Spinal manipulative therapy (SMT) has been recognized as an effective treatmentmodality for many back, neck and musculoskeletal problems. One of the major issues of the use of SMT is its safety, especially with regards toneck manipulation and the risk of stroke. The vast majority of these accidentsinvolve the vertebro-basilar system, specifically the vertebral artery (VA) betweenC2/C1. Here, we present first ever data on the mechanics between C2/C1 during cervicalSMT performed by clinicians. VA strains obtained during SMT are significantly smaller than those obtained duringdiagnostic and range of motion testing, and are much smaller than failure strains. We conclude from this work that cervical SMT performed by trained clinicians doesnot appear to place undue strain on VA, and thus does not seem to be a factor invertebro-basilar injuries. KEY POINTS FROM THIS STUDY: 1) Spinal manipulative therapy (SMT) is an effective treatment modality formany back, neck and musculoskeletal problems. It has received wide-spreadacceptance in a variety of disciplines including chiropractic, physiotherapy, nursing,and mainstream health care. 2) One of the major issues of the use of SMT is its safety, especially withregards to neck manipulation and the risk of stroke. 3) The estimate of stroke associated (but not necessarily caused) by SMT issmall – about one in a million. 4) It has been assumed [wrongly as per this study] that the VA experiencesconsiderable stretch during extension and rotation of the neck, which may lead toocclusions and damage to the VA, predisposing the patient to stroke. 5) Recent evidence indicates that vertebral artery damage is unlikely to occurbetween C1 to the foramen magnum, and between VAs origin from the subclavianartery to C6. 6) Here, we review the results of existing studies on human VA strains duringhigh-speed, low-amplitude SMTs administered by qualified clinicians and comparethem to the strains encountered during full range of motion (ROM) tests, andfurthermore, add the summarized results of unpublished works from strainsmeasured from all sections of 8 VAs using data from 3 clinicians, resulting in a totalof 3,034 segment strains obtained during SMTs and 2,380 segment strains obtainedduring full ROM testing. 7) Tests were performed on 12 human cadavers, 2 embalmed and 10 fresh. AllROM and SMT testing was performed by three licensed chiropractors. The SMT wasa diversified lateral/rotational maneuver applied to the recorded spinal regions. 8) The force measurements were performed and measured in such a mannerthat if anything at all, the strains during SMTs measured here are likely greaterthan those one would obtain in a patient. 9) The mean [not maximum] failure strains of VAs are reported previously to be58%. All maximum values documented here are clearly below the mechanicalfailure strains of VAs with the largest measured value of 22.9% for ROM testing inthe V3 region representing about 39% of the failure strain. 10) These authors found that the length change of the VA during SMT is muchsmaller than that observed during the range of motion testing. 11) These authors did additional experiments to conclude that repeatedtreatments, as may occur in clinical practice, could not cause micro-structuraldamage and failure strain to the vertebral artery. 12) In summary, the maximal strain values for the ROM testing at eachsegmental level were always greater than the corresponding strain values for theSMTs, suggesting that neck SMTs impose less stretch than turning your head, orextending your neck while looking up at the sky. 13) The results of this study suggest that neck SMTs impose stretches on the VAthat are maximally 55% of the passive range of motion achieved with normalmovements of the head and neck and are maximally 22% of the observed meanmechanical failure strain of human VAs. Therefore, based on the mechanical testsperformed here, one should be able to conclude that stretching of VA during neckSMTs does not cause any damage of the VAs. 14) The VA is never really strained during spinal manipulative treatments butthat the VA is merely taking up slack as the neck and head are moved during SMT,but that there is no stress and thus no possibility for microstructural damage. 15) These authors also argue and state that the increased rate of stretch of theVA during SMT was not able to injury the VA. 16) The results from this study demonstrate that average and maximal VAstrains during high-speed low-amplitude cervical spinal manipulation aresubstantially less than the strains that can be achieved during ROM testing for allvertebral artery segments. 17) We conclude that cervical spinal manipulations, as tested here, are safe froma mechanical point of view for normal, healthy VA.