Isometric endurance test of the cervical flexor muscles - Reliability and normative reference values

Objective To obtain reference values for the isometric endurance test (IET) of the cervical flexor muscles, investigate its reproducibility, and compare the results with the maximal isometric strength test (MIST) of the cervical flexor muscles. Design Cross-sectional non-comparative study with single group repeated measurements. Methods Altogether 219 healthy females aged 20–59 years volunteered to participate in the study. The IET was performed in the supine position and MIST seated. The reproducibility was evaluated by the intraclass correlation coefficient (ICC) and an analysis described by Bland and Altman. The relationship between the two measuring methods was evaluated by Pearson's correlation coefficient. Results The mean (SD) IET time was 60 (33) seconds with no significant differences between the age groups of each decade. The ICC for intrarater repeatability was 0.80. However, the Bland-Altman analysis suggested moderate variation in repeated measurements. Pearson's correlation coefficient between the IET and MIST was 0.56. Conclusion Normative reference values for the IET are presented. Although the ICC showed good repeatability, one should consider that the change at follow-up visits has to be considerable to be clinically relevant. The correlation between the endurance time and maximal flexion strength was moderate. Thus IET of the cervical flexor muscles may be used in the clinic like the Biering-Sorenson test has been used to assess fatigue of the trunk extensor muscles.