|Stretching: A Research Retrospective
Len Kravitz, Ph.D.
A primary function of muscles is to create tension and produce force for movement of the body's skeleton system. The intrinsic property of muscles and joints to go through a full or optimal range of motion is referred to as flexibility. It is developed through the use of various stretching procedures. Presently, uncertainty exists about some proposed benefits of flexibility including its effect on injury avoidance, muscle soreness prevention, muscular strength training and performance improvement. This review article will attempt to clarify these issues with existing evidence-based science and present a current research update on this component of fitness.
Determinants of Flexibility
Andersen (2006) suggests that the foundational determinants of flexibility are a multi-factorial cluster of elements . Recognizing and understanding these determinants of flexibility will help the exercise professional better determine the adequate flexibility goals for each client. It is also acknowledged that flexibility is a characteristic that is specific to each joint or group of joints, and thus not an absolute characteristic of all of the joints of the body. Further inspection of the association of age, gender and physical activity to flexibility is relevant to exercise professionals.
Age and Flexibility
Flexibility has been shown to decrease up to 50% in some joint areas with chronological age. From a population base of 1000 elderly men and women, Bassey et al. (1989) showed that shoulder abduction was reduced gradually and consistently with age, and is about 25% less than norms from a younger population. Eingauf et al. (1987) examined the changes in spinal mobility for 109 women aged 20 to 84 years. The results of this study indicated that spinal mobility decreased by 20%, 33% and 50% for anterior flexion, lateral flexion and extension, respectively. Brown and Miller (1998) showed that sit-and-reach range of motion decrease approximately 30% for women 20 to 70 plus years of age. Buckwalter (1997) proposes that a gradual deterioration in cell function in the cartilage, ligaments, tendons and muscles with age is the mechanism for this loss of flexibility. Misner and colleagues (1992) add that collagen, a main constituent of connective tissue, becomes dense (and stiffer) with aging. However, Bassey and associates suggest that this loss of motion can be minimized with regular stretching and range of motion exercise.
Gender and Flexibility
It has been shown that due to minor differences in joint structures and connective tissue anatomy, women have slightly greater range of motion than men for most joint motions. With a sample of 190 male and female subjects ranging in age from 18 to 88 years, Bell and Hoshizaki (1981) measured 17 joint actions in 8 specific joints. It was found that women did have greater overall flexibility then males. In assessment of the upper body joints (shoulder, elbow, wrist, trunk and neck) of a group of 41 subjects (22 young male and female subjects aged 25 to 35 years and 19 mature male and female subjects aged 65 to 80 years), Doriot and Wang (2006) also found females to have significantly greater range of motion in several joint actions. However, Doriot and Wang note that the effect of gender on range of motion is much less than that of age.
Physical Activity and Flexibility
For the most part, physically active individuals have greater flexibility in the joints they regularly utilize as compared to their physically inactive counterparts. Voorrips et al. (2009) confirmed with a population of 50 mature women (mean age 71 years) that those subjects who regularly did more walking (and more physically activity) had greater flexibility in the hip and spine (as assessed by the sit-and-reach test) than their less active counterparts. Kerrigan et al. (2001) declare these data suggest a very meaningful application with fall prevention. The researchers showed that hip tightness is associated with more falls, when comparing 16 elderly (8 men and 8 women; average age = 77 years) subjects with a history of falling to 23 healthy non-fallers (10 men and 13 women; average age = 73 years). The authors specifically recommend hip extension stretching as a necessary intervention for fall prevention in this population. Misner and colleagues (1992), in a 5-year long-term study with 12 women aged 50-71 years showed that regular exercise (15-30 minutes of stretching and 30-60 minutes of walking or water aerobics) 3 times per week for 5 years increased shoulder and hip range of motion significantly (3% - 22% in various joint actions). The authors add that exercise also helped the subjects perform activities of daily living much more efficiently. Indeed, ACSM (2006) recommends that preventive and rehabilitative exercise programs should include activities that promote the maintenance of flexibility.
What are the stretching methods to increase flexibility?
There are several known methods (and variations within each method) to increase flexibility including passive stretching, static stretching, ballistic stretching, dynamic stretching, proprioceptive neuromuscular facilitation (PNF) techniques, contract-relax stretching and resistance stretching.
Passive stretching is usually performed with a partner who applies a sustained stretch to a relaxed joint. Partner stretching requires close communication between client and exercise professional, and a slow application of the stretch in order to prevent a forceful manipulation of the body segment and possible injury.
Ballistic stretching involves a bouncy approach to reach the target muscle's endpoint of motion. A concern with ballistic stretching usage is that it is often performed in a jerky, bobbing fashion that may produce undesirable tension or trauma to the stretched muscle and associated connective tissues. It may produce a potent stretch reflex (see Side Bar 1. Question #10 for detailed explanation) that will oppose the muscle lengthening.
Dynamic stretching incorporates active range of motion movements that tend to resemble sport-specific or movement-specific actions. For instance, a volleyball player might do some shoulder flexion/extension actions prior to playing in a volleyball game. Thus the rhythmic nature of a controlled dynamic stretch has a functional application due to its similarity to the primary movement task. Dynamic stretching is often incorporated in the 'active' phase of Group X class warm-ups.
Static, or hold stretching is probably the most commonly used flexibility techniques and is very safe and effective. With this technique, a muscle or muscle group is gradually stretched to the point of limitation (a mild even strain), and then typically held in that position for a period of 15 to 30 seconds.
PNF stretching 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 (Sharman, Cresswell, and Rick, 2006). There are several variations of PNF stretching. The contract-relax method involves initially contracting the target muscle followed by relaxing and stretching the target muscle with an assist from a partner or applied force such as a towel or rope. A variation of the contract-relax method is to perform a contraction of the OPPOSING muscle (muscle on opposite side of joint) during the stretching phase of the target muscle to take it to a new endpoint of motion (further increase in stretch). This method is referred to as the contract-relax agonist-contract method (Sharman, Cresswell, and Rick, 2006). Importantly, traditional PNF techniques involve doing the stretches in diagonal or spiral motions to promote movement through various planes of motion. See Focus Box 1 to read about the proposed neurophysiological mechanisms of PNF stretching.
Contract-relax techniques follow the same 'contract-relax' methodology of PNF stretching, although the movement pattern tends to involve a single-joint motion through one plane.
Resistance stretching has gained much media attention and exercise professional interest. This is the stretching/strengthening technique that 4-time Olympic swimmer sensation Dara Torres endorses as an important component of her training success. Resistance stretching focuses on contracting the target muscles as they are lengthened. In the first phase of this technique, the target muscles are placed in the shortened position. In this position the client CONTRACTS the target muscle(s). While contracted, the muscle is taken through a full range of motion (lengthened). So, resistance stretching incorporates a strengthening component through the entire range of motion. In essence, it is a carefully performed ECCENTRIC contraction (with a skilled professional). The originators of resistance training incorporate some very detailed rotational patterns (to challenge the muscle in multiple planes) with this particular technique. For more information and resources on resistance stretching the IFJ reader is referred to this web site: http://www.innovativebodysolutions.com/
Controversial Issues of Flexibility
The most controversial issues with flexibility include injury avoidance, muscle soreness prevention, muscular strength training and performance improvement. Several hundred studies have been conducted on these topics from randomized, controlled studies (the most powerful form of evidence) to those from general observation (the least certain type of evidence). To specifically address these controversial questions, recent review articles published in influential peer-reviewed publications were selected for this article. The efficacy of this approach is that review articles rigorously evaluate and summarize findings across a number of scientific studies and provide the overall 'state of knowledge' on a specific topic.
The impact of pre-exercise stretching on injury risk
Perhaps one of the most exhaustive and comprehensive research reviews on the impact of stretching and sport injury risk was completed by Thacker et al. (2004). The authors conclude that pre-exercise stretching does not prevent injury among competitive or recreational athletes. Thacker and colleagues add that the theories explaining why pre-exercise stretching prior to exercise may not prevent injury propose there is an alteration in joint connective tissue compliance (ability of the tissue to extend appropriately in response to applied pressure). In some cases this alteration may lead to greater joint instability. Thacker and colleagues point out that studies incorporating a pre-exercise combination of resistance exercise, body conditioning and warm-up show promise for better injury prevention. Perhaps this will be a new direction for fitness professionals to pursue.
The impact of stretching on preventing or reducing muscle soreness
Herbert and de Noronha (2009) summarize that stretching before and after exercise has not been shown to impart any additional protection from muscle soreness. Therefore, stretching does not lesson some of the mechanisms of muscle soreness including damage to the ultra structure of muscle, accumulation of calcium ions, cell inflammation, swelling and activation of pain receptors.
The impact of stretching on muscular strength
Although stretching and strength training are recommended as part of any comprehensive training program, better ways to include them in a training session are now being realized. When viewing the acute (immediate) effects of stretching before strength training, Rubini, Costa and Gomes (2007) note that the research indicates that static and PNF stretching (which is what most of what studies have utilized) have shown decreases in MAXIMAL strength from 4.5% to 28%. Yet most of this research has used more than one stretching exercise for the same muscle group with total stretching times of 120 to 3600 seconds, which is much more than recommended for optimal flexibility increases (according to ACSM (2006), 4 stretches of 30 seconds totaling 120 seconds is optimal). Rubini, Costa and Gomes add that when the total flexibility session is shorter (from 30 seconds to 480 seconds) the research shows little or no compromise from stretching right before maximal force production. Importantly (and practically), exercise enthusiasts do not train daily to their maximal voluntary contraction, where compromises in strength are observed in the research. Interestingly, Rubini and colleagues highlight there is no scientific consensus in the research for the underlying mechanism explaining the force production loss in muscle after stretching.
The impact of stretching on performance
The studies investigating flexibility and sport performance center attention on the areas of jumping ability, torque (rotary force), running economy and maximal force production. Shrier (2004) reviewed 23 studies, which when combined have included static, PNF, and ballistic stretching techniques with both genders (from children through adults) of untrained individuals to highly competitive athletes. The findings of this research, which is supported by other reviews (Haff, 2006), reveals that regular stretching, when performed at times other than BEFORE performance, may elicit positive long-term performance outcomes. However, pre-performance stretching may educe insignificant or negative performance outcomes.
This article symbolizes an important triumph for applied research. For many decades, coaches, athletes and others have touted numerous benefits of flexibility. As seen in similar disciplines, the empirical beliefs of key pioneers often guide the field. However, as observed with flexibility, many of these beliefs have not been shown to be accurate when challenged through the benchmark of scientific investigation. This does not minimize the importance of flexibility as a component of fitness, yet it better directs the exercise professional how to incorporate it into program designs for clients.
Focus Box 1. Proposed mechanisms of PNF stretching: A controversy continues!
The proposed mechanism of PNF stretching involve two neurophysiological phenomenon referred to as autogenic inhibition and reciprocal inhibition (Sharman, Cresswell, and Rick, 2006). Autogenic inhibition, also called the 'reverse myotatic reflex,' refers to a reduction (or inhibition) in excitability of a contracting muscle. This inhibitory input comes from the sensory receptors in the tendons (of the target muscle) known as the Golgi tendon organs. Thus, in the contract phase of the contract-relax PNF stretch, the target muscle (to be stretched) is contracted, which elicits autogenic inhibition. This is followed by a stretch to the target muscle (thus having inhibited neural input and allowing for greater range of motion). Reciprocal inhibition involves the 'agonist-contract' phase of the contract-relax agonist-contract PNF method. When the opposing muscle contracts, it is felt that inhibitory messages are signaled to the target muscle to relax (thus allowing for a lengthened stretch). Therefore, the thought is the more the muscle can be relaxed through autogenic inhibition and reciprocal inhibition, the more the muscle can be lengthened, and the greater the gains in flexibility (Haff, 2006).
The controversial plot 'thickens'
In a contemporary review article, Gordon Chalmers (2004) argues that the historical explanation of autogenic and reciprocal inhibition are convenient explanations, but recent decades of research denote that a much more complex neuromuscular response is in play. He states that the research shows that possible viscoelastic properties of muscle (see Focus Box 2 for more on viscoelastic) and an unexplained phenomenon known as 'stretch tolerance' are possible mechanisms observed with acute changes in range of motion from PNF stretching. Shrier (1999) continues that stretching may elicit an 'analgesic effect', resulting in an increased pain threshold (or pain tolerance) without actual changes in muscle stiffness to stretch. This 'analgesic effect' hypothesis is referred to as 'stretch tolerance'. Clearly more sophisticated research is needed to identify and understand this new hypothesis with stretching.
Focus Box 2. What are the 'viscoelastic' properties of muscle?
Muscle is composed of viscous (sarcoplasm), elastic (muscle filaments) and nonelastic (connective tissue) fibers. The elastic properties of muscle are very similar to a spring (Chalmers, 2006). A spring will lengthen in direct proportion to an applied force, and return to its natural resting length upon release of the force. When a muscle is stretched, the passive resistance to the elongation of the stretch is referred to as 'stress relaxation.' With sustained holding of the stretch, the muscle will gradually elongate, a viscoelastic property known as 'creep.' Clalmers explains that when a stretch is sustained, there is a reduction (about 30%) in the 'stress relaxation' (thus a transitory relaxation in muscle tension). Woods, Bishop and Jones (2007) underscore that the greater increases in muscle length are within the first 15 minutes after completing three, 30-second static stretches. Viscoelastic properties of muscle show that a muscle will provide its greatest resistance to stretch when the stretch is applied rapidly; thus the scientific rational for slow stretching of a muscle (Chalmers, 2006). With regular, long-term stretching there are proposed viscoelastic changes in the surrounding connective tissue and tendon (attached to the target muscle), which slightly increase the elasticity of the muscle-tendon unit (Kubo, et al. 2001). De Deyne (2001) concludes that other cellular and molecular biological changes occur in muscle including the addition more sarcomeres (structural unit of a muscle cell) to muscle.
Side Bar 1. Frequent Questions and Answers on Flexibility
1) How long should you hold a stretch for flexibility improvement? In review of the existing scientific literature, ACSM (2006) recommends holding a stretch from 15 to 30 seconds.
2) What are the optimal times to repeat a stretching? According to ACSM (2006), two to four repetitions is optimal as further repetitions do not elicit additional benefits.
3) How many days per week should someone stretch? The range of motion demands of each person differs, but ACSM (2006) suggests 2 to 3 days per week as a minimum. ACSM continues that 5 to 7 days per week of some type of stretching routine would be the 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? The specialized nerves that communicate information about the musculoskeletal system to the central nervous system are called proprioceptors. Proprioceptors (also called mechanoreceptors) are the source of all proprioception, which is the perception of one's own body position and movement. Proprioceptors are 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 these range of motion techniques, Thacker et al. (2004) noted that all methods have been shown to be very effective in improving range of motion with no clear best method. In fact, several studies show PNF to be superior to static and dynamic where as other studies have show equal effectiveness of several stretching methods (Haff, 2006). However, Sharman, Cresswell, and Rick (2006)
contend that since PNF stretching improves passive and active range of motion, 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 tendon of muscle-tendon units) only respond to high forces. In fact, the Golgi tendon organs are sensitive to very low forces, and a contraction of as little as 20% to 70% of maximal contraction will suffice (Sharman, Cresswell, and Rick, 2006). The lower intensity of contractions will help to reduce the risk of any type of injury from the PNF stretching.
8) Does the research suggest the best application of PNF stretching? Evidence-based research provides the following recommendations (Sharman, Cresswell, and Rick, 2006).
a. Static contraction duration of the target muscle; 3 to 15 seconds
b. Contraction intensity of target muscle from 20%-70% (see #7 for explanation). Sharman, Cresswell, and Rick (2006) note that there is evidence that progressive increases of intensity (within the 20%-70% range) may provide greater gains in range of motion.
c. Opposing muscle contraction intensity: no studies have investigated this phase of PNF stretches
d. One complete repetition seems to be sufficient (i.e., ONE contract-relax or ONE contract-relax agonist-contract)
9) Will heat packs before stretching enhance the range of motion? Knight and colleagues (2001) compared active stretching, static stretching, hot packs (superficial heat before stretching), and ultra sound (deep heat before stretching) of the plantar-flexor muscles with ninety-seven (59 women, 38 men) subjects who had limited dorsiflexion range of motion. All groups increased active and passive range of motion with the deep heat intervention being the most effective.
10) Please explain 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 muscle contraction. 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.
Side Bar 2. Warm-up versus stretching: Let's clear up the confusion
The workout warm-up and flexibility (stretching) component of a workout should not be confused. Warming up before exercise is the essential preparation needed to augment the workout. Thacker and colleagues (2004) state that the warm-up is physical activity that increases blood flow (vasodilatation) to working tissues, velocity of nerve impulses to muscles, and delivery of oxygen and foodstuffs for energy liberation. This increase in blood flow also enhances the removal of waste products from muscle. Combined altogether, Thacker et al. submit that these changes prepare the body for vigorous exercise by enhancing the muscle's metabolic (reactions to make ATP) properties and enhancing the mechanical efficiency of muscle contraction and force production. Safran et al. (1988) demonstrated (from their research with an animal model) that warming up is an imperative pre-exercise injury prevention component because it increases the elasticity of the muscle-tendon unit. Contrariwise, the purpose of stretching, as described throughout this article, is to increase of range of motion about a joint and group of joints. Woods, Bishop and Jones (2007) state that the warm-up should be at a relative intensity of 40-60% of a person's VO2max, perhaps inducing some mild sweating without fatigue.
Side Bar 3. Flexibility Guidelines for the Exercise Professional
Here are some useful stretching guidelines for exercise professionals to consider with their clients' stretching programs. Note, some guidelines are NOT appropriate all methods. For example, the resistance stretching technique involves muscle contracting throughout the range of motion, which is quite different from traditional stretching methods.
1. Assess your client's flexibility in order to pinpoint strengths and weaknesses.
2. Design a stretching program that enables a client to boost her/his physical activity and/or sport participation.
3. Make sure the muscles are appropriately warmed up before taking them through any muscle stretching technique.
4. Perform stretching minimally 2 to 3 times per week, and ideally 5 to 7 days per week.
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.
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 stretches. Instruct the client to EXHALE slowly as they extend to the endpoint of the stretch. Explain that as a person exhales the diaphragm muscle and thoracic cavity muscles are relaxing, thus promoting a more effective relaxation of the target muscles during the stretch.
10. Stretch the muscles in various positions, as stretching in different planes may improve the overall range of motion at the joint.
11. Incorporate some stretches that attempt to relax the target muscle before going into the stretch. This may be accomplished by taking the muscle out of a weight bearing and/or body stabilization position prior to the stretch.
12. Stretch after each vigorous workout 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 (or method) for the target muscles.
Len Kravitz, Ph.D., is the Program Coordinator of Exercise Science and Researcher at the University of New Mexico where he recently won the "Outstanding Teacher of the Year" award. Len was honored with the 1999 Canadian Fitness Professional International Presenter of the Year and the 2006 Canadian Fitness Professional Specialty Presenter of the Year awards and chosen as the American Council on Exercise 2006 "Fitness Educator of the Year.
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