By Dr. G. John Mullen, PT, DPT, CSCS of Swimming Science, COR, Creator of Swimmer’s Shoulder System, Swimming Troubleshooting System , and Mobility for Swimmers System , Swimming World correspondent
SANTA CLARA, California, June 28. THE following is an excerpt from the Mobility for Swimmers System , a comprehensive guide for improving mobility. Order your copy today!
Perhaps the most basic question that must be asked about stretching is whether it actually increases joint range of motion. Fortunately, the literature appears to confirm that stretching does increase range of motion, as follows:
Joint mobility — Harvey (2002) reviewed whether static stretching would generate lasting increases in flexibility across multiple joints. Their teams found that in 13 studies, four were rated as moderate quality and nine were rated as poor quality. The results suggest that regular stretching can increase joint range of motion by approximately 8 degrees for more than a day after stopping stretching.
Hamstring mobility — Decoster (2005) reviewed various hamstrings stretching and found 28 studies. Although they noted a relatively poor overall study quality, they concluded that hamstring stretching does increase joint mobility.
Calf mobility — Radford (2006) performed a systematic review that investigated the effects of static calf muscle stretches compared with no stretching. Five trials were included in their meta-analysis, which showed that calf muscle stretching increases ankle dorsiflexion, particularly when performed for more than 30 minutes total stretching duration.
In summary, stretching does appear to increase range of motion in different muscle groups. Such increases in flexibility appear to last more than one day at least.
However, understanding what exactly changes during stretching is not well understood. Many believe this improvement is from a change in muscle tissue. Yet, the muscle may actually be one of the structures not altered during stretching training! Understanding which structures in the body are pliable and can change is essential for prescribing individualized mobility programs.
Think of all the factors influencing mobility:
Active structures: Muscles with proper extensibility are generally associated with an optimal usable range of motion.
Passive structures: Ligaments can limit the range of motion due to their role as joint stabilizers. On the other hand, ligaments that are too loose can be problematic, causing joint instability.
Neural structures: Sometimes there will be a lack of usable range of motion despite adequate extensibility of the muscles and ligaments. In this case, the nervous system can send sensation-limiting range of motion.
Other factors: Elasticity of skin, adhesion between the muscle fibers and adhesions between the muscle and fascia, or even psychological factors.
Range of motion is a highly complex topic, which is poorly understood. At this time a few theories are used to explain the improvements in range of motion.
Mechanical property theories
Four types of mechanical theory exist, where the mechanical properties of the muscle tissue itself are altered by stretching, as follows:
1. Viscoelastic deformation The term “viscoelastic” describes a material that behaves both elastically and viscously (i.e. like a liquid in that the exact response to tensile force is rate and time dependent). Some researchers have suggested viscoelastic properties of muscles are responsible for the ability of stretching to increase range of motion. A stretched muscle does seem to lose its resistance to stretch after being held in a stretched position for a period of time. However, these results only last a short period. This cannot be the reason for the long-term effect of stretching, which seems to last for more than one day. A muscle and its tendon have both viscous and elastic mechanical properties. These viscous properties within the myotendinous unit (MTU) will elongate in response to a slow sustained force and will resist rapid changes in length (Taylor 1990). While under stretch, the force generated by the viscous material to resist elongation decreases over time, termed stress relaxation (Taylor 1990; McHugh 1992). Therefore, during extended stretch periods, the MTU will elongate, called creep, the likely cause of improved range of motion.
2. Plastic deformation The term “plastic” describes a material that changes shape permanently in response to an external force. Elastic materials are thought to behave plastically when extended past their elastic limit. Cipriani (2012) reviewed the literature and the results did not support a plastic deformation but rather a viscoelastic deformation, which was temporary and not permanent. Plastic deformation loses credibility, as research suggests improved range of motion progressively decreases once a static stretching program is discontinued. MTU have also been demonstrated to increase passive torque and stiffness as elongation of the muscle occurs (Magnusson 1998). Over time, stretching programs have been shown to increase passive torque (Magnusson 1996). If a muscle is stretched for a long time, reductions in passive torque occur, resulting in short term improvements in flexibility (Magnusson 1997; Magnusson 1998).
3. Increased sarcomeres in series Eccentric training is able to change the optimum length of a muscle. Increasing the number of sarcomeres in a series alters the length-tension relationship, as each sarcomere influences the length-tension relationship (Brughelli 2007). The length-tension relationship indicates muscle perform best at a specific position, which likely changes with an increased amount of sarcomeres in a series. However, only animal studies have shown changes in the number of sarcomeres in a series, not in human stretching studies.
4. Neuromuscular relaxation Static stretching may also alter the stretch reflex, increasing the ability of the muscle to relax. Yet, this adaptation appears to be short term, not altering the passive torque curves, likely not responsible for the changes in flexibility.
Mechanical properties of muscle do not appear to be responsible for the changes in range of motion that are observed following stretching protocols.
Sensation theory
Most studies not accept the sensation theory as the main source of range of motion improvements following stretching. The sensation theory suggests the sensitivity for pain decreases following stretching.
These researchers have therefore formulated the hypothesis that stretching increases range of motion by reducing the sensation for assessing muscle length (Magnusson 1998; Magnusson 1996). A few studies have noted the onset of stretch sensation increases following periods of stretching of three to eight weeks in many studies (Halbertsma 1994; Magnusson 1996; Weppler 2010).
This suggests stretching does not increase the mechanical properties of muscle, just the onset of stretch sensation.
The implication of the sensation theory is not clear, as a reduction in sensation at a given joint angle may alter proprioception or joint sense and increase the risk of injury. Specifically, this joint ROM might be described as a “safety margin” during which the individual is aware of a strong sensation of pain and acts to reduce this pain. Reducing the size of this safety margin could potentially be detrimental to the risk of injury in certain sports, although studies are needed to assess this hypothesis.
Genetic Theory
Genes seem to play a role in all life functions. However, the degree of contribution and if these genetics are simply the gatekeepers will not well understand. While we don’t know much about genetics, it seems certain genes expression is more common in people with flexibility. One study found people with a CC genotype of the COL5A1 gene increasing their tendon extensibility at the knee extensors, but not plantar flexors in Japanese men (Kubo 2013). These results bring to light the influence, but also the complexity of genes and stretching. Genes may be influenced by gender, race, and nationality, as well as specific to each muscle, clearly more research is necessary.
Epigenetic Theory
Epigenetics are the light switches which appear to turn on and off genetic material. These items may be altered during stretching, but we unfortunately don’t know much about them at this time. This may be why sustained stretching theory alters muscle length. For example, you may not have certain genes turned on until you begin stretching on a regular basis, then this continual stretching may turn on certain genes for flexibility. This theory is more of a result of mobility, not the cause, but nonetheless must be considered as science progresses and there may be treatments or training to specifically switch on these epigenomes.
Clearly, range of motion is a highly complex topic requiring more research. However, it seems the sensation theory is the main cause of improved range of motion.
Dr. G. John Mullen received his Doctorate in Physical Therapy from the University of Southern California and a Bachelor of Science of Health from Purdue University. He is the owner of COR PT, strength and conditioning consultant, creator of the Swimmer’s Shoulder System, and chief editor of the Swimming Science Research Review.