Research Review by Dr. Shawn Thistle©


Jan. 2007

Study Title:

The relationship between posture and back muscle endurance in industrial workers with flexion-related low back pain


O’Sullivan PB et al.

Publication Information:

Manual Therapy 2006; 11: 264-271.


Industrial workers have a lifetime incidence of low back pain (LBP) of approximately 60%, with yearly incidence as high as 31%. Such subjects represent a unique patient population because of the repetitive and sometimes physically challenging nature of their work. Chiropractors and manual therapists often work in close vicinity to, or have formal business arrangements with, various industrial companies and as a result are responsible for treating these patients.

Previous research has investigated potential predictors of LBP, which are thought to include: prolonged sitting, prolonged standing, reduced back muscle endurance, altered muscular coordination, increased lumbar mobility, decreased lumbar side flexion, reduced lumbar lordosis, psychosocial issues…the list could go on.

However, these factors often cannot account for new cases of LBP. To illustrate, Adams et al. (Spine 1999) suggested that these factors could only account for 12% of serious LBP cases! We should also remind ourselves that spinal range of motion does not seem to be a valuable predictor of future or current spinal problems.

Further, lumbar spine posture has yet to be associated with incident LBP. Therefore, the aim of this preliminary study was to determine if a relationship exists between spinal posture, activity levels, back muscle endurance and LBP in a specific sub-group of industrial workers.

This study included 55 subjects in total and was designed as a cross-sectional study. Twenty-one matched male control subjects with no history of significant LBP and no backache in the previous three months were compared to a group of twenty-four male workers from the same plant with a history of flexion-provoked LBP.

Average patient age in both groups was about 38y. LBP subjects reported a flexion injury to the lumbar spine with ongoing pain for the preceding 18 months while performing flexion motions or tasks.

Subjects were excluded if they reported traumatic onset of the LBP, or had pain provoked by lumbar extension (a subsequent study will investigate these patients). Also excluded were those with radicular symptoms, previous spinal surgery, pending workers compensation cases, and those who had undergone specific stabilization rehabilitation for their condition.

The measures utilized in this study included:
  • a questionnaire designed for this study which asked about activity levels (the authors could not find a previously validated instrument that met their needs so instead designed one)
  • a structured interview to determine time spent per week sitting, standing, lifting, and participating in vigorous activity
  • posture was measured using photo-reflective balls attached to bony landmarks – images taken digitally while sitting, slump sitting, standing, and lifting were then digitized and measured to determine joint and spine angles
  • Biering-Sorensen test to measure back muscle endurance (NOTE: the authors’ description did not include the 'second chance' described in other studies where once the patient deviates ~10° from the starting position they are verbally instructed to return to neutral while the measurement continues – upon deviating again the test is complete)
  • a lifting task whereby a 12Kg box with handles was lifted 5cm off the floor – patients were given no instructions on how to lift, rather were instructed to lift as they normally would

Pertinent Results:

  • the LBP group had significantly lower back muscle endurance than the control group
  • when comparing the usual sitting, standing, and lifting lumbar flexion angles, there was no difference between groups
  • however, LBP subjects sat in their “usual” position with greater posterior pelvic tilt than controls
  • when comparing usual versus maximal slumped sitting, LBP subjects sat closer to their end range flexion angle than did controls
  • there was a significant inverse relation between low back muscle endurance and increased posterior pelvic tilt during “usual” sitting
  • there was a significant inverse relation between back muscle endurance and sitting closer to end range flexion
  • low back muscle endurance was negatively correlated with longer periods of sitting, and general inactivity

Conclusions & Practical Application:

This small preliminary study provides evidence that a relationship exists in male industrial workers between reduced lumbar muscle endurance, habitually posturing the lumbar spine near end range of flexion, reduced levels of activity, and time spent sitting. These findings support previous research indicating that reduced back muscle endurance is a common finding in those with flexion-related LBP.

By sitting nearer to end range flexion, these patients are adapting what some call a “passive posture” – meaning that tissue load is transferred to passive tissues (discs and ligaments) as a result of decreased muscle activity. The exact cause of reduced back muscle endurance is still not clear, but some potential factors include disuse through inactivity, impaired motor control, or prolonged passive loading. Interestingly, LBP patients in this study reported more pain with flexion activities, yet sat closer to end range flexion than controls.

Recent literature has suggested that clinicians should attempt to identify patients as having flexion or extension intolerance (or preference, depending on how you look at it). Such classification, although in its infancy from a science perspective, has been indirectly utilized for years and directly espoused by treatment methods such as McKenzie and Cox. I attended a recent seminar with Stu McGill at the Canadian Memorial Chiropractic College last winter where he stated that many flexion-intolerant patients actually seek out static flexion postures (seated or standing), which they report as being more comfortable.

Many of these patients (chronic low back problems who are also chronic “slouchers”) assume these positions because they feel good, while not realizing that they are perpetuating the problem. The astute clinician needs to recognize this pattern, and immediately initiate spine-sparing strategies to re-educate the patient.

The authors acknowledge the obvious shortcomings of using their own questionnaire as a measure in this project. Overall, I do not feel that this detracts significantly from the value of this study.

It is important to note that the same authors are also conducting a similar study for extension-related LBP patients. This type of research is important as it will assist us in recognizing certain patterns or movement intolerance, while guiding appropriate and effective rehabilitation programs.