Clinical Biomechanics 14 (1999) 217-219 Brief Report Reproducibility of isometric strength: measurement of neck muscles Jari J. Ylinen”“, Asghar Rezasoltanib, Mikko V. Julin”, Hilkka A. Virtapohja”, Esko A. MdkG” “Department of Physical and Rehabilitation Medicine, Central Hospital of Central Finland, Keskussairaalantie 19, 40620 .&iiskylli, Finland hLIkES - Research Centre for Sport and Health Sciences, Jyviiskylii, Finland ‘Department of Health Sciences. University of Jyviiskylii. JvviiskylG, Finland Received 1 December 1997; accepted 26 June 1998 Abstract Objective. To evaluate the repeatability of an advanced method of measuring the strength of functionally different groups of neck muscles using an isometric apparatus. Design. A repeated measures design was used within and between sessions. Background. Chronic neck pain has been associated with poor isometric neck strength. In rehabilitation, strength measure- ments may thus help to evaluate the basic condition and show whether or not treatments and exercises have been beneficial for improving strength. Previous tests have concentrated on testing strength while trying to bend forwards and backwards, but no biomechanics device, able to test rotator muscles, has been presented. Method. The new isometric measurement device was used to test flexion, extension and also rotation strengths of normal subjects (n = 33). Results. Intratester repeatability was good with regard to all measured parameters. Conclusions. With both tests performed on the same day and test comparisons between days, intratest repeatability was good with regard to all measured parameters. Relevance Isometric testing of extremities and back muscles has been shown to be reliable and is commonly used to assess basic levels of strength. Our findings suggest that it is also suitable for measuring different parameters of neck muscle strength. 0 1999 Elsevier Science Ltd. All rights reserved. Keywords: Neck strength; Isometric contraction; Measurement device; Muscle testing 1. Introduction The main function of the neck muscles is to maintain postural balance, and thus, isometric testing seems to be an appropriate method to evaluate neck strength. There are only few studies of neck strength, and of the training methods for improving it. The important factor impeding research has been the lack of afford- able and reliable measurement equipment. Rotational movement of the neck is initiated at the atlanto-axial joint in the horizontal plane [l]. Only minor lateral shifting occurs between the atlas and axis - *Corresponding author. in rotation [2]. A CT study of the cervical spine showed the mean unilateral rotation to be 72” [3]. Three- dimensional imaging of the upper cervical spine with biplanar radiography showed nearly 80% of all axial rotation occurring between C, and Cz [4]. The centre of atlanto-axial rotation was located in the centre of the dens in these studies. Rotational strength of the cervical spine has been difficult to measure due to its complex structure. Measuring the strength directly with a pressure gauge placed on the forehead and/or occipital area would seem to be of value. However, if the gauge is placed directly on the skull, the moment arm will vary greatly, depending on the size of the skull. A new device has been constructed with the 026%0033/99/$ - see front matter 0 1999 Elsevier Science Ltd. All rights reserved. PII: SO268-0033(98)00063-l