*Several studies have vindicated the beneficiary effects of an adjustment or known as spinal manipulation. Chiropractic scientific evidence include but is not limited to:
Biomechanical effect; Herzog et al. (1990) investigated the biomechanical effects of spinal manipulation. They found that the preparatory load applied during spinal manipulation to reach end range of motion of the joint produced a force between 20 N to 180 N; and the high velocity-low amplitude thrust applied during spinal manipulation at the end range of motion of the facet joint produced a force of between 220 N to 550 N with a duration of between 200 ms to 420 ms. Nathan and Keller (1994) measured intervertebral lumbar motion by inserting pins into the lumbar spinous processes and applying a high velocity-low amplitude thrust to the lumbar spinous process via a mechanical manipulation device, known as an activator adjusting instrument. Their finding included peak axial displacement of +- 1.06 mm; peak shear displacement of +- 0.48 mm; and peak rotational displacement of +- 0.89 degrees of the lumbar vertebral segments. They concluded that the physical movement of the vertebral segment induced by spinal manipulation may play a role in the therapeutic effects of spinal manipulation. Milan et al. (2012) performed a literature review on the effect of spinal manipulation on spinal range of motion (ROM). Fifteen articles were retained; nine articles on cervical spine ROM, two articles on temporomandibular joint ROM, three articles on lumbar spine ROM, and one study on sacroiliac joint manipulation on hip joint ROM. Milan et al. (2012) finding’s included a significant increase in ROM post-spinal manipulation for five articles of the cervical spine ROM and two articles of the temporomandibular joint ROM. It is important to note that the majority of these studies included asymptomatic participants with no electromyography (EMG) evidence of associated paraspinal muscle weakness or hypertonicity, as these participants are likely to have already full ROM pre-spinal manipulation. In addition, the effect of global and local muscle dysfunction on ROM was not considered.
Decrease muscle spasm; DeVocht et al. (2005) investigated the effect of spinal manipulation of the lumbar spine on EMG of localized tight muscle bundles in the segmental paraspinal muscles. They found that an immediate transient decrease in EMG post the spinal manipulation, but also found in some participants a spike in EMG post the spinal manipulation following an electromyographic response latency. Herzog et al. (1999) demonstrated in their study that spinal manipulation applied to the cervical spine, thoracic spine, lumbar spine and sacroiliac regions in asymptomatic individuals increased their associated paraspinal EMG following an electromyographic response latency. Colloca and Keller (2001) reported in their study following an electromyographic response latency that the EMG amplitude increased to reach peak amplitude within 50 ms to 100 ms post-spinal manipulation in symptomatic individuals.
Decrease muscle weakness and increase muscle strength; Keller and Colloca (2000) investigated the effect of spinal manipulation on segmental paraspinal muscle strength in participants with low back pain, by measuring the EMG during paraspinal extension isometric maximal voluntary contraction, before and after spinal manipulation. They found a significant transient increase in paraspinal EMG post the spinal manipulation, compared to a placebo spinal manipulation which showed no significant change. Suter and McMorland (2002) applied spinal manipulation to C5/C6/C7 segments with evidence of motor inhibition of the biceps brachii muscle using an interpolated twitch technique and EMG. Their results showed a significant reduction in biceps brachii muscle inhibition and an increase in biceps brachii muscle force post-spinal manipulation. Dunning and Rushton (2009) found a significant increase in biceps brachii muscle activity post-spinal manipulation of the ipsilateral C5/C6 segment in asymptomatic patients for neck pain and bilateral upper extremity pain. Suter et al. (2000) found a significant decrease in inhibition of the knee extensor muscles post manipulation of the ipsilateral sacroiliac joint in symptomatic patients with sacroiliac syndrome, anterior knee pain and EMG evidence of motor inhibition of the knee extensor muscles. Du Plessis (2014) investigated the effect of C5/C6 spinal manipulation on the EMG and muscle strength of the biceps brachii in participants with chronic neck pain over a three week period. Three measurements were recorded spanning the three weeks. The mean dynamometry increased from the first reading recorded of 20.67 kg to the second reading recorded of 21.49 kg, and increased from the second to the third reading recorded of 22.99 kg, with a significant increase in the biceps brachii muscle strength over the three weeks (p = 0.005). The mean EMG amplitude increased from the first reading recorded as 150.76 mV to the second reading recorded as 151.66 mV, and increased from the second to the third reading recorded as 152.02 mV, with a significant increase in the biceps brachii muscle activity over the three weeks (p = 0.000).
Reduce pain and central sensitization; Chu et al. (2014: 220) carried out an evidence-based review with meta-analysis on the peripheral responses to cervical and thoracic spinal manipulation. Data were extracted, and within-group and between-group effect sizes were calculated for outcomes of skin conductance, skin temperature, pain, and upper extremity range of motion during upper limb neurodynamic tests. Their results included eleven studies of which statistically significant changes were seen with increased skin conductance, decreased skin temperature, decreased pain, and increased upper extremity range of motion during the upper limb neurodynamic tests. Bialosky et al. (2009) measured thermal pain sensitivity in patients with chronic low back pain pre- and post-spinal manipulation compared to control groups, and found only significant inhibition of temporal summation post manipulation of the lumbar spine. The control groups included exercising with a stationary bicycle and performing lumbar extension exercises, of which both have been reported in the literature to produce hypoalgesia and have been described in the literature for the treatment of low back pain. Bialosky et al. (2009) concluded that spinal manipulation can provide a novel counter-irritant, resulting in inhibition or neuroplastic changes associated with central sensitization at the dorsal horn of the spinal cord. Glover et al. (1974) examined areas of lumbar skin that were painful to pinprick in patients with low back pain. Glover et al. (1974) found that after manipulation of the lumbar spine, the diameter of the area where the pinprick evoked pain was reduced compared a control group which received detuned short-wave therapy. Terrett and Vernon (1984) quantified the decrease in pain sensitivity after spinal manipulation. They used a graded, electrical stimulation to cutaneous paraspinal tissues, to establish a model for pain sensation. A minimal electrical stimulation (pain threshold) and a maximal electrical tolerable stimulation (pain tolerance) necessary to evoke pain were assessed by a blinded observer, in patients with tender areas in the thoracic spine. Terrett and Vernon (1984) found that manipulation of the thoracic spine significantly increased the pain tolerance levels by 1.5-fold within 30 seconds post the manipulation. Also, within 9.5 minutes after the manipulation, the pain tolerance levels progressively increased by 2.4-fold.
Study by Dr Aldo Victor
Victor (2016) investigated whether spinal manipulation (SMT) facilitates linear golgi tendon organ (GTO) Ib afferent activity as part of the convergent input on the homonymous motor neuron pool excitability. Each subject in Victor’s (2016) study performed modified stretching of their biceps brachii muscle based on the principle of the autogenic inhibition phase of proprioceptive neuromuscular facilitation (PNF) stretching, for the purpose of inducing linear summation of Ib afferent activity. A SMT intervention and a placebo SMT intervention by using an Activator II Adjusting instrument (AAI) with a force setting of zero force were applied to the ipsilateral C5/ C6 spinal segmental levels during the modified stretching of each participant’s biceps muscle at a standardized time interval. Electromyography (EMG) and dynamometry readings of each subject’s biceps muscle were captured during the full duration of the modified stretching. Statistical analysis made use of the mean values of each variable (EMG Root Mean Square / RMS / amplitude and muscle force) for the 500 millisecond period before and after all interventions (Victor 2016). The findings of Victor (2016) showed immediately post SMT a decrease in the EMG amplitude by 9.03 % (p = 0.39) in the face of an increase in muscle force by 4.76 % (p = 0.155) with a summation of percentage difference between the EMG amplitude and muscle force of 13.79 %. The immediate post placebo AAI interventions showed negligible decrease in both EMG amplitude (< 1.87 %) and muscle force (< 1.98 %) with a summation of percentage difference between the EMG amplitude and muscle force of less than 1.93 %.
The findings of Victor's 2016 study opened a door for further postulations; through
repeated SMT over time it can be suggested that through habituation
(neuroplasticity), facilitated joint Ib inhibitory pathways caused by altered
arthrokinetic reflex arcs (namely arhrogenic muscle inhibition) can be desensitized by the stimulatory
effect of spinal manipulation on the mechanoreceptors in the facet joint’s
tissue and result in improved homonymous ɑlpha motor neuron functioning. This theory is vindicated by several studies in the
literature, that demonstrated a significant incremental increase in muscle
force and EMG amplitude post SMT applied several times over a time period of
several weeks (Du Plessis 2014), as well as studies that demonstrated a
transient increase in EMG following the EMG response latency and a significant increase
in muscle strength and/ or decrease in arthrogenic muscle inhibition (AMI) immediately post SMT (Olsen 2015: 86;
Dunning and Rushton 2009: 512; Pickar 2002: 364; Suter and McMorland 2002: 541;
Suter et al. 2000: 385). In addition, the
findings of Victor's 2016 study
clinical implications for rehabilitation practitioners and physical therapists.
AMI presents a unique challenge to musculoskeletal therapists as it may hinder
any treatment or rehabilitation outcomes (Rice and McNair 2010: 250; Rossi et
al. 2002: 523). Any treatment that specifically aids in the reduction of AMI
will be an important tool to therapists in improving treatment and
rehabilitation outcomes that would have been impeded by the inhibition (Rice
and McNair 2010: 250; Rossi et al. 2002: 523). For optimal
management of patients with muscle weakness suspected to be of arthrogenic
nature, the application of SMT to the segmentally innervated facet joints can
be a beneficial approach before tradition strength rehabilitation or training
is initiated. Spinal manipulation may therefore then be included in the conservative treatment
protocols for muscle weakness (Victor 2016).
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