Reasons for testing

Spine surgeons do not routinely measure spine strength, or more specifically, trunk muscle strength when assessing their patients. Simple manual testing is an option, but such tests suffer from subjectivity, lack of uniformity, and lack of quantification. More specific trunk muscle strength tests are time-consuming, often require bulky and expensive equipment, and the information gleaned is generally felt to be of limited value in routine practice. More commonly, such tests are undertaken by rehabilitation clinicians or as part of research, especially in relation to low back pain, where it is accepted that trunk muscle strength is clinically relevant [1, 2]. Some research has suggested that spine strength measurement tools may be useful in identifying risk factors, planning treatment, and measuring progress.

Types of tests

Trunk performance can be evaluated in terms of endurance and strength. Broadly speaking, endurance tests are manual examinations conducted with minimal equipment, while strength measurements are mechanized and usually computerized. These can be isometric (isotonic), isokinetic, or isodynamic (isoinertial), depending on whether the test is against a fixed resistance or through a range of movement, and whether the movement is at a fixed or variable velocity.

Extension

The best-known static endurance test is the Biering-Sorensen test, which measures how many seconds a subject is able to keep the unsupported upper body in a horizontal position. The subject lies prone with the lower body supported either on a Roman chair with supports, or on a bench with straps. The subject initially has the waist flexed with the head hanging down and is asked to raise the trunk to the horizontal position with hands crossed over the chest. The test is continued until the subject can no longer maintain the horizontal position [3].

Numerous studies have shown that the test is accurate and reliable, although it is unclear whether it can discriminate between subjects with and without low back pain [4, 5]. Ito et al have described a variation of this test where the subject lies prone on the floor with a small pillow under the abdomen to reduce lumbar lordosis, and is asked to extend their trunk to keep the sternum off the floor for as long as possible [6].

A dynamic test can be performed by asking the subject to repeatedly extend and count the number of repetitions [7, 8].

Flexion

These tests evaluate flexor (abdominal) endurance. The subject lies supine and is asked to maintain about 25° upper trunk flexion (a partial crunch) or hip flexion (a partial straight leg raise). Alternately, both the upper trunk and hips can be flexed simultaneously. Time to fatigue is measured. A variation of the original Kraus-Weber fitness test, this measure is less commonly used and has been less thoroughly evaluated for accuracy and reliability. Dynamic endurance can be measured by counting the number of partial crunches that can be performed until fatigue [7].

Isometric

The simplest strength test is isometric, where maximum force exerted against static resistance in a particular plane (eg, trunk flexion) is measured. The test can be repeated at different angles to generate a strength curve. The Tergumed system is one commonly employed isometric measurement system. Four devices consisting of fixed weight-resistance systems are used to measure flexion, extension, lateral flexion, and rotation. The subject is seated with appropriate restraints and asked to perform a maximal voluntary contraction for 6 seconds. The MedX system is a similar device. Due to the static nature of the testing, results from isometric devices are of limited value when assessing dynamic function.

Isokinetic

Isokinetic testing provides a more dynamic assessment, where force or torque is measured throughout a range of movement at various constant, preset velocities. As muscular output increases, the machine resistance increases, and acceleration is not possible. One of the best known isokinetic trunk testing machines is the Cybex, which consists of three separate machines to measure trunk extension/flexion, torso rotation, and lifting. Peak torque or force is measured at various speeds, and the software derives average power and various rations including extension/flexion and left/right rotation. Consistency of effort is derived by calculating an average points variance. Other machines that measure only trunk extension/flexion include the KIN/COM and Biodex devices. Although measurement through a range of movement more closely simulates normal function, controlling the speed of motion is somewhat unnatural, limiting the real-life relevance of the results.

Isodynamic

Isodynamic testing employs a constant force or maximum torque during movement, thereby allowing for changes in acceleration and velocity of motion in proportion to the muscular effort of the user. The Isostation B-200 device is a triaxial dynamometer that simultaneously measures lumbar movements in three axes. Although theoretically of more clinical relevance, there are practical limitations to this device and a paucity of published studies.