range of motion techniques utilized.
The Benefits of Stretching
An examination of the reported research and empirical evidence support the following benefits of stretching:
1) An increase in functional range of motion (Taylor, Dalton, Seaber, & Garrett, 1990).
2) Reduction of low back pain and injury (Bach, Green, & Jensen, 1985; Farfan, 1973).
3) Reduction in the incidence and severity of injury (Safran, Garrett, Seaber, Glisson, & Ribbeck, 1988).
4) Improvement in posture and muscle symmetry (Corbin & Noble, 1980).
5) Delay in the onset of muscular fatigue (DeVries & Adams, 1972).
6) Prevention and alleviation of muscle soreness after exercise (DeVries, 1961).
7) Increase in the level of certain skills and muscular efficiency (Beaulieu, 1980).
8) Promotion of mental relaxation (DeVries, Wiswell, Bulbulion, & Moritani, 1981).
9) An opportunity for spiritual growth, meditation and self-evaluation (Alter, 1988).
10) Personal enjoyment and gratification.
Flexibility training has more recently been recognized amongst the other components of fitness as a means to better unify one's mind, body and spirit. Taking a similar approach to the harmony of the mind, body and spirit in yoga techniques, many health practitioners are using flexibility training as a vehicle to facilitate mental and physical relaxation as well as stress reduction.
Joint Considerations With Flexibility
The points in your body where two or more bones meet are called joints or articulations. There are three types of joints: synarthrodial joints allow no movements (such as in the skull), amphiarthrodial joints allow limited movement (such as in the spine), and diarthrodial joints allow considerable movement (such as in the arms and legs). The diathrodial joints are of greatest concern in flexibility training.
The diarthrodial or synovial joints function is to hold bones securely together while permitting considerable movement. The adjacent ends of the bones are covered with a weight-bearing or articular surface known as the articular cartilage. This cartilage absorbs shock and prevents direct wear on the bone. The composition characteristic of the articular cartilage is something between a solid and a liquid.
The ligamentous sleeve called the capsular ligament is attached firmly to both bones of the joint, enclosing the joint entirely. The capsular ligament is lined with a thin synovial membrane which secretes a synovial fluid into the joint cavity. This synovial fluid provides nourishment to the articular cartilage and serves as a lubricant to the joint. It also converts the compression stress placed upon the joint from physical activity to a hydrostatic stress which limits the potential dangers to the joint.
In addition to the capsular ligament, each joint typically has several other ligaments which serve to help bond the bones together. The ligaments are strong fibrous bands which are made from the same tissue found in the joint capsule. Besides helping to bind the bones, ligaments serve to prevent dislocation, and limit some ranges of movement.
Factors that Influence Flexibility
Distinctive connective tissues associated with any joint contribute to joint flexibility. With the muscles relaxed, and reflex mechanisms minimally involved, Johns and Wright (1962) have found the relative contributions of soft tissue to joint stiffness to be the following: joint capsule, including ligaments (47%), muscles and their fascial sheaths (41%), tendons (10%), and the skin (2%). Other factors which influence flexibility are:
1) Age. There tends to be a decrease in flexibility with aging (Chapman, 1971). This is largely attributed to a loss in elasticity in the connective tissues surrounding the muscles which go through a normal shortening process resulting from a lack of physical activity. Due to this loss of joint mobility, older persons are more susceptible to injury from vigorous physical activity. Regular exercise, including stretching exercises, can minimize the effect of this age- related decrease in range of motion.
2) Gender. Females tend to be more flexible than males of the same age throughout life (Holland, 1968). This difference is generally attributed to anatomical variations in joint structures.
3) Type of joint. It is very well established that flexibility is specific to each joint. For example, trained dancers demonstrate superior flexibility of the ankle and legs but only moderate flexibility in their upper torsos. The degree of range of motion at the joint is also affected by joint structure (e.g., ball and socket, hinge, condyloid) and the type of movement that the joint exhibits (i.e., flexion-extension, rotation, adduction-abduction, pronation-supination, protraction-retraction and circumduction).
4) Exercise history. Participation in regular exercise involving full range of motion generally enhances flexibility; on the other hand, a sedentary lifestyle often results in diminished flexibility (Beaulieu, 1980).
5) Temperature. An increase in body temperature via a warm-up or the participation in physical activity will increase range of motion (Sapega, Quendenfild, Moyer, & Butler, 1981). A lowering in body temperature is associated with a decrease in flexibility.
6) Body build. The evidence leads one to conclude that factors including arm and leg length, arm span, height, and weight do not significantly affect range of motion (Alter, 1988).
7) Resistance training. Resistance trainingin which exercises are executed through a full range of motion may help to improve a person's flexibility (Massey & Chaudet, 1956; Wickstrom, 1963).
8) Pregnancy. During pregnancy, the pelvic joints and ligaments are relaxed and capable of greater range of motion (Bird, Calguneri, Wright, 1981). The hormone responsible for this change in range of motion is relaxin. After pregnancy, relaxin production decreases and the ligaments tighten up.
Warm-up vs. Stretching
The warm-up and stretching portion of a class should not be confused. The warm-up is physical activity that raises the temperature of the blood, muscles, tendons and ligaments. The goal is to prepare the body's freely moveable joint structures for vigorous physical activity while reducing the risk of injury (Safran et al., 1988). The warm-up is best accomplished with a full-body rhythmic activity such as low-to-moderate intensity aerobics, stationary cycling, walking or jogging. This segment, approximately 5 minutes in length, should be intense enough to increase body temperature, but not so demanding as to lead to fatigue. Often included after this full-body movement phase of the warm-up are some stretching exercises that go through a functional range of motion, holding positions usually no longer than 10 seconds.
Stretching exercises, to increase range of motion, are best presented after the cardiovascular cool-down or after the muscle toning section of class. The temperature of the soft tissues is most likely elevated, making this time in the workout ideal for increasing flexibility.
Methods of Stretching
The types of stretching programs commonly used are classified in four general categories: passive, ballistic, static and proprioceptive neuromuscular facilitation (PNF). Passive stretching techniques are usually performed with an outside force such as a towel or partner who applies a stretch to a relaxed joint. Partner stretching requires close communication between partners, and the slow application of the stretch in order to prevent injuries due to rigid or forceful manipulation of the body segment. Ballistic stretching was quite popular in the 1970's, but is used primarily by athletes due to a greater risk of injury and lesser efficiency compared with other stretching techniques. With ballistic and passive stretching there is a need to control numerous factors to insure safety, limiting the applications of these techniques.
At present, the two most accepted methods of improving flexibility are the static and PNF techniques. To date, neither technique has been demonstrated to be superior for improving range of motion. Each method operates on the premise that to increase flexibility and prevent risk of injury, the muscle being stretched should be as relaxed as possible.
Static, or hold stretching, is probably the most commonly used flexibility technique and is very safe and effective. With this technique, a muscle or muscle group is gradually stretched to the point of limitation, and then typically held in that position for a period of 15 to 30 seconds. Taylor (Taylor et al., 1990) reported significant improvement in flexibility using four sets of 15-20 seconds per stretch.
PNF stretching techniques are also very effective for increasing flexibility. The PNF techniques were developed by Dr. Herman Kabat in the 1950's as part of his therapeutic work with patients suffering from paralysis and muscular diseases. In the early 1970's, Holt introduced modifications of Kabat's work that were adopted by several athletic teams (Holt, Travis, & Okita, 1970). Over the years these PNF concepts and modifications, when carefully introduced, have been applied by many personal trainers and fitness instructors with their students. With proper instruction, the PNF techniques have also been shown to be safely implemented with students (Kravitz, 1980).
Two commonly used PNF stretching techniques, contract-relax and contract-relax agonist contract, may be readily modified and used either individually or with a skilled partner. In the first phase of both techniques, the target muscle group is placed on stretch. The next phase involves a less than maximal voluntary contraction in the pre-stretched muscle group for 4 to 6 seconds. The contraction is "isometric" because movement of the body segment is resisted by the individual or partner. In the third phase of this technique, the contracted muscle group is first relaxed, and then stretched to a new point of limitation. With the contract-relax agonist contract technique, the client now contracts the opposing muscle(s) for 4 to 6 seconds against a resistance. In the final step, the agonist contraction is released and the target muscle group is taken to a final stretch. Researchers have found the contract-relax agonist contract technique to be superior to the contract-relax technique for improving range of motion (Etnyre & Lawrence, 1988).
Sensory Response to Stretching
When a muscle is stretched, receptors within the muscle, known as muscle spindles are stimulated, and send a message to the spinal cord that the muscle is being extended. If the muscle is overstretched, or stretched too fast, the spinal cord sends a reflex message to the muscle to contract. This is a basic protective mechanism, referred to as the stretch reflex, to help prevent over-stretching and injury. This reflex helps to explain the risk in ballistic stretching. The speed of bouncing during ballistic stretching may elicit an equally responsive contraction of the muscle, leading to strain in the musculotendinous area and microscopic tearing of muscle fibers.
Located in the musculotendon junction is another sensory receptor called the golgi tendon organ. When excessive tension (force) is created in the muscle from either a deep stretch or a muscle contraction, the golgi tendon organ triggers a reflex known as the inverse stretch reflex. This reflex inhibits muscle contraction and relaxes the muscle. Thus, the golgi tendon organ is part of a defense mechanism which prevents the muscle from developing too much tension, which may otherwise lead to injury.
In short, the muscle's sensory receptors (muscle spindle and golgi tendon organ) provide a means of monitoring and maintaining an optimal and safe operating range of motion for the muscles. The muscle spindle causes a stretch reflex making the muscle contract when too much stretch or too fast of a stretch is initiated, while the golgi tendon organ produces an inverse stretch reflex which relaxes the muscle when too much tension is being produced.
Research has noted that the stretch reflex can be inhibited in muscles that are passively stretched (Basmajian, 1985). Subjects have been able to relax muscles consciously when normal stretch reflexes would be expected to occur. Thus, instructors may help students stretch by encouraging them to focus on relaxing the muscles before and during a stretch.
A little-known factor about stretching, according to Siff (1992), is that any form of stretching that exerts pressure on the soles of the feet or palms of the hands will produce a strong reflex extension of the limb(s) concerned. This is known as a positive supportive reaction, and it serves to stabilize the limb. This reaction is one neurological reason to avoid forward unsupported stretches.
Technique in Stretching
The results of a recent study demonstrate the importance of technique in a stretch. Sullivan et al. (1992) found that the effect of the pelvic position, i.e., anterior pelvic tilt vs. the posterior pelvic tilt, in a hamstring stretch significantly affected the range of motion at the hip joint. (The anterior pelvic tilt proved to be the preferred anatomical position.) The implications of this research are clear. Instructors who are knowledgeable in anatomy and kinesiology of muscle attachments and joint movements may have greater success in designing flexibility programs for their students.
Alter (1988) defines a flexibility training program as a planned, deliberate and regular program of exercises that can progressively and permanently increase the usable range of motion of a joint or set of joints, over a period of time. Although stretching techniques are continually evolving, presently there are no universally agreed upon guidelines to follow for prescribing the type, duration, and number of repetitions of any given stretching technique. It is certain that for flexibility to increase, careful application of a slow and progressively increasing stretch just past the point of limitation, but not to the point of pain, is necessary. Depending on a client's fitness level, goals, limiting factors of flexibility, and other exercise program participation, an individualized flexibility program can be designed using the following guidelines:
1. Assess your client's flexibility in orderto pinpoint strengths and weaknesses.
2. Design a program that stretches the specific muscles used by the client during physical activity or sport participation.
3. Warm-up before stretching to increase the body temperature and range of motion.
4. Perform stretching exercises daily.
5. Stretch all the major muscle groups as well as opposing muscle groups.
6. Focus on the muscles involved in the stretch, minimizing the movement of other body parts.
7. Hold stretches between 15 and 30 seconds. Research suggests that four sets of 15 to 20 seconds per stretch will result in optimal gains (Taylor et al.,1990).
8. Stretch to the limit of movement, not to the point of pain. This is referred to as the "endpoint" of the stretch.
9. Keep the breathing slow and rhythmical while holding the stretches.
10. Stretch the muscles in various positions, as stretching in different planes may enhance muscle relaxation and improve overall range of motion at the joint.
11. Attempt to relax the target muscle before going into the stretch.
12. Stretch after each vigorous workout to reduce the potential of delayed-onset muscle soreness and to encourage mind and body relaxation.
13. If the stretch yields pain in the joint area, back off the movement and make sure the stretching technique is correct. It may be necessary to try another position or a different stretching exercise for the target muscles.
Alter, M. J. (1988). Science of stretching. Champaign: Human Kinetics Books.
Armstrong, R. B. (1984). Mechanisms of exercise-induced delayed onset muscular soreness: a brief review. Medicine and Science in Sports and Exercise, 16, 529-538.
Bach, B. K., Green, D. S., & Jensen, G. M. (1985). A comparison of muscular tightness in runners and non-runners and the relation of muscular tightness to low back pain in runners. Journal of Orthopedic Sports Physical Therapy, 6, 315-323.
Basmajian, J. (1985). Muscles alive. Baltimore: Williams and Wilkins.
Beaulieu, J. E. (1980). Stretching for all sports. Pasadena: Athletic Press.
Bird, H. A., Calguneri, M., & Wright, V. (1981). Changes in joint laxity occurring during pregnancy. Annals of the Rheumatic Diseases, 40, 209-212.
Chapman, E. A. (1971). Effects of exercise upon joint mobility of young and old men. Doctoral Dissertation, University of Southern California.
Corbin, C. B., & Noble, L. (1980). Flexibility: A major component of physical fitness. The Journal of Physical Education and Recreation, 51, 23-24, 57-60.
DeVries, H. A. (1961). Electromyographic observations of the effects of static stretching upon muscular distress. Research Quarterly, 32, 468-479.
DeVries, H. A., & Adams, G. M. (1972). EMG comparison of single doses of exercise and meprobamate as to effects of muscular relaxation. American Journal of Physical Medicine, 51, 130-141.
DeVries, H. A., Wiswell, R. A., Bulbulion, R., & Moritani, T. (1981). Tranquilizer effect of exercise. American Journal of Physical Medicine, 60, 57-66.
Etnyre, B. R., & Abraham, L D. (1988). Antagonist muscle activity during stretching: a paradox re-assessed. Medicine and Science in Sports and Exercise, 20, 285-289.
Farfan, H. F. (1973). Mechanical disorders of the low back. Philadelphia: Lea and Febiger.
Holland, G. J. (1968). The physiology of flexibility: A review of the literature. Kinesiology Review, 49-62.
Holt, L. E., Travis, T. M., & Okita, T. (1970). Comparative study of three stretching techniques. Perceptual and Motor Skills, 31, 611-616.
Johns, R. J., & Wright, V. (1962). Relative importance of various tissues in joint stiffness. Journal of Applied Physiology, 17, 824-828.
Kravitz, L. R. (1980). A comparative analysis of variable time-controlled proprioceptive neuromuscular facilitation slow reversal-hold-relax stretching to active assistive stretching. Masters Thesis, San Jose State University.
Kraus, H., & Hirschland, R. P. (1954). Minimum muscular fitness tests in school children. Research Quarterly, 25, 178-188.
Massey, B. A., & Chaudet, N. L. (1956). Effects of systematic, heavy resistance exercise on range of movement in young adults. Research Quarterly, 27, 41-51.
Safran, M. R., Garrett, W. E., Seaber, A. V., Glisson, R. R., & Ribbeck, B. M. (1988). The role of warmup in muscular injury prevention. The American Journal of Sports Medicine, 16, 123-129.
Sapega, A. A., Quendenfild, T. C., Moyer, R. A., & Butler, R. A. (1981). Biophysical factors in range-of-motion exercises. The Physician and Sportsmedicine, 9, 57-65.
Siff, M. C. (1992). Biomechanical analysis and modeling of components of flexibility and exercise stretching. International IDEA Research Symposium. Las Vegas, NV.
Sullivan, M. G., Dejulia, J. J., & Worrell, T. W. (1992). Effect of pelvic position and stretching method on hamstring muscle flexibility. Medicine and Science in Sports and Exercise, 24, 1383-1389.
Taylor, D., Dalton, J. D., Seaber, A. V., & Garrett, W. E. (1990). Viscoelastic properties of muscle tendon units - the biomechanical effects of stretching. American Journal of Sports Medicine, 18, 300-309.
Wickstrom, R. L. (1963). Weight training and flexibility. Journal of Health, Physical Education and Recreation, 34, 61-62