AJCC 2003

October 2003

Quantifying Spinal Muscle Activity & Strength

by Christopher J. Colloca, D.C.

Qualitative assessments determine the nature, as opposed to the quantity of the elements composing a test or measure. Inspection, palpation, and visual observations of patient structure or function are all examples of qualitative assessments used by clinicians. Whether the clinician is judging muscle strength by his or her kinesthetic sense, visually estimating range or quality of spinal motion through observation, or attempting to define tissue characteristics through palpation, such qualitative assessments can only estimate the clinician’s perceived judgment.

Quantitative assessments, in contrast, express a numerical value relative to the proportionate quantities of a test or measure. In the context of spine measurements, range of motion can be described in units of degrees, spinal displacements can be described in units of inches or centimeters, and physiological changes can be expressed, for instance, in units of temperature (degrees) or electrical signals (volts) or other relevant descriptors. Quantitative measures thus allow us to objectify clinical assessments in order to understand and communicate information in absolute terms as opposed to those that are ambiguous.

Traditional spinal assessments including orthopedic and neurologic tests aim to identify nerve root compression and therefore are invalid in assessing common spinal disorders absent of neurological involvement. This situation is further complicated by the fact that diagnostic imaging including MRI assessments are not helpful in identifying somatic lesions determined responsible as pain generators (Osti and Fraser, 1992). Indeed, the pain generator for a majority of patients with spinal disorders have been identified as the spinal joints including the intervertebral disc in the lumbar spine (Kuslich et al., 1991) and the zygapophyseal joints in the cervical spine (Barnsley et al., 1995). Nociception arising from spinal joints has been found to result in alterations in paraspinal muscle function (Solomonow et al., 1998; Solomonow et al., 1999; Solomonow and Krogsgaard, 2002; Holm et al., 2002), and subsequently the stabilizing muscular capabilities of the spine. The ability to quantify muscle activity or strength of the spine, thus has been deemed an important parameter in managing patients with spinal disorders (Szpalski et al., 1996).

Dynamometry

Traditionally, measurements of motor strength in clinical practice are performed to assess extremity joint muscles or to evaluate a patient for nerve root compression. Qualitative grading criteria (grades 0-5) are commonly used to describe the strength of the muscle in study. Obviously, the subjective nature of qualitative assessments provides less accurate and thus reliable measures of muscle strength. New technology is providing low-cost equipment to quantify muscle strength in clinical practice (Figures 1-2). Specific to assessments of muscle strength of the spine, maximum values for strength and force plots can be obtained can from muscle groups in performance of various spinal motions (Jordan et al., 1999). Quantifying spinal muscle activity and strength, thus can be a significant indicator of spinal function useful to clinicians in both diagnosis and management via tracking patient outcome (Mannion et al., 2001).

Figure 1. Manual muscle testing of trunk extension with a hand-held dynamometer. With this particular dynamometer (PowerTrack, JTech Medical Industries), maximum force is digitally displayed on a wrist mounted LCD panel, or alternatively, strength curves are plotted by computerized software. (Photograph courtesy of J-Tech Medical Industries, Salt Lake City, UT) function useful to clinicians in both diagnosis and management via tracking patient outcome (Mannion et al., 2001).

Figure 2. Isometric dynamometry testing. (A) Lateral flexion of the cervical spine is shown where the restrained but subject can exert an isometric exertion in the lateral flexion direction against a fixed resistance that contains the dynamometer. Note that the sub-ject’s feet are not in contact with the floor to avoid recruitment of stabilization muscles in accomplishing cervical lateral flexion task. (B) Isometric trunk strength during lifting is a common assessment of work capacity. (Photographs courtesy of J-Tech Medical Industries, Salt Lake City, UT)

Figure 3. Isometric trunk extension maximal voluntary contraction is performed as electromyographic signals are monitored from the erector spinae musculature.

Electromyography

Surface electromyography (sEMG) enables a clinician to monitor the level of electrical activity that produces a certain muscular tension based upon changes in amplitude and frequency of the sEMG signal. Particular to measuring EMG-force relationships, the average number and firing rate of motor units contributing to an actual muscle contraction can be measured and thus related to the quantity of actual force produced. EMG thus, is not a direct assessment of muscle force, but of muscle electrical activity, and other relationships need to be established (calibration of electrical output and force produced) before reasonable muscle force estimates can be made.

Efforts have been made to normalize surface EMG recordings in an attempt to facilitate comparisons between individuals. Because there is not an exact 1:1 relationship between myoelectric signal and muscle contraction force, a standard of reference must be established for such comparisons and comparisons among muscles or activities. This process, a form of force calibration, is referred to as normalization (De Luca, 1993). Various factors are responsible for changes of the myoelectric signal such as slight change in electrode locations, tissue properties, or temperature. After applying the electrodes at the appropriate site, a normalization test is performed where contractions are performed within the context of the type of examination. The most common method of normalization is to perform one reference contraction, usually an isometric maximal voluntary contraction (MVC) (Figure 3). The myoelectric values subsequently obtained are expressed as a percentage of the MVC. Because of the variability of MVC’s, research has demonstrated significantly reduced errors in using submaximal MVC’s for normalization techniques (Lehman, 2002).

The use of EMG as a biomechanical analysis has been found to reveal impairments that have not been routinely identified with standard clinical tests (Larivie’re et al., 2002). A recent study from Germany (Danneels et al., 2002) examined trunk muscle recruitment patterns during strength and coordination exercises in healthy back pain subjects. The results showed that, in comparison with the healthy subjects, the chronic low back pain patients displayed significantly lower EMG activity of the multifidus muscle during the coordination exercises, indicating that, over the long term, back pain patients have a reduced capacity to voluntarily recruit the MF in order to obtain a neutral lordosis. During the strength exercises, the normalized activity of both back muscles was significantly lower in chronic low back pain patients. The authors emphasized their findings as important factors for symptom generation, recurrence or maintenance of low back pain. In 2000, Keller and Colloca reported their results of an average 21% increase in trunk muscle activity during isometric lumbar extension efforts in patients receiving spinal manipulation (Keller and Colloca, 2000). Such functional improvements were not observed in patients receiving sham manipulation or in controls. Promising studies such as these fortify the body of knowledge supporting the role of quantitative assessments as objective outcome measures useful in the diagnosis and management of patients with spinal disorders.

References

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