Within-day and between-day reliability of a novel test to evaluate back extensor strength in healthy individuals.

Abstract

Background Proximal junctional kyphosis (PJK) is a common complication of long spinal fusion. The prevalence of PJK ranges between 6 and 41% and frequently requires reoperation. One of the potential causes is thought to be posterior muscle dissection within the fused range at the time of posterior fusion. Various measurement protocols have been tested to evaluate extensor muscle strength, endurance and power in adults. However, it is not universally accessible and is rarely used clinically due to the high cost, requirement for considerable user expertise, demands on the functional capacities of the patient and protracted testing time. Currently, the Biering–Sørensen test is the most widely used isometric test for assessing extensor muscle endurance. But, it was deemed unsuitable and difficult for older patients with chronic low back pain undergoing spine surgery. In this study, we designed a simple method that allows us to perform lumbar spine extensor strength tests in a comfortable seated position. This study aimed to assess the within-day and between-day reliability of this novel test for evaluating back extensor strength in healthy individuals. Methods In this study, we examined 79 healthy subjects (33 males and 46 females), who were 25–63 years of age. The subjects were placed in a seated position on the novel chair and secured tightly with a lap belt. In each session, the subjects were asked to extend their backs against the force transducer at maximum capacity, maintaining the extension for 5 seconds. The maximal force delivered over that period was recorded. The subjects had a practice trial followed by three forceful extensions with pausing intervals of 30 seconds. Measurements were obtained by one of three trained raters, and the average force of all three trials was recorded. A follow-up session was carried out within 14 days. Intra-class correlation coefficients (ICCs) were used to assess within-day and between-day reliability. Results The mean force in the initial session was 314.6±118.3 N, and it was 318.6±123.6 N in the follow-up session. The ICCs for within-day reliability and between-day reliability were 0.89 [95%CI: 0.83–0.92] and 0.88 [95% CI: 0.81–0.93], respectively. There was a strong correlation between the average measures of the initial and follow-up sessions (r = 0.80; p < 0.001; R2 = 0.62). Conclusion The use of the static dynamometer chair is a reliable, non-invasive, cost-effective test that facilitates the assessment of the strength of lumbar spine extensors in healthy adults.