Understanding Flexibility's Role in Influencing Range of Motion

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Understanding Flexibility's Role in Influencing Range of Motion

The debate surrounding flexibility and range of motion appears to be centered on a few factors such as type of stretch (static stretching or PNF/neuromuscular stretching), duration of the stretch required to increase range of motion, and the mechanics of the stretch (how does stretching increase range of motion). Researchers have sought to determine what form of flexibility creates the greatest range of motion around a joint, an understanding of the mechanics of stretching, and an understanding of the duration of stretch required to see a significant change in range of motion.

The majority of the research studies that met the inclusion criteria (see Table 2) point to increased range of motion when using an acute or chronic static stretching program. In fact, according to several studies, when increasing range of motion, PNF stretching (a form of static stretching that uses reciprocal inhibition - contracting the agonist muscle to reduce the motor neuron activity to the antagonist muscle) created the largest gains in range of motion. (2-4) Sady (1982), found PNF stretching superior to all types of flexibility when used to increase range of motion when tested on the trunk, shoulder, and hamstrings. (2) In a study by Decicco (2005), PNF stretching, whether performing the contract-relax-contract technique or the hold-relax-contract technique, created significant range of motion differences over a 6-week span when compared to traditional static stretching. (3) In addition, a study by Schuback (2004), concluded that when tested head-to-head against static stretching implemented in a home program, PNF stretching reigned superior. In this study, they found that self-applied PNF stretching using a contract-relax-contract method increased range of motion more than self applied static stretching. (4)

More research needs to be done to fully understand the mechanisms behind stretching. Current theories equate increased range of motion to structural changes and/or changes in stretch sensation. In fact, the truth may lie somewhere in combination. Some theorize that structural changes take place when performing static stretching such as decreased stiffness of the muscle tendon unit and passive resistive force (5) while others hypothesize that the muscle structure does not actually change, but the stretch sensation leads to increases in range of motion. The greater stretch tolerance might lead to increased range of motion around a joint. (6) Using an examination of muscle stiffness (ratio of change in force to change in angle) as a guide, theoretically, if muscle stiffness increased in combination with increased range of motion, then changes in muscle structure would have occurred. The muscle stiffness theory hypothesized by Reid and McNair in their 2004 published study, found that a six-week hamstring-stretching program performed one time per day, held for 30 seconds, repeated three times, increased knee extension range of motion, passive resistive force, and stiffness. (5) This was also shown in the research of Gadjosik (1991, 2001) (7,8) "who suggested that the increases in hamstring length may be due to possible increases in the number of sarcomeres in series". (5) While these findings require more study, many postulate these changes to be influenced by the intensity of the stretching protocol. In the study performed by Bjorklund, a decrease in stretch sensation of the rectus femoris muscle was found to increase range of motion of knee flexion; there was no increase in range of passive knee flexion, and hence, passive stiffness was thought to be unaffected. (6) This study corresponded to the work of Magnussen (1996) which found evidence of sensory adaptation from stretching. (9) This leads researchers to believe that sensory adaptation is an important factor in increasing range of motion from static stretching. Researchers did not rule out changes in muscle stiffness and stated that changes in sensation may precede changes in stiffness. (6) Of interest is the reference to the muscle being stretched. As discussed in Bjorklund's study, "angle of pennation, cross-sectional area and shortening velocities are important factors for the passive properties of a muscle". (6) Overall, more research on various muscles, intensities, and protocols will help researchers understand the underlying mechanism behind stretching and its role on increasing range of motion.

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While various stretching protocols exist, researchers have begun to understand the effect of stretching duration on increasing ranges of motion. In an important study by Bandy (1994), the duration of holding a stretch was measured against shorter or longer static stretching protocols. Thirty seconds of holding a stretch, once a day, five days a week for six weeks resulted in increased range of motion at a joint. (10) However, in contrast to Bandy's findings, some researchers argue about the implementation of static stretching. In a study done by Bazett-Jones (2005), acute bouts of static stretching (using 3 sets of 30 second holds) showed no effects on increasing range of motion (11) as did the research of Youdas (2003) which challenged the chronic effects of static stretching finding no significant changes in range of motion in the gastrocnemius after implementing a 6-week stretching regimen (30 second duration stretches, performed once a day, 5 days a week). (12) However, it should be mentioned that Bandy used ROM-limited individuals, the other two studies used healthy individuals. Given the differences in research results, it is important to analyze a few different factors that may affect the relationship between static stretching and range of motion.

Although much more research must be done to understand some of the underlying factors behind stretching and increasing range of motion, there are a few determinants that NASM uses to understand the role of static stretching on increasing range of motion. First, is the determination of muscle imbalances that may affect range of motion. In a study done by Clark (1999), researchers found that when testing the range of motion of the hamstrings, stretching the opposite muscles, the quadriceps, increased range of motion at the hips. (13) The quadriceps were found to be tight, which created an anterior pelvic tilt of the hips, in effect increasing the length of the hamstrings which decreased range of motion for hip flexion. This study showed that flexibility and range of motion requires a thorough and integrated assessment process looking at all aspects or barriers to range of motion and not an isolated look at a singular muscle. Second, the implementation of the method on certain muscles is a factor. When looking at the contrasting studies of Bandy and Youdas, Youdas implemented an exact replica of the methodology of Bandy, but performed the stretches on the gastrocnemius muscle, looking at range of motion of dorsiflexion versus Bandy's study on the hamstrings. While Youdas found that the stretching regimen did not produce the same results as Bandy (no significant increase in range of motion), there is a significant point to consider (but the population difference: ROM-limited versus healthy should be taken into consideration). The calf muscles may require a greater or longer stretch protocol due to their daily positioning (the calf muscles are potentially more frequently engaged during daily activities and can be influenced by the footwear of individuals) and functional requirements. This is not to say that the hamstrings are not enlisted as frequently during daily activities, but the question behind the functional muscle differences and influence of stretch on muscle differences remains. Given the constant strain on the calf muscles, a stretching intervention program for increasing dorsiflexion might require a greater amount of stretching each day than the hamstrings which Bandy tested in his study. Of importance is also the inclusion criterion of Youdas' study. The study included healthy subjects with no lower-extremity dysfunction assessed by gait observation. Would the same static stretching protocol have influenced individuals with ankle restrictions such as increased pronation? Additionally, research has still to consider the integrated effects of flexibility. An isolated stretching regimen may not create the required effects on increasing and sustaining range of motion. In fact, it is NASM's position that a proper flexibility program would require the incorporation of a strengthening program for the antagonist muscle immediately after performing flexibility. Each joint moves as a result of length-tension and force-couple relationships surrounding a joint. If one muscle is short and overactive, its functional antagonist may be long and underactive. Given this relationship, a proper flexibility program would require implementing a corrective strengthening program to help a joint increase range of motion and re-establish normal length-tension and force-couple relationships. Overall, NASM concurs with current research which has found static stretching to increase range of motion; however, using clinical evidence to support their stance, NASM believes that a proper flexibility program would also require implementation of a corrective strengthening program to enhance range of motion. See Table 6 for application guidelines in implementing a flexibility program to increase range of motion.


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