A recent systematic review published in the Journal of Manipulative and Physiological Therapeutics has taken aim at an issue that is potentially close to many a chiropractors heart: the effectiveness of spinal manipulative in increasing muscle strength in healthy people. Following a spate of chiropractic-based studies showing increased muscle strength (or rather cortical drive to the muscles) post-adjustment, the results may not be all that surprising: they suggest that spinal manipulative therapy is superior to no intervention or sham manipulation when it comes to “augmenting the percentage change in isometric strength gain .”
The systemic review took in 909 identified records in a thorough database search. From here, duplicates were removed and exclusion criteria were applied leaving only 3 articles to assess on this topic. Due to the heterogeneity of the articles, the authors suggested the results be interpreted cautiously.
Here’s the kicker though – None of the work produced by Haavik, Murphy or the New Zealand College of Chiropractic was included in the systemic review after the exclusion criteria was applied. This is despite investigation on muscle strength pre and post-adjustment (though admittedly in most of these cases researchers were not just looking at muscle strength, but also at whether the increase in strength was due to changes in the brain or the spinal cord). In fact, the studies included were all pre-2016.
The three papers surviving the 906-item cull were:
The reason for the inclusion of just these three articles was that they had data for measures of isometric strength. This was not available in other studies, hence the recent line of investigation emerging from New Zealand researchers was not included. Researchers remarked that:
“The findings from this review have elucidated, at least in part, the facilitatory effects of SMT on muscle strength rather than the analgesic effects. Previous studies that investigated injured participants were associated with several possible factors such as pain muscle inhibition and afferent dysfunction.Thus, the differential effects of SMT on muscle strength could not be ascertained.Previous studies have shown that SMT could improve muscle strength of patients with active pain such as back pain and neck pain,but the underlying mechanism remained questionable. It is uncertain whether the gain in strength was due to the removal of pain inhibitionor the motor facilitation effectsof the SMT. Hence, the focus of healthy participants in this review could rule out the possibility of the pain inhibitory removal hypothesis and corroborate the motor facilitatory mechanism of SMT. Further studies are warranted to verify this assertion.”
For the sake of the study, the term “Spinal manipulation” was defined as high velocity low amplitude thrust techniques which also eliminated previous studies which “demonstrated that low-velocity end-range mobilization may also increase muscle strength .” Thus, this systemic review focused purely on the percentage change in isometric muscle strength after HVLA spinal manipulation.
There were a number of limitations cited, including selection bias, English language narrowing the field, and the quality of the studies included. However, researchers remarked that:
“We contend that the pooling of results from both upper and lower limbs herein is considered valid because our decision was corroborated by Haavik et al’s recent study  which demonstrated an increase in neurophysiological measures over the upper and lower limb muscles, suggesting an increase in cortical excitability after spinal manipulation.”
While the researchers did advise cautious interpretation of results and pointed to a need for more research, it is certainly encouraging to see that a systemic review like this could not rule out the effect of SMT on muscle strength. For those who would contend that chiropractic care should be limited to neck and back pain, this review should be yet another crack in the wall.