Flexibility and Stretching

Flexibility and Stretching

Endurance, strength, balance, and flexibility are the four common areas of fitness considered to be the pillars of general health (Nat’l Inst. Aging). Flexibility is an odd choice to lump in with the others because there is no end goal that is easily defined in the context of health and fitness. It is difficult to be too strong, too well-conditioned, or too good at balancing, but it is possible to be too flexible. Muscles are supposed to stop short of complete extensibility, and resistance to stretching helps protect the ligaments that keep your bones knitted together.

What these recommendations probably intend is that people should train to improve their mobility—the ability to move their body through complete ranges of motion without undue struggle or pain. Flexibility plays a role in mobility, being a function of the things that inhibit range of motion around a joint—muscle extensibility, tension applied to the muscle, its cross-sectional area, and time under tension (Weppler 2010). But how much is enough? And should you actually stretch to improve flexibility? If so, when is the best time to stretch?

What Flexibility Means

Flexibility or muscle extensibility is related to the range of motion around a joint due to muscular inhibitions. The measurement of end-range joint angles depends on how far your muscles let the joint move. Few people lack the innate protective mechanism that stops joints from extending far enough to tear the connective tissues that attach bones to bones. Improving flexibility past a certain point may be harmful, depending on the mechanisms of improved flexibility.

There is some debate about whether muscle extensibility improvements come from an increase in muscle length or a change in sensation that allows the person to stretch farther. A person stretches past the end-range of their extensibility either because the muscle length changed, there was a decrease in muscle stiffness, or 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 would mean altering the function of the muscle 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. 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 changes 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 2010). This analysis gives us the “sensory theory” of muscle extensibility, which has steadily gained ground (Freitas 2017).

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. The most common question: whether static stretching or dynamic stretching as part of a warm-up provides any benefits or causes any significant detriments in performance and the possibility of injury (Behm 2016).

There are two basic kinds of stretching with some variations: static stretching and dynamic stretching. 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. Ample studies have shown that static stretching for longer periods (45–60 seconds) has a negative impact on muscular performance (Lima 2019). Light, short-duration static stretching has little negative effect, but also little positive effect on improving one’s range of motion. Regular long-term static stretching also tends to increase range of motion around joints, with the results varying by frequency, duration, and the individual’s background. Unsurprisingly, people with dance or gymnastics backgrounds have better results from stretching (Lima 2019).

A variation of static stretching is PNF stretching, which is performed in a contract-relax pattern or a contract-relax-agonist-contract (CRAC) pattern. PNF stretching also improves range of motion, similar to static stretching, but is difficult to perform by yourself. “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” (Behm 2016). PNF has shown similar negative impacts on performance in the same manner of static (Lima 2019).

Dynamic stretching is active stretching or movement through different ranges of motion around joints, gradually increasing as you warm up. Dynamic stretching tends to mimic sports or training 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. Similarly, combining short-burst static stretching with dynamic stretching in a warm-up has shown none of the negative effects associated with static stretching alone.

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]” (Id.). This suggests that if extensibility changes from static stretching are purely sensory, then performance changes are as well. More likely, however, there is more to the puzzle than has been thoroughly examined.

So, should you stretch? And if so, how should you go about it?

Stretching as part of a warm-up for other activities is a great way to raise your body temperature, make your muscles bendier, and get ready for the activity itself. But your warm-ups should stick to dynamic stretches and movements that mimic the activity you are warming up for. Most studies that have shown the benefits of dynamic warm-ups have examined highly specific warm-ups. That is partially why we like to warm up for barbell training with very light sets of the exact movements we will be using to train.

Some activities, particularly some sports, require more flexibility than you can get from just practicing or playing them. In those cases, you may want to improve your flexibility as a way to improve your performance. A regimen of regular static stretching or PNF stretching can help you improve your range of motion. Holds of 60 seconds or longer, regularly performed for four to eight weeks seem to lead to changes in extensibility. Just be aware that your results may vary, and static stretches should come after your training or practice, not as part of your regular warm-ups.

The body adapts to what you do with it. Barbell training’s big movements are beyond what most people will encounter in their daily lives. 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. More flexibility training should be reserved for special cases and sports that require it.

To get started with these big movements and improve your mobility while building strength, visit the Barbell Logic Beginning Barbells Page. It has in-depth lifting guides, cues, and troubleshooting tips for each of the big barbell lifts.

References

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)

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

Camila D. Lima, Cassio V. Ruas, David G. Behm, Lee E. Brown, “Acute Effects of Stretching on Flexibility and Performance: A Narrative Review,” J. of Sci. in Sport and Ex. (2019)

National Institute on Aging: Exercise and Physical Activity (https://www.nia.nih.gov/health/exercise-physical-activity)

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.“)

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