Lower back pain is a very complex problem and may have many causes.  One cause is when spinal tissue failure occurs as a result of high compression forces applied through the spine leading to spinal injuries.  High spinal compression forces may lead to micro fractures in the vertebral endplates, compression fractures of the vertebral bodies and damage to the spinal discs (1-3).

Activities which cause high spinal compression forces include:

• Lifting heavy weights.
• Lifting lighter weights in weak postures which increase the functional weight of the object and thus the load on spinal tissues (e.g. lifting from the floor or above shoulder height).
• Sustained spinal bending postures (with or without a load in the hand).
• High repetition spinal bending postures (with or without a load in the hand).
• Exposure to whole body vibration in vehicles that experience vibrational acceleration including shocks between 2-6g (11).

(1-4,11).

Certain body postures also create higher compression forces through the spine than others.  For example, bending the spine while lifting, increases the pressures on the spinal discs by more than 100%.   Spinal bending combined with twisting increases spinal disc pressures by more than 400%.  On the other hand, when people recline backwards in a chair, even while adopting a slouching posture, spinal disc pressures reduce by 50-80% – a posture most of us adopt when we’re getting tired during extended bouts of sitting.  Sitting up straight in a chair actually creates twice the spinal compression compared with reclining backwards in a chair – something to tell your granny or your teacher when they criticize your reclined slouching posture!

(3,4).

In 1979, it was noted that when heavy lifting was performed while holding one’s breath (for a few seconds), the intra-abdominal pressure was raised, the spinal extensor muscles activity reduced and both led to reduced compression loading on the lumbar spine, reducing the risk for spinal injury.  However, if the heavy lifts extended for longer than a few moments, the breathe was released and the intra-abdominal pressure fell to much lower levels, reducing this spinal support mechanism substantially (5).  This reduction in spinal compression due to raised intra-abdominal pressure was supported by research published in 2003, 2006 and 2010 and showed that the greatest benefit occurred when the body was in flexed (bent) postures (6-8).

The question arises as to how raised intra-abdominal pressure reduces spinal compression and helps to protect the spine from spinal compression failure leading to spinal injury and lower back pain.

Both abdominal and spinal extensor muscle contraction cause an increase in the spinal compression forces.  However, the abdominal muscle contractions (0- 40% MVC) also assist in raising the intra-abdominal pressure, and when doing so, the net forces on the spine result in reduced spinal compression.  In these circumstances it was also found that there was a reduction in the activity of the erector spinae muscles, with a greater reduction in these muscles’ activity corresponding to a greater increase in intra-abdominal pressure (8).

Furthermore, a 2013 published study revealed that chronic lower back pain sufferers who were experiencing a remission from their pain still exhibited lower levels of agonistic abdominal muscle activity and higher levels of antagonistic paraspinal muscle activity when compared to healthy individuals when performing spinal flexion (stooping/bending) with or without handling a load.  This alteration in their abdominal and spinal muscle recruitment activity/ patterns could result in increased spinal loads (not measured in their study) and possibly contribute to the recurrence of lower back pain in individuals where these altered recruitment patterns have become the norm (9).  On the other hand, research published in 2011 showed that activation of the core muscles showed no improvement in spinal stability, casting doubt on the mechanism in which core muscle rehabilitation is used to assist in the treatment of chronic lower back pain (10).

References:

1. Chaffin D.B.; Park K.S (1973). A longitudinal study of low-back pain as associated with occupational weight lifting factors. Am Ind Hyg Assoc J. 34(12):513-25.
2. Freivalds A.; Chaffin D.B.; Garg A.; Lee K.S. (1984). A dynamic biomechanical evaluation of lifting maximum acceptable loads.  J Biomech. 17(4):251-62.

3. Adams M.A.; McNally S.D.; Chinn H.; Dolan P. (1994). Posture and the compressive strength of the lumbar spine. J Biomech. 27(6):791-791.

4. Nachemson A.L. (1981). Disc pressure measurements. Spine. 6(1):93-7.

5. Hutton, W. C.; Cyron, B. M.; Stott, J. R.R. (1979). The compressive strength of lumbar vertebrae. J Anatomy. 129(4): 753-758.
6. Daggfeldt, K.; Thorstensson, A. (2003).  The mechanics of back-extensor torque production about the lumbar spine. J Biomech. 36(6): 815-823.
7. Arjmand, N.; Shirazi-Adl, A. (2006). Role of intra-abdominal pressure in the unloading and stabilization of the human spine during static lifting tasks. European Spine Journal. 15:1265–1275.
8. Stokes I.A.; Gardner-Morse M.G.; Henry S.M. (2010). Intra-abdominal pressure and abdominal wall muscular function: Spinal unloading mechanism. Clinical Biomechanics. Nov;25(9):859-66.
9. D’hooge, R.; Hodges, P.; Tsao H.; Hall L.; MacDonald D.; Danneels L. (2013). Altered trunk muscle coordination during rapid trunk flexion in people in remission of recurrent low back pain. J of Electromyograhy and Kinesiology. Feb;23(1):173-81.
10. Stokes I.A.; Gardner-Morse M.G.; Henry S.M. (2011). Abdominal muscle activation increases lumbar spinal stability: analysis of contributions of different muscle groups. Clinical Biomechanics. Oct;26(8):797-803.
11. Bazrgari, B.; Shirazi-Adl, A.; Kasra, M. (2008). Seated whole body vibrations with high-magnitude accelerations—relative roles of inertia and muscle forces. Journal of Biomechanics. 41:2639-2646.