Flexibility and Stretching

Stretching and flexibility are common recommendations for general health, but their benefits are not quite so clear. What these recommendations probably intend is that people should train to improve mobility. While that may seem like semantics, mobility and flexibility do not carry the same connotations for training; one is stretching, and the other is movement. While flexibility plays a role in someone’s mobility, it is rarely the limiting factor and almost never improved through flexibility training—i.e., stretching.

Flexibility and Stretching

Connotations are important when talking about health and fitness. Words like endurance, strength, balance, or flexibility have baggage.[1] They mean something to the reader that the one giving the advice may not intend. For example, these four types of exercises are common recommendations for general health. Flexibility is an odd choice to lump in with the others. Flexibility is not a technical term, but it brings the image of static stretching and someone who is “bendy” (also not a technical term) to the minds of most. When people hear flexibility, they think stretching. Yet, for improvements in health, fitness, and performance, stretching is rarely a crucial exercise.

Instead, what these recommendations probably intend is that people should train to improve their mobility. While that may seem like semantics, mobility and flexibility do not carry the same connotations for training; one is stretching, and the other is movement.[2] While flexibility plays a role in someone’s mobility, it is rarely the limiting factor and almost never improved through flexibility training—i.e., stretching.

Flexibility is a function of the things that inhibit muscle length: muscle extensibility, tension applied to the muscle, its cross-sectional area, and time.[3] Baseline extensibility allows us to move our joints enough that we can complete daily tasks without pain or injury. While it is difficult to be too strong, too well-conditioned, or too good at balancing, it is possible to be too flexible. Muscles are supposed to lack complete extensibility, resistance to stretching helps protect the ligaments that keep your bones knitted together. Any stretching you do should be conducive to your purposes, and not approached with a more-is-better attitude.

What Flexibility Means

Flexibility is related to the range of motion around a joint due to muscular inhibitions. A better term is muscle extensibility, since the measurement of end-range joint angles depends on how far your muscles let the joint move. Few people lack the protective mechanism that stops your joints from extending far enough to tear the connective tissues that attach bones to bones.

There is some debate about whether muscle extensibility improvements come from an actual increase in muscle length or modification in a person’s sensation that allows them to stretch further. Applying pressure to a joint, forcing it toward its end-point, will cause the agonist muscles to elongate under tension. Plotting muscle length as a function of tension shows how far a muscle will stretch at a given amount of pressure. Muscle extensibility focuses only on the end-range of the measurement: you can stretch father because your muscle length changed, from a decrease in stiffness muscle stiffness, or because more pressure was applied to the limb. If extensibility is improved by an increase in muscle length (a mechanical change in the muscle), then the muscle should also show increased lengths at lower applied tensions—it will move through all given ranges of motion more easily.

A mechanical change means altering the function of the muscle or neuromuscular operation through stretching. Muscle fibers are composed of myofibrils, which can contract, relax, and lengthen. These myofibrils are made up of sarcomeres, the smallest contractile unit of a muscle. Within each sarcomere are contractile proteins—actin, myosin, and titin. Nerves connect the spinal column to the muscle at the neuromuscular junction. When an electrical signal crosses the neuromuscular junction an into the muscle fibers, it stimulates the release of calcium, which in turn, allows the myosin heads to attach to actin. The myosin head pulls the actin at its new attachment, causing the filaments to which the actin and myosin at attached to slide across each other. The muscle contracts, the sarcomere shortens, and the action generates force. The force of contracting muscles travels through connective tissue (tendons) to attachment points on the skeleton to cause or resist movement at the joints.

Muscles are also sensory organs. Muscle spindles are sensitive to changes in muscle length and are often involved in creating tension to resist stretching. At the tendon, the Golgi tendon organ senses changes in tension and the rate of change of the tension. This sensor may cause a muscle to relax as a protective mechanism.

If stretching causes a mechanical change in muscle length, it would be due to some long term change in these basic muscle functions. One theory says that muscles lengthen due to their viscoelastic properties, allowing us to maintain a stretched (deformed) state for extended periods of time. The “viscous” nature of muscles allows them to change but is countered by their “elastic” properties (tendency to return to their original length). Other theories have examined the possibility of permanent deformation of the connective tissues, increases in the number of sarcomeres after stretching, and the possibility of neuromuscular relaxation.[4] Weppler and Magnusson examined each of these theories and suggested that they are inadequate to explain real and prolonged changes in muscle extensibility:

“If the increases in muscle extensibility observed after stretching were due to an increase in length of the muscles caused by any of these mechanical explanations, there should have been a lasting [increase] in passive torque/angle curves. Instead, the only change observed… was an increase in end-range joint angles and applied torque.”

They explained away mechanical explanations for the measurements taken in a variety of studies that showed increases in muscle extensibility. The studies were left with only two things: (1) observed increases in end-range joint angles and (2) the standards for measurements themselves. In the examined studies, extensibility measurements end-points based on the subjects’ sensations (pain onset, self-determined maximum stretch, or maximum pain tolerated). “The only observable explanation for these results was that subjects’ perception of the selected sensation occurred later in the stretch application” (Weppler). This analysis gives us the “sensory theory” of muscle extensibility, which has steadily gained ground.[5]

Is Stretching Good for You?

The mechanical vs. sensory theories are unsettled, but we know other things about the effects of stretching on performance. Instead of looking at the mechanism for observed changes in muscle extensibility, other studies have looked at the ancillary effects of stretching exercises on performance. In particular, whether static stretching, dynamic stretching, and PNF prior to physical activities provide any benefits or any significant detriments regarding performance and the possibility of injury.[6]

Static stretching is what most people think about when they think about stretching, holding an extended position for several seconds. This is the hallmark of the gym-class warmup. Studies are tending to show that static stretching has a negative impact on maximal muscular performance in “a clear dose-response relationship.” Light, short-duration static stretching has little negative effect, but extended stretches can lead to significant impairments.

Dynamic stretching is active stretching, movement through different ranges of motion around joints, gradually increasing as you warm up. Dynamic stretching tends to mimic impending activities and has shown benefits to performance similar to those you get from general and specific warmups: it helps raise your core temperature and gets you ready for the activity itself. While dynamic stretching does not come with the same performance hindrances as static stretching, there is no “robust evidence” for performance enhancements either.

The latest and greatest form of stretching by enthusiasts is PNF stretching, performed other in a contract-relax pattern or a contract-relax-agonist-contract (CRAC) method. According to the same review, “Despite its efficacy increasing ROM, PNF stretching is rarely used in athletic preactivity routines, possibly because (i) there is normally a requirement for partner assistance, (ii) it may be uncomfortable or painful, and (iii) muscle contractions performed at highly stretched muscle lengths can result in greater cytoskeletal muscle damage.” They speculate, however, that PNF may affect performance negatively in the manner of static stretching since it includes a static stretching phase. There are not enough reviews on it to draw any conclusions.

Again, the mechanisms for any performance issues with static stretching are elusive. But the authors suggest that it is not due to changes in muscle length: “Given our current understanding, changes in muscle length are unlikely to be an important mechanism influencing the force reduction after [static stretching].” This seems to suggest that if extensibility changes from static stretching are purely sensory, then performance changes are as well. Or, more likely, there is more to the puzzle than has been thoroughly examined.

If there is a takeaway from this detour on flexibility, it seems to be a common one: your body adapts to what you do with it. Barbell training’s big movements are beyond what most people will encounter in their daily lives, improving mobility while building strength. Yet, mobility training is a general fitness attribute, “common to virtually all athletic or physical performance endeavors” (Sullivan and Baker). It is unlikely that someone who regularly squats, presses, bench presses, and deadlifts will suffer from a loss of mobility due to deficiencies in strength, balance, or flexibility. It is also unlikely that someone who suffers from a lack of mobility will improve it by stretching alone. Activities like barbell training, Yoga, and T’ai Chi are better choices, as they will improve flexibility in the context of mobility for valuable athletic developments.


References

[1] See, e.g., National Institute on Aging: Exercise and Physical Activity (https://www.nia.nih.gov/health/exercise-physical-activity)

[2] Jonathon M. Sullivan & Andy Baker, “The Barbell Prescription,” p. 28 (2016)

[3] Cynthia Holzman Weppler, S. Peter Magnusson, “Increasing Muscle Extensibility: A Matter of Increasing Length or Modifying Sensation?” Phys Ther, 90, 438-449 (2010).

[4] Cynthia Holzman Weppler, S. Peter Magnusson, “Increasing Muscle Extensibility: A Matter of Increasing Length or Modifying Sensation?” Phys Ther, 90, 438-449 (2010).

[5] See, e.g., Freitas, Mendes, Le Sant, Andrade, Nordez, Milanovic, “Can Chronic Stretching Change the Muscle-tendon Mechanical Properties? A Review,” Scand J Med Sci Sports, 28(3), 494-806 (2017) (“The results of the present meta-analysis support the sensory theory. However, we are unaware if the loading applied to the [motor tendon unit] might not be sufficient to trigger structural or mechanical adaptations.“)

[6] See David G. Behm, Anthony J. Blazevich, Anthony D. Kay, and Malachy McHugh, “Acute effects of muscle stretching on physical performance, range of motion, and injury incidence in health active individuals: a systematic review,” Appl. Physiol. Nutr. Metab., 41, 1-11 (2016)

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