A recent Dutch study challenges this idea about strengthening or activating core muscles to reduce pelvic pain. Using ultrasound to measure the thickness of the TVA among a sample of post-pregnant women with and without pelvic pain, Dr. Jan M.A. Mens from Erasmus University Medical Centre and Dr. Annelies Pool-Goudzwaard from the University of Amsterdam found that women in pain (n=43) had a 31 percent increase of TVA thickness when they performed an active straight leg raise test (ASLR) compared to 11 to 13 percent increase among those without pain (n=39). (1)
There was no significant difference between both groups at rest. For many years, some physiotherapists and other manual therapists have told their patients with pelvic pain to “strengthen their core muscles” to reduce and manage their pain and function. This idea comes from early studies of pelvic pain that the pain is mechanical in nature due to too much strain upon the ligaments in the lumbopelvic region. Co-contraction of the transverse abdominal muscle (TVA) and pelvic floor muscles compresses the region which may protect against strain and lowers the risk of pelvic pain.
However, pelvic pain—or pain itself—is much more complex than just mechanical issues.
Each participant laid supine and raised one leg while an examiner placed an ultrasound probe between the iliac crest and last rib to scan the TVA. The thickness of the muscles was measured from between both aponeuroses perpendicular to the muscle fiber direction. The subjects performed the test three times, and the results were averaged.
“We measured thickness increase of the transverse abdominal muscle at rest and during ASLR. In patients with long lasting pregnancy-related pelvic girdle pain thickness at rest was the same as in parous women without signs of pelvic pain (3.1 mm),” explained Mens in an online interview with Massage & Fitness Magazine. “During ASLR, TVA thickness increased about 12% in the group without pain. However, in the women with PGP thickness increase was 31%. Previous studies showed that TVA thickness increase is a good measure of activity of the muscle. These studies showed that during activity, assessed on EMG, TVA shortens and thickens.
“We found a large difference in TVA thickness increase (31% versus 12%) in a group of 43 patients and 39 controls. This means that our sample was large enough. Even two groups of 20 would have bene sufficient. In a new study we recommend more blinding, knowing that blinding is very difficult, because the investigator is able to observe the difficulty of the participant to raise the leg. It seems more practically to assess intratester reliability.”
The researchers cited that early research in the topic had mixed results, which were based on case-control studies. While a 2009 review supported the theory that co-contraction of the TVA and pelvic floor muscles can reduce pain (2), later research did not. (1) Some of the limitations in previous studies include mixed sample critieria, such as severity of pain, duration, and cause. Samples in the control group can be biased if the subjects are “not well-defined due to differences in level of fitness.” (1)
“In addition, colleagues might have detailed knowledge of the objective of the study and this knowledge could influence ‘spontaneous, automatic recruitment’. Furthermore, in most studies, maximal voluntary muscle contraction was investigated instead of spontaneous, automatic recruitment of muscles during a well-defined task,” they wrote.
Mens and Pool-Goudzwaard also cited research by Professor Peter O’Sullivan and Beales that “suggested that motor control impairments in long-lasting [pelvic girdle pain] show a large variation: ‘non-specific’ PGP disorders are represented by a number of sub-groups with different underlying pain mechanisms rather than a single entity.” (3,4) That sums up pain is not a result of a single cause.
They proposed two hypotheses why there is such a large difference in TVA activation.
1. Delayed contraction of the TVA.
Electromyographic (EMG) studies have shown that people in pain tend to have a delayed activation of the TVA compared to those with no pain. For some people, even small movements in the sacroiliac joint (SI) can elicit pain, and a delayed contraction due to displacement or fear of pain can further delay TVA contraction.
2. Imaging location.
The ultrasound scan showed only one region of the TVA muscle and is not a representative of the entire muscle since it stretches from the thorax to the pelvic floor. Different parts of the TVA can have varying degrees of activation among healthy adults. And so, it is possible that by examining more than one region of the TVA, there would be a more accurate comparison between the pain and no-pain groups.
While the study had better sample groups than previous studies and had examined automatic activation of the core muscles rather than attempting to have the subjects consciously “activate” them, there were a few limitations.
As mentioned in the second hypothesis, the location of the scan does not represent the entire muscle. It is possible that if the examiner had scanned the TVA from a different location, the results may be different.
Another limitation is the lack of blinding where the examiner already knows who has pain or not. That may affect how the muscle is measured and perceived on the screen.
Finally, because the women in the pain group suffered for more than six months, this data should not be extrapolated to populations with less severity and/or duration of pain.
“Physiotherapists should realize that it is not logically to stimulate the use of TVA in situations where it is already more active than usual,” Mens said. “[It seems more] logically to stimulate the TVA in low load, pain free exercises. Moreover, in case treatment with exercises is not successful and TVA is hyperactive during ASLR, it is time to switch from treatment strategy—for example, to prescribe painkillers or NSAIDs and/or corticosteroid injections into the sacroiliac joint.”
Overall, therapists should reconsider core muscle strengthening and activation for those with pelvic pain. While there is some evidence that exercise of various types can help manage pain, core exercises (or sometimes called “motor-control exercises”) have been shown to be no better than other forms of exercises, like running or weightlifting, for low back pain. Meanwhile, what may likely work better is to help those with pain is to help them find their best way to move and exercise. If they can do so, they are more likely to stick with it for a long time.
1. Mens JMA, Pool-Goudzwaard A. The transverse abdominal muscle is excessively active during active straight leg raising in pregnancy-related posterior pelvic girdle pain: an observational study. BMC Musculoskeletal Disorders. 2017;18:372. doi:10.1186/s12891-017-1732-9.
2. Koppenhaver SL, Hebert JJ, Parent EC, Fritz JM. Rehabilitative ultrasound imaging is a valid measure of trunk muscle size and activation during most isometric sub-maximal contractions: a systematic review. Aust J Physiother. 2009;55:153–69.
3. O’Sullivan PB, Beales DJ. Diagnosis and classification of pelvic girdle pain disorders. Part 1: a mechanism based approach within a biopsychosocial framework. Man Ther. 2007;12:86–97.
4. O’Sullivan PB, Beales DJ. Diagnosis and classification of pelvic girdle pain disorders. Part 2: illustration of the utility of a classification system via case studies. Man Ther. 2007;12:e1–12.