It would be great to find scientific studies that verify and explain why so many people have found Yin Yoga to be beneficial, or to warn us against doing the practice due to injuries and pathologies that could arise. Unfortunately, there are no studies to my knowledge of Yin Yoga that prove or disprove its physiological efficacy.[1] If there are no studies, then we must evaluate the practice through analogy and reasoning: are there other investigations that use similar techniques to Yin Yoga that can help understand what might be happening? Fortunately, yes there are. While these are not the highest levels of proof we would love to see, they do lend credence to the claims many people make about Yin Yoga and matches their own personal experiences. Several of these analogies are captured in the YinYoga.com Forum, in the section called The Science behind Yin Yoga, but recently another branch of therapy caught my eye, because it also uses techniques very similar to those employed in a yin class. The therapy is called static progressive stretch, or SPS for short.

The JAS Ankle system utilizes Static Progressive Stretch to achieve permanent restoration of joint ROM in three 30-minute sessions per day.
The JAS Ankle system utilizes Static Progressive Stretch to achieve permanent restoration of joint ROM in three 30-minute sessions per day.

SPS is one of a group of therapeutic interventions used to help patients regain lost range of motion while decreasing pain,stiffness and swelling. It is used with patients who have undergone surgery or trauma that required immobilization of a joint. The immobilization may have been necessary to protect the joint from excessive stress during the initial healing phase, but immobilization can lead to significant complications and pathologies, including contracture of the joint.[2] Ideally, static progressive stretch should begin within 90 days of trauma, within the adaptive phase of wound healing, which is when the collagen fibers are most responsive to remodeling forces.

In order to recover lost range of motion the first option is to use physical therapy to passively stretch the joint with progressively greater loads of force to take the joint beyond its limited range of motion.  However, physical therapy is limited in the number and duration of sessions. You can only visit your therapist so many times a week, and certainly not 3 or 4 times every day. Thus stretching devices may be considered as an adjunct to, or in place of, physical therapy.

There are several possible devices that can be used, but of interest for those teaching or doing Yin Yoga is static progress stretch (SPS).[3] With these devices the joint is taken to its tolerable limit of movement (end range) for a period of time. Once the sensation ebbs, the patient then is free to increase the stress, taking the joint to a new end range, and again held there. Through this progressive approach, at each setting the joint is subjected to a constant level of movement, and as the joint is held at this position, the stress level within the tissues lessens. [4] Thus we have the name static progressive stretch. These devices have been used for restrictions in mobility of ankles, knees, shoulders, elbows and wrists.

 

Comparing Yin Yoga to SPS

The Over-stepping Dragon also stresses the ankle joint.
The Over-stepping Dragon also stresses the ankle joint in a static and progressive way.

The above should sound familiar to Yin Yoga students and teachers. Remember the 3 Principles of Yin Yoga?

  1. Come to your edge 
  2. Become still
  3. Hold for time.

These are the same criteria used with SPS orthoses:[5] we find the first tolerable edge in the brace where the patient feels a stress, but it is not too much. This is the first end-range position. Then, the patient leaves the joint at this position (stillness) until there is a sense that the edge has moved. If and when that happens, the patient advances the orthosis to the next end-range of movement available. This, in Yin Yoga, is called “playing your edge” and it is up to the student to determine whether it is a good idea to go further or not. Finally, the device is left on for 15 to 30 minutes. This is a bit longer than we would hold a single Yin Yoga posture, but it does show the importance of holding for time.

One study looked at the outcome of using an SPS orthosis for 30 minutes, 1 to 3 times a day on ankle range of motion. On average, it took 10 weeks for the ROM to plateau. The mean improvement in total plantarflexion and dorsiflexion was 17°, however the range of increases experienced by different subjects varied from 2° to 44°. Some people benefited far more than others.[6] (This is not unusual given the reality of human variation: people hit their final end range of motion due to bony compression. For example, in dorsiflexion (pulling your toes to your shin), when the tibia (your shin bone) comes into contact wit the talus bone (the first or top bone of your foot), you have reached compression and no further movement is possible. Some people have shapes to these bones that cause compression to arise very quickly and thus have very limited ROM in dorsiflexion even when they have no injuries.)

Another study found an average improvement in knee ROM of 25° with the range of improvements of 8-82°.[7] One study found an average improvement in elbow ROM of 26° with a range of 2-60°.[8] Yet another study looking at the shoulder found SPS improved average shoulder passive abduction by 26° and external rotation by 21° more than physical therapy alone provided.[9] (It should be pointed out that all of these studies have methodological problems and should only be relied upon to give indications of what might be possible through using SPS orthosis rather than proving they absolutely work for everybody.[10]) In some of these studies the stress was held for as little as 15 minutes, but in those cases it was repeated up to 8 times a day.

There are interesting correlations to Yin Yoga in the way SPS devices are used:

  • The stress is static and passive, which is the same in Yin Yoga: we don’t continually move and the muscles are not used to create the stress.
  • A tolerable depth is obtained
  • The duration of the stress is on the order of 30 minutes. In a Yin Yoga class, we do not hold any one particular posture that long, but throughout the whole class, the cumulative time spent stressing a joint could be 15~30 minutes.
  • If the sensations decrease, the patient decides to go deeper and adjusts the device to allow more range of movement; in the same way, in Yin Yoga the student controls the depth in a pose. If the sensations ebb away (the edge “moves”) then the student allows herself to go deeper.

Potential Problems?

One large retrospective study in 2019 looked at the incidence of problems and complaints arising from the use of SPS. This study reviewed several studies over a 10-year period, which covered 167,751 patients. The total number of complaints was 212, which is far less than 1%. The majority of the complaints were not very significant, such as some discomfort while wearing the device. There were no serious injuries reported.[11]

This is a very telling point! Many teachers are hesitant to teach Yin Yoga out of a fear of possibly hurting their students. The fear is that by holding a stress on a joint for a long period of time, near the end range of motion, the student could injure her joint. However, most teachers would probably agree the risk of injury is much higher in a joint that is already damaged than in a normal, healthy joint. And, yet, all these studies have found that applying a stress at the end range of motion of a joint to a recovering joint actually improves the mobility while decreasing inflammation and pain, and with no significant risk to the patient. This mirrors most students experience with Yin Yoga as well. As long as students do not try to go past their end range of movement and do not linger in the postures if pain arises, then the risk of hurting themselves is very low. Certainly, there is always some risk in any physical activity, but as long as a student practices with intention and attention, the risk appears to be very low while the benefits seem to be quite good.

Footnotes:

[1]There are studies of Yin Yoga specifically, but they address the psychological benefits of the practice, not the physiological affects. For example, see Frida Hylander et al, “Yin yoga and mindfulness: a five week randomized controlled study evaluating the effects of the YOMI program on stress and worry” in Anxiety, Stress, & Coping Volume 30, 2017 – Issue 4.

[2] Other events can also cause contracture: nerve injury, stroke, muscle weakness or spasticity, and disease.

[3] Other devices include serial casting, static braces, or dynamic splints that provide low-load, prolonged stretch. Dynamic splints use spring loading or elastic bands to provide low-intensity tension (less than that exerted by a physical therapist) and are designed to be worn over relatively long periods (for example, 6 to 8 hours at a time or overnight). With these devices the angle of the joint may be slowly changing over time. With an SPS orthosis, the angle of the joint remains constant until the patient resets it, and the device is generally only worn for up to 30 minutes at one time.

[4] This is technically called “relaxation”, which is a phenomenon related to creep, however in creep the stress is held constant while the length increases. With relaxation, the length is held constant thus the internal resistance of the tissues decreases.

[5] An orthosis is a corrective brace. The plural is orthoses.

[6] Costa CR, McElroy MJ, Johnson A, et al. Use of a static progressive stretch orthosis to treat post-traumatic ankle stiffness. BMC Res Notes. 2012; 5:348.

[7] Bonutti PM, Marulanda GA, McGrath MS, et al. Static progressive stretch improves range of motion in arthrofibrosis following total knee arthroplasty. Knee Surg Sports Traumatol Arthrosc. 2010; 18(2):194-199.

[8] Ulrich SD, Bonutti PM, Seyler TM, et al. Restoring range of motion via stress relaxation and static progressive stretch in posttraumatic elbow contractures. J Shoulder Elbow Surg. 2010; 19(2):196-201.

[9] Ibrahim M, Johnson A, Pivec R, et al. Treatment of adhesive capsulitis of the shoulder with a static progressive stretch device: a prospective, randomized study. J Long Term Eff Med Implants 2012; 22(4):281-291.

[10] See Static Progressive Stretch (SPS) and Patient-Actuated Serial Stretch (PASS) Devices, published by the Amerigroup RealSolutions in Health Care, 10/17/2018; accessed at https://medicalpolicies.amerigroup.com/medicalpolicies/policies/mp_pw_c176280.htm on May 14, 2019.

[11] Sodhi N, Yao B, Anis HK, Khlopas A, Sultan AA, Newman JM, Mont MA. Patient satisfaction and outcomes of static progressive stretch bracing: a 10-year prospective analysis. Ann Transl Med 2019;7(4):67. doi: 10.21037/atm.2018.08.31