Stretching and Flexibility Part 2: Why does stretching cause changes in ROM? Does strengthening do the same? Stretch vs strength after injury

In our last post you were left hanging with "you stretch and there is a change in range of motion." How does this happen? 

Static stretching does show a short-term effect of improved ROM/flexibility. The greatest change occurs 15-30 seconds into the hold of a stretch and no more than 2-4 repetitions are needed (Page 2012).

Does static stretching change the muscle-tendon tissue properties? 

SparkNotes version: no. If this theory was true then we would see a lasting shift to the right in the passive torque/angle curves. As we covered in the first post, this is not the case likely due to viscoelastic properties. 

Konrad et al. looked at a six-week static stretching program to improve ankle dorsiflexion ROM in healthy subjects. They found that while ROM increased significantly, "other functional (passive resistive torque, maximum voluntary contraction) and structural (fascicle length, pennation angle, muscle stiffness, tendon stiffness) parameters were unaltered." 

Furthermore, Freitas et al. conducted a systematic review and meta-anaylsis including a total of 26 studies that including stretching protocols lasting longer than two weeks and had at least two sessions per week. They concluded, "stretching interventions with 3‐ to 8‐week duration do not seem to change either the muscle or the tendon properties." These studies did find a difference in muscle extensibility/ROM, though. 

This demonstrates that static stretching can alter ROM, but it is not due to changes in muscle or tendon structure. Therefore, we must look at non-muscular structures that could play a role. By we, I mean Nordez et al, because this is exactly what they did. 

Other structures

It has been shown that stretching of the lower limb increases ROM of the cervical spine. I cannot name any muscle that attaches from the lower limb to the head (although I challenge you to do so). If there is a structural explanation, the results should then be attributed to the involvement of continuous structures connecting the lower limb and the spine, such as the myofascia and the peripheral nervous system. (Konrad et al) Results and conclusions were similar in a paper by Andrade et al that showed maximal dorsiflexion angle significantly decreased by flexing the hip from 150 to 90°. The limiting tissue of the posterior chain in dorsiflexion would be the gastroc or calf muscle. As this muscle does not attach across the hip, hip position should not have a change in ROM. Therefore, the change may be due to peripheral nerves and/or fascial tissue.

Let's start with fascial tissue. Fascia is a complex tissue that has yet to have an exact agreed upon definition but is a type of tissue that is found nearly everywhere in our body. It functions to constantly transmit and receive mechano-metabolic information that allows all body systems to operate in an integrated manner (it's pretty important). The different layers of fascial tissue seem to rely on hyaluronic acid to "slip" one over the other during movements. Fascial tissue properties and ability to move over each other can change with aging, muscle injury, surgical treatments, and emotional responses. The complexity of this tissue is likely what gives us the perception of "stiffness" when we first wake up in the morning. The changes associated with fascia are demonstrated in the figure below:

Image from: Zugel et al. 2018

The peripheral nervous system will be kept short and sweet. These nerves move in neural sheaths and our ROM will change if the neural sheath is put on tension. An example is the "slump test." If you sit in a slumped position with your leg out in front of you, you will notice a change in your knee extension and/or hip flexion flexibility compared to sitting up straight with your head up. 

Fascial tissue is densely innervated with nerve connections related to the autonomic (sympathetic/parasympathetic) nervous system. Our sympathetic system is aroused by mental stress and plays a role to protect us during times of perceived danger. Diaphragmatic breathing has been proposed to activate the parasympathetic nervous system, allowing for a relaxation effect. If you've ever had the experience of connecting your breath to a static stretch, you may find that the deep breath tends to "unlock" you and allow you to go further into the ROM. Is it possible that the initial restriction is due to sympathetic tone signaling fascia to protect us from a perceived dangerous movement? 

The sensory theory

I think the most credible explanation up to this point is the sensory theory. This theory is consistent with what we established above in that there's an increase in ROM after stretching, but there is no change to muscle or tendon length. ROM is then improved due to alterations of our sensory perception or "stretch tolerance." In other words, it seems to be yet another thing that we don't know much about. Our nervous system tends to be the governor of all with GTOs and muscle spindles playing some type of role with feedback communication. In fact, the person's belief of static stretching may be playing a role as well. It's pretty difficult for subjects in clinical research to not know they are stretching. Weppler et al state, "Because there is no way to keep subjects from knowing that they are participating in a stretching study, subjects may demonstrate an increase in extensibility because they expect this to be the result of stretching. Increased extensibility then may be due to a psychological alteration in sensory perception or to a willingness of subjects to tolerate greater torque application." It remains unknown if our increased tolerance to stretch is a peripheral or central nervous system adaption.

Difference between Strengthening and Stretching 

The main difference between these two is that resistance training does create changes in both muscular and tendon properties. Research shows strengthening improves physiological cross sectional area, hypertrophy, increases in tendon stiffness (a good thing), flexibility, neural adaptations, and comes with the side effect of being swole (Brumitt et al.). The benefits of these adaptations are improved force production and increased capacity. These, in turn, build higher resilience to injury and may lead to improved performance. More to come on this in part 3!!!

Should you stretch or strengthening after injury occurs? 

In our post switching from RICE to PEACE & LOVE after injury we talked about the importance of mechanotransduction for healing. In short, this is the physiological process where cells sense and respond to mechanical loads that stimulate healing and tissue repair. Performing a strengthening exercise vs. a static stretch likely leads to a greater tissue response as the load used for strengthening is often greater than a stretch. If I'm a tendon or muscle, I have no idea if you are stretching or strengthening me. All I know is that some load is being applied and this load, if appropriate, can stimulate healing via mechanotransduction. For example, it has been reported that athletes with hamstring strains recover faster by performing more intensive stretching (4x per day) than by performing less intensive (1x per day) stretching (Malliaropoulos et al). The difference between the groups was overall volume or load of stretching. However, recovery was defined by improvements in ROM and we know that there are many other factors to considered when using the word "recovered."

Despite positive outcomes in these types of studies and improvements in flexibility, it is difficult to isolate the effectiveness of the stretching component of the treatment plan because the protocols usually include strengthening and other interventions as well (Page et al). Research shows us that optimal load is key for recovery, however implementing optimal load is a bit of a trial and error process. Furthermore, questions remain on if the load should be pain-free or pain-threshold oriented (Hickey et al). If your injury is very irritable and a muscular contraction is not well-tolerated, a stretch may be the starting point of optimal load. At the same time, isometric contractions and slow heavy resistance training have shown to be beneficial in some musculoskeletal conditions. Lastly, when injury occurs we must admire the complexity of the movement system and appreciate all the factors that may have played a role. For some people optimal load may be no load initially. 

If you are not injured then we would assume that your musculoskeletal and neural structures are ready to accept load and static stretching is likely too low of a load to create physiological adaptations leading to greater performance via force production. 

So, we beg the question, should static stretching and flexibility as a whole be retired as a major component to fitness? Part 3 coming soon! 

Stay Curious, Stay Connected, Stay Strong. 

Dr. Dillon Caswell, PT, DPT 

References: 

1. Mine and Dr. Jason Tuori's, PT, DPT, CSCS thoughts 
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