IN VIVO MUSCLE ELASTICITY MEASUREMENT: A PROGRESS REPORT

Edward F. Owens, Jr., M.S., D.C.
Ronald S. Hosek, Ph.D.

Life Chiropractic College

Fifth Advances in Conservative Health Science Conference
Davenport, Iowa 1986

Abstract

Last year at this conference a new technique for the measurement of muscle tone was described (1). This presentation is a report of the progress of that project. The earlier paper described an instrument that probes the soft tissue of the spine, while measuring the indentation of the tissue and the force needed to produce it.

A tissue test produces a graph of force versus depression as shown in Figure 1. The graph demonstrates that the force of indentation increases with increasing penetration in a non-linear fashion. Also, there is hysteresis between the upper (penetration) and lower (retraction) portions of the graph.

Using muscle elasticity as measure of muscle tone is not a new technique. It has been used in one form or another since 1960 to monitor the changes in tone of limb muscles. Frames are used to flex a joint while measuring both the force needed to produce movement and the angulation of the joint (2,3,4). By subtracting the forces due to inertia of the limb and device, friction in the joint and elasticity of connective tissue, the elasticity of the muscles that cross the joint can be assessed.

Some interesting information that has been gathered using this technique is that structural changes occur in the muscles of individuals with spasticity, due either to Parkinsonism or cerebral palsy (5,6,7). The changes apparently take at least a year to develop and can be measured using this type of device. There are also histological changes that can be seen in these subjects. The spastic muscle appears to become more fibrotic and also change from type II (fast twitch) to type I (slow twitch-tonic) muscle (8). Histological studies of scoliotic subjects suggest that these muscle changes also occur in spinal muscles (9).

A way of monitoring changes in spinal muscle elasticity would provide a non-invasion means of assessing the fibrosis and functional type of the muscle. Using this technique we hope to track changes that occur in muscle due to subluxation or correction of subluxation. It might also eventually provide a means of grading the chronicity or severity of subluxations.

The last year has been spent in refining the muscle tonometer and in writing software to make data taking easier and faster. In use, the current probe position is displayed on the computer screen as the probe is positioned over the point to be tested. In the most recent experiments we tested the elasticity at 10 points across the spine at the levels of L1 and T12. Five points were on the left and five on the right, each point being 12 mm from the next.

With the stylus in place over the tissue, the force of the stylus was monitored as the probe was brought into contact with the skin. In order to minimize movement artifacts, the subject was instructed to hold his breath at mid-inspiration for the duration of the test (about twenty seconds). The tissue was compressed until a preload of 1 newton was shown on the computer screen. The computer then activated the stylus motor to depress the tissue by seven millimeters over a course of 3.5 seconds. A point of force and position data were taken at every .21 millimeters along the trajectory of the depression. The probe was then retracted at the same rate while data was being taken.

At this depth of probe penetration some discomfort was experienced by some subjects at certain points, especially those directly adjacent the spinous process. If the pain was unbearable, the subject could exhale to reduce the contact with the stylus.

Gathering data in several places produces a family of curves that could be graphed on the same axis. To show the results more clearly, we analyzed each test separately for maximum force produced at each location, the energy stored in the tissue during indentation, the energy recovered during retraction and the maximum slope of the FORCE-DEPRESSION curve.

Figure 2 shows the reduced results from a series of 10 probes performed at the level of the first lumbar vertebra on a human subject. Each of the three histograms shows a calculated factor versus its position across the spine. Positions one through five are located at 12 millimeters from the increments on the left side of the subject, beginning 60 millimeters from the spinous process, and positions six through 10 are on the right side, with position six being closest to the spine. The spinous process itself was not tested.

The top histogram shows that the maximum force encountered during indentation of the tissue was not uniform at every point across the subject's trunk. The two points located on either side of the spinous process showed significantly greater tone, with a slightly higher force encountered on the left side.

The middle histogram represents the integral of force over distance for each tissue test. The taller cell at each position is a display of the energy stored during indentation of the tissue, and the shorter cell is the energy recovered during retraction of the stylus. The energy stored at that location. Also, the difference in height of the two cells would represent the hysteresis, or energy lost due to the viscoelastic nature of the tissue.

The lower histogram shows the calculated maximum slope of each tissue test. This calculation proved to be erratic and very sensitive to noise in the force versus depression data.

Individual tests have been found to be repeatable using foam samples and human subjects. As a test of the repeatability of the reduced data we performed the same series of ten tests on the same subject at the same locations. Figure 3 shows the reduced results for that series of tests. The two tests are very similar in the results shown for Maximum Force and Energy Stored, but differ in the Max Slope calculations. We think that this calculating is just too sensitive to noise. Some other way of determining modulus of elasticity needs to be used. The data could be smoothed or we could use a curve fitting routine to find an equation for the force-depression relationship.

We performed a total of 210 test runs on 9 different subjects. The average of the reduced data is shown in Figure 4. The reduced and averaged data show a tendency toward harder tissue at the spine and no evidence of one side being favored over the other.

In summary, the tonometry device gives reproducible results, is sensitive to changes in muscle tone that occur at different locations and can be used to scan a subject for areas of hypertonus. Future work will aim at characterizing more completely the pattern of muscle tone that occurs along the human spine, tracking the relationship between muscle tone and subluxation and developing a computer model to help interpret the measured force versus depression relationship.

GRAPHS NOT SHOWN

References

  1. Owens, E.F. and Hosek, R.S. "In Vivo Measurement of Muscle Tone". Presented to the Fourth Annual Conservative Health Science Research Conference, Palmer College of Chiropractic, October, 1985.

  2. Jimenez-Pasbon, E. and Nelson, R.A. "Quantitative measurement of muscle tone in cats". Neurology (Minneap), 15:1120-6; (1965).

  3. Duggan, T.C. and McLellan, D.L. "Measurement of Muscle Tone: a method suitable for clinical use: Electroencephalography and Clinical Neurophysiology, 35:654-658, (1973).

  4. Ma, S.P. and Zahalak, G.I. "The mechanical response of the active human triceps brachii muscle to very rapid stretch and shortening". Biomechanics, Vol. 18, No. 8, pp. 585-598, (1985).

  5. Hufschmidt, A. and Mauritz, K.H. "Chronic transformation of muscle in spasticity: a peripheral contribution to increased tone". Journal of Neurology, Neurosurgery and Psychiatry, 48:676-685, (1985).

  6. Tardieu, C., et.al. "Muscle hypoextensibility in children with cerebral palsy: I. clinical and experimental observations". Arch Phys Med Rehabil., Vol. 63, p 97-102, (1982).

  7. Diez, V. and Berger W. "Normal and imparied regulation of muscle stiffness in gait: a new hypothesis about muscle hypertonia". Experimental Neurology, 79, 680-687, (1983).

  8. Edstrom, L. "Selective changes in the sizes of red and white muscle fibers in upper motor lesions and Parkinsonism". Journal of Neurological Science, 11:537-550, (1970).

  9. Spencer, G.S.G., and Zorab, P.A. "Spinal Muscle in Scoliosis Part I. Histology and Histochemistry". Journal of the Neurological Sciences, 30:137-142, (1976).