The aim of this study was to develop a clinically simple and amenable test of back extensor strength and evaluate its convergent test retest reliability. We designed a static dynamometer chair to perform this test in a comfortable seated position. Our novel chair showed excellent test retest reliability results in healthy population.
Previously published reviews13,14 have demonstrated a level of inter-instrument validity between handheld and isokinetic muscle strength testing for upper and lower limb. However, few studies have investigated trunk extensor strength using HHD 15,16 as well as established reliability of trunk extensors strength in a comfortable seated position 12,17
Moreland et al used a variant of the prone Biering-Sørensen test to investigate the interrater reliability of maximal isometric back extensor strength on 39 subjects 15.The authors did their trials with a 30 seconds rest period in between consecutive measurements. However, their test had several weak points. The dynamometer was stabilized by the examiner which made it in open chain and highly dependent on examiner strength. In addition, the dynamometer fixation was affected by the grip strength, gender and lean body mass18. Furthermore, Biering-Sørensen test has significant source of inter-individual variation because it’s affected by hip extensor activation, along with the mass of the upper extremities and torso.5
Valentin et al showed that test retest reliability of a modified Biering-Sørensen test improved by using external belt fixation of the HHD in patients with osteoporosis and low trauma vertebral fractures16. However, using the prone position in their test produced discomfort which limited maximal extension production in some patients. In fact, repeated testing was difficult for one participant because of back pain following the first session. In addition, one participant had difficulty raising the chest from the examination table because of muscle weakness and another one terminated testing due to dyspnea in prone position. These findings supported the fact that inducing pain is an important discouragement for clinical testing.19
To overcome the problem of testing in discomforting position, Harding et al. assessed healthy individuals in standing position using a closed chain wall fixation and determined the relationship between extensor muscle strength and bone mineral density (BMD). However, since most spine patients are either old or assessed post-operatively, testing them for maximal trunk extension in standing position makes patients uncomfortable and puts at risk the patients with poor balance. In addition, this position does not eliminate the hip extensors and gluteal muscles which might give misleading results. Furthermore, testing subjects in standing position may not be feasible in many kyphotic individuals. 20
A limited number of studies looked at examining lumbar spine extensors strength in comfortable seated position to overcome all the pervious concerns. Park et al, designed a similar chair with HHD device that is attached to it, to measure lumbar spine extensors strength. The chair test retest results were reliable with Intraclass Correlation Coefficient 0.82 (0.65–0.91 Ci 95%). Also, when they compared the chair with isokinetic dynamometer machine PrimusRS, it showed good validity. However, their study included only a few patients (30 patients in total)12. Yang et al, examined lumbar spine extensors strength in three different postures: prone, standing and setting using HHD. For sitting position, they used a chair without a back support that is fixed to the wall, then they fixed the HHD device to the wall separately. They compared the results of the test with isokinetic dynamometer for validation. Their conclusion for the test in a sitting position was reliable with interclass Correlation 0.90 (0.83–0.94 CI 95%), however, has a low validity 17.
Our protocol involves testing subjects in seated position and secured tightly with seat belt which helps to eliminate other muscles function and isolate the extensor muscles. In addition, this technique maximizes the comfort and safety of tested subjects, especially for old, post-operative patient or individuals with poor balance.
The HHD was fixed on the chair and the chair was stabilized against the wall which eliminates examiner-based variability, making the test feasible for all patient including kyphotic patients, is not influenced by upper body mass and doesn’t induce pain during or after testing in our tested sample. Furthermore, compare to other tests our novel technique is cost effective and the simplicity of design and small size make it easy to access and transport in the clinic setting.
Testing patient using our novel protocol showed excellent reliability in measuring the lumber extensor muscle strength and its results are reproducible with each time subjects take the test. These promising results on healthy adults, without a history of spine disease or LBP is a first step in an investigation into the changes in spinal extensor muscle power following spine surgery.