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Table 2 Mechanistic themes identified based on the sentences used in the selected studies

From: Potential mechanisms for lumbar spinal stiffness change following spinal manipulative therapy: a scoping review

Mechanistic theme and studiesSpecific sentences used in the studiesRelated measurement
Change in muscle activity
 Wong et al. [10]a“decreases in spinal stiffness may permit increased disc diffusion and increased segmental motion enabling increased LM thickness ratios.”LM recruitment (thickness ratio)
 Shum et al. [20]“mechanical deformation of pain receptors of soft tissues may elicit activity of the paraspinal muscles, which will stiffen the motion segment … in people with back pain”
“The decrease in spinal stiffness and the increase in spinal mobility after mobilization may also be due to the decreased muscle activity of the erector spinae”
N/A
 Fritz et al. [9]a“the effectiveness of SMT could relate to a mechanical impact on spinal stiffness and subsequent neurophysiologic consequences facilitating muscle activity”
“effects of SMT may be mediated by … enhancement in LM recruitment”
LM recruitment (thickness ratio)
 Haussler et al. [22]“Manual therapy techniques may also … cause reflex muscle relaxation, altered motor function”N/A
 Ferreira et al. [23]“reductions in involuntary muscle activity associated with resolution of pain may be responsible for the reductions in spinal stiffness”N/A
 Allison et al. [25]“may be due to … muscle relaxation”N/A
Increase in mobility
 Wong et al. [10]“decreases in spinal stiffness may permit increased … segmental motion”N/A
 Shum et al. [20]a“changes in bending stiffness may be the mechanical mechanism responsible for the … improvement in spinal mobility.”ROM
 Stamos-Papastamos et al. [21]a“some links were observed between changes in stiffness and changes in lumbar ROM”ROM
 Haussler et al. [22]a“Manual therapy techniques may also … improved spinal flexibility”
“applying a mechanical thrust (i.e. SMT) caused a direct physiological increase in passive spinal mobility”
“indicative of producing a beneficial effect of increased passive spinal mobility or flexibility”
Dorsoventral displacement
 Ferreira et al. [23]“it has been hypothesised that there is a relationship between spinal pain, reduced voluntary movement and abnormal spinal stiffness, and that restoration of normal spinal stiffness will result in a reduction of symptoms and a return of voluntary movement”N/A
 Allison et al. [25]“Posteroanterior mobilization of the lumbar spine has been advocated as a treatment technique to restore spinal mobility on the basis that it will decrease spinal stiffness”N/A
Decrease in pain
 Shum et al. [20]a“Large amplitude oscillations (grade III) may stimulate mechanoreceptors, leading to a decrease in pain”
“changes in bending stiffness may be the mechanical mechanism responsible for the reduction in pain”
Pain intensity (VAS)
 Stamos-Papastamos et al. [21]“It was hypothesized that a direct comparison of the 2 techniques [manipulation and mobilization] on the same asymptomatic subjects could possibly clarify the interaction of bending stiffness and ROM, without pain being present. Moreover, any changes on stiffness and ROM … could not be due to pain relief.”N/A
 Ferreira et al. [23]a“it has been hypothesised that there is a relationship between spinal pain, reduced voluntary movement and abnormal spinal stiffness, and that restoration of normal spinal stiffness will result in a reduction of symptoms and a return of voluntary movement”
“change in stiffness was associated with back pain”
“changes in stiffness occur as symptoms improve in patients with low back pain, and these changes are not directly due to the application of spinal manipulative therapy”
Pain intensity (VAS)
 Allison et al. [25]“may be due to changes in symptom response”N/A
 Goodsell et al. [26]a“Importantly, it has been theorized that hypomobile spinal joints may occur in association with low-back pain, and the stiffness of these joints may be altered by use of manipulative therapy.”
“the treatment did not produce a sufficient change in pain for a change in stiffness to be detected”
Pain intensity (VAS)
Increase in pressure pain threshold
 Shum et al. [20]“The reduction in pain and stiffness … may also be due to changes in the pain threshold”N/A
 Haussler et al. [22]a“SMT also increased the amplitude of applied force, indicative of increased tolerance to pressure”Applied force
 Haussler et al. [24]a“The increases in dorsoventral vertebral mobility and the amount of applied pressure to the back after SMT”Applied force
Change in spinal tissue behavior
 Edgecombe et al. [19]“the force applied during SMT application is believed to affect the local spinal tissues”
“the observed increase in stiffness was the result of viscoelastic change caused from insufficient fluid recovery”
N/A
 Wong et al. [10]a“decreases in spinal stiffness may permit increased disc diffusion”Intervertebral disc diffusion
 Allison et al. [25]a“Repeated loading of the spine causes creep and relaxation of spinal connective tissues, changing the resistance to the applied load. In some cases, micro-failure of tight connective tissue structures may decrease the resistance to movement and increase the range of movement in a restricted spinal segment”
“the initial displacement under load may have changed, reducing the length of the non-linear region of the force–displacement curve or resulting in small movements of the spine into more extension.”
Stiffness measurement in multiple locations
Change in the central nervous system or reflex pathways
 Shum et al. [20]“The reduction in pain and stiffness … may also be due to changes in the … and sympathetic nervous response”
“the procedure may elicit activation of descending inhibitory mechanisms”
N/A
 Fritz et al. [9]“An SMT force has been shown to stimulate peripheral afferents, altering central nervous system (CNS) input, and enhancing motoneuron excitability.”N/A
 Haussler et al. [22]“Manual therapy techniques may also stimulate peripheral joint receptors and central nervous system pathways”N/A
 Allison et al. [25]“reflex modulation of the sensory and motor pathways … may be modified”N/A
Correction of a vertebral dysfunction
 Haussler et al. [24]“back stiffness is one of the primary clinical indicators of vertebral dysfunction”N/A
  1. aStudies which suggested potential mechanisms based on data
  2. Italicized sentences were the authors’ suggestions based on data
  3. LM Lumbar multifidus, SMT Spinal manipulative therapy, ROM Range of motion, VAS Visual analog scale