It is most likely that spinal adjustments exert a reflex effect on pain and muscle tension through joint and muscle receptors.
There are specialised receptors in our joints and muscles that help allow us to detect where the arms, legs, neck and head are in relation to each other and in space. The term used for this is proprioception. This phenomenon lets us walk in the dark and know where our limbs are, to close our eyes and put our finger on our nose, to detect small movement in our joints and many other tasks. It is thought that minor loss of this ability may play an important role in why we can injure ourselves.
It makes sense then to see what we can do to improve our proprioception.
Loss of proprioception leads to further injury
- Position and movement sense may be impaired in the presence of joint disease1 and normal proprioception is thought to be essential for muscular coordination.2
- Loss of normal synchronisation of firing patterns can alter joint mechanics resulting in repetitive microtrauma.3
- Altered joint and muscle receptor input into the nervous system can result in disruption to patterns of muscle activation, leaving the person vulnerable to further injury or chronic pain.4
- Deficits in proprioceptive input may change perception and reorganise spinal motor reflexes so they no longer protect the spine from mechanical injury.5
- With loss of proprioception the person can become reliant on vision.6
- Chronic low back pain patients have poorer balance and lumbar spine position sense than controls7 as do patients with arthritis of the knees.8
- Low Back Pain patients have less refined position sense and lower acuity in repositioning the back presumably because of altered paraspinal muscle spindle feedback and central processing of this sensory input.5
Causes of Disturbed proprioception
- Tissue injury
- Loss of motion
The nerve fibres that transmit pain are very thin. The fibres that carry mechanical information (proprioception) are larger. These larger mechanical nerve fibres are more sensitive to lack of oxygen.9 Reduced joint receptor activity reduces the ability to inhibit pain in the spinal cord.10
The Adjustment and the Nervous System
It has been discovered that the controlled forces used in chiropractic adjustments are so effective because:
- The greatest level of nerve firing occurs at or near the extremes of joint motion.11
- The fast stretch of an adjustment will fire cells that create a relaxation of muscles.12
The speed of the adjustment appears to be a critical factor and not the severity or amplitude. As a result, all of our adjustments have been modified to revolve around high velocity, low amplitude thrusts.
In 1999 ten people with no pain underwent 11 different high speed, low amplitude spinal manipulations in all different regions of the spine. These people were measured using 16 pairs of surface electrodes placed on the back and limb muscles. It was found that:
- Each spinal manipulation produced consistent reflex responses in a target-specific area
- In observations of symptomatic patients, it was shown that local muscle spasm disappeared immediately after spinal manipulation.13
Stimulation of joint receptors causes a reflex muscle response
A reflex exists in the human spine from receptors in ligaments, discs and facet joints to the multifidi (deep muscles of the back) and possibly other muscles.14
Stimulation of these receptors within the disc (spacer between vertebrae) and facet joints15 elicits reactions in the deep muscles of the back that are so important in spinal control (multifidi & longissimus).
“…(it has been) proposed that the focus of the treatment regimen must be to restore the reflex system” – Indahl
The reflex appears to be triggered when there is relative motion of two vertebrae, causing multifidi to become active. The aim of a chiropractic adjustment is to specifically create this motion between two adjacent vertebrae.
It is the co-contraction of front and back muscles that is the major stabilising mechanism of the spine. Sustained activities of the spinal muscles at jobs lead to muscle fatigue. The fatigue in the receptors “results in an exponential decrease in reflexive muscle activity, exposing the spine to possible injury and pain”.16
The Central Effects of an Adjustment
It has been proposed that the chiropractic adjustment may act more centrally in the nervous system. Nerve fibres transmit pain responses into the spinal cord. These messages pass onto projection nerve cells in specific locations of the spinal cord. Most of the fibres then cross over to the opposite side of the spinal cord.
Thirty percent of the information regarding pain reaches the part of the brain that relays information on so that we are aware of where the painful sensation is coming from. The remaining 70% travels to an evolutionary older part of the brain. Much of this information goes to a region known as the periaqueductal gray (PAG). The PAG:
- Can block out pain in severely wounded soldiers
- Can produce almost complete analgesia if stimulated
- Is the site of action for opiate analgesics
We know that the PAG is important in helping to regulate the amount of pain that a person is experiencing. An inability to activate this part of the brain may be involved in chronic pain or make particular people susceptible to chronic pain.
The PAG receives input from the spinal cord at all levels. However, the majority of its input comes from the upper cervical area (upper neck).17 Chiropractors have known for over 100 years that the most influential area of the spine, with regard to the most impact, is the upper cervical area.
For many years, B.J. Palmer, the developer of chiropractic, practiced only upper cervical adjusting. It was known as “Hole-In-One”.
- Swinkels, A & Dolan, P (1998) Spine 23: 590-597.
- Gill & Callaghan (1998) Spine 23: 371-377.
- Lephart, et al. (1997) Am. J. Sports Med. 25: 130-137.
- O’Sullivan et. al. (1997) JMPT 5: 20-26.
- Brumagne et. al. (2000) Spine 25: 989-994.
- Nies & Sinnott (1991) Spine 16: 325-330.
- Radebold et. al. (2001) Spine 26: 724-730.
- Hassan, B et. al. (2001) Ann. Rheum. Dis. 612-618.
- Nansel & Szlazak (1997) JMPT 20: 219-224.
- Seaman, D & Winterstein, J (1998) JMPT 21: 267-280.
- McCloskey (1978) Physiologic Reviews 58: 763-820.
- Murphy (1997) Am. J. Clin. Chiro. 7: 23-24.
- Herzog et al. (1999) Electromyographic responses of back and limb muscles associated with spinal manipulative therapy. Spine 24: 146-153.
- Solomonow, M et. al. (1998) Spine 23: 2552-2562.
- Indahl, A. et. al. (1997) Spine 22: 2834-2840.
- Solomonow, M et. al. (1999) Spine 24: 2423-2433.
- Keay, KA, Feil, K., Gordon, BD, Herbert, H and Bandler, R. (1997) Spinal afferents to functionally distinct periaqueductal gray columns of the rat: An anterograde and retrograde tracing study. J. Comparative Neurology 385: 207-229.