Patricia Dolan

Electromyographic median frequency changes during isometric contraction of the back extensors to fatigue.

Study Design. This was a cross‐sectional study involving 229 healthy, back pain‐free individuals. Objectives. To examine the relationship between electromyographic manifestations of fatigue and endurance time during isometric contraction of the back extensors to fatigue. Summary of Background Data. Despite the widespread use of electromyography to monitor muscle fatigue, its relationship with endurance time has not been well investigated. Methods. Using skin‐surface electrodes, electromyographic signals were recorded from thoracic (T10) and lumbar (L3) regions of erector spinae during an isometric endurance test, and the rate of change in median frequency of the electromyographic power spectrum (MFGRAD) was calculated. Results. MFGRAD was significantly higher at L3 than at T10. The best predictor of endurance time was given by the greater MFGRAD observed at either region. MFGRAD calculated over a submaximal time period (50% total time or 60 sec) also correlated significantly with endurance time. Women displayed a significantly longer endurance time and lower MFGRAD than men. Conclusions. Endurance appears to be limited by the most fatigable region of the muscle group. MFGRAD is a suitable technique for monitoring back muscle fatigue, even when it is determined over a submaximal time period. The back extensors of women are less fatigable than those of men when the same task is performed. [Key words: fatigue, electromyography, endurance, erector spinae, median frequency] Spine 1994;19:1223–1229

Sustained loading generates stress concentrations in lumbar intervertebral discs.

Study Design Cadaveric motion segment experiment. Measurements on each specimen were compared before and after creep loading. Objectives To show how sustained “creep” loading affects stress distributions inside intervertebral discs. Summary of Background Data The central region of an intervertebral disc acts like a hydrostatic “cushion” between adjacent vertebrae. However, this property depends on the water content of the tissues and may be lost or diminished after creep. Methods Twenty‐seven lumbar motion segments consisting of two vertebrae and the intervening disc and ligaments were loaded to simulate erect standing postures in life. The distribution of compressive stress in the disc matrix was measured by pulling a miniature pressure transducer through the disc in the midsagittal plane. Profiles of vertical and horizontal compressive stress were repeated after each specimen had been creep loaded in compression for 2‐6 hours. Results Creep reduced the hydrostatic pressure in the nucleus by 13‐36%. Compressive stresses in the anulus were little affected when the profiles were measured at 1 kN, but at 2 kN, localized peaks of compressive stress appeared (or grew in size) in the posterior anulus after creep. Conclusions Increased loading of the apophysial joints causes an overall reduction in intradiscal stresses after creep. In addition, water loss from the nucleus causes a transfer of load from nucleus to anulus. Stress concentrations may lead to pain, structural disruption, and alterations in chondrocyte metabolism. Disc mechanics depend on loading history as well as applied load.

Diurnal Variations in the Stresses on the Lumbar Spine

Two complementary experiments were performed, the first on living people and the second on cadaveric spines. In the first experiment, electronic inclinometers were used to measure the range of lumbar flexion of 21 volunteers in the early morning and in the afternoon. The results showed that the range of movement increased by 5° during the day. In the second experiment, cadaveric lumbar motion segments were creep loaded to simulate a day’s activity and their bending properties were measured before and after creep. The results showed that creep loading reduces the spine’s resistance to bending (the effect being particularly marked in the disc) and increases the range of lumbar flexion by 12.5°. The results of the two experiments were combined to show that in life, forward bending movements subject the lumbar spine to higher bending stresses in the early morning compared with later in the day. The increase is about 300% for the discs and 80% for the ligaments of the neural arch. It is concluded that lumbar discs and ligaments are at greater risk of injury in the early morning.

The stages of disc degeneration as revealed by discograms

One hundred and thirty-nine discs from cadaveric lumbar spines were injected with a mixture of radio-opaque fluid and dye. Discograms were taken and the discs were then sectioned in the sagittal plane. Examination of the sections revealed that injected fluid did not at first mix with the disc matrix but pushed it aside to form pools of injected fluid. The location of these pools, and hence the appearance of a discogram, depended on the stage of degeneration of the disc. It is concluded that useful clinical information can be obtained from discograms.

Recent advances in lumbar spinal mechanics and their clinical significance

Of the many problems associated with low back pain, those which are most amenable to biomechanical investigation are identified. Recent advances in lumbar spinal mechanics are then reviewed in five sections dealing with mechanical function, mechanisms of failure, movements in vivo, loading in vivo, and the biological consequences of mechanical loading. The discussion suggests that mechanical fatigue damage may frequently be the underlying cause of low back pain, even when degenerative changes are evident in the tissues, and the review ends by suggesting some priority areas for future research.

Repetitive lifting tasks fatigue the back muscles and increase the bending moment acting on the lumbar spine

During manual handling, the back muscles protect the spine from excessive flexion, but in doing so impose a high compressive force on it. Epidemiological links between back pain and repetitive lifting suggest that fatigued muscles may adversely affect the balance between bending and compression. Fifteen volunteers lifted and lowered a 10 kg weight from floor to waist height 100 times. Throughout this task, the bending moment acting on the osteoligamentous lumbar spine was estimated from continuous measurements of lumbar flexion, obtained using the 3-Space Isotrak. Spinal compression was estimated from the electromyographic (EMG) activity of the erector spinae muscles, recorded from skin-surface electrodes at the levels of T10 and L3. EMG signals were calibrated against force when subjects pulled up on a load cell, and correction factors were applied to account for changes in muscle length and contraction velocity. Fatigue in the erector spinae muscles was quantified by comparing the frequency content of their EMG signal during static contractions performed before, and immediately after, the 100 lifts. Results showed that peak lumbar flexion increased during the 100 lifts from 83.3 +/- 14.8% to 90.4 +/- 14.3%, resulting in a 36% increase in estimated peak bending moment acting on the lumbar spine (P = 0.008). Peak spinal compression fell by 11% (p = 0.007). The median frequency of the EMG signal at L3 decreased by 5.5% following the 100 lifts (p = 0.042) confirming that the erector spinae were fatigued, but measures of fatigue showed no significant correlation with increased bending. We conclude that repetitive lifting induces measurable fatigue in the erector spinae muscles, and substantially increases the bending moment acting on the lumbar spine.

Time-dependent changes in the lumbar spine's resistancc to bending

Abstract Objective. To show how time-related factors might affect the risk of back injury. Design. Mechanical testing of cadaveric lumbar motion segments. Background. High bending stresses acting on the lumbar spine are associated with injuries to the intervertebral discs and ligaments. Since these soft tissues are viscoelastic, the bending stress (‘bending moment’) must depend on the speed of movement and the duration of loading, but this has not previously been quantified. Methods. Forty-five cadaveric lumbar segments, consisting of two vertebrae and the intervening disc and ligaments, were loaded in combined bending and compression in order to simulate movements and postures in living people. The relationship between flexion angle and bending moment was determined at different loading rates, and after sustained loading in bending and in compression. Results. Rapid flexion movements increased the peak bending moment by 10–15% compared to slow movements. On average, repeated flexion over a period of 5 min reduced the peak bending moment by 17%, and 5 min of sustained flexion reduced it by 42%. Two hours of compressive creep loading reduced the height of the intervertebral discs by 1.1 mm, increased the range of flexion by 12%, and reduced peak bending moment by 41%. Conclusions. The scale of these changes suggests that, in life, the risk of bending injury to the lumbar discs and ligaments will depend not only on the loads applied to the spine, but also on loading rate and loading history.

Fatigue of the erector spinae muscles : a quantitative assessment using frequency banding of the surface electromyography signal

Study Design The authors investigated fatigue-induced changes in the frequency content of the surface electromyographic (EMG) signal from the erector spinae muscles. Objectives The objective of the study was to understand the EMG changes in fatiguing muscle and to obtain a rellable index of fatigue. Summary of Background Data Power spectral analysis has been used increasingly in recent years to monitor muscle fatigue, but parameters other than the mean or median frequency have received little attention. Methods Thirty-five healthy volunteers participated. They pulled upward with constant force on a handlebar attached to a floor-mounted load cell while the EMG signal from the erector spinae was recorded at the levels of T10 and L3 at 1024 Hz; 1.0-sec “windows” of the signal were analyzed using fast Fourier transforms, and the resulting power spectra were divided into 10 frequency bands between 5 Hz and 300 Hz. The median frequency, total power, and peak amplitude of the spectra were also calculated. Changes in the frequency content of the EMG signal were examined during submaximal contractions of different intensity and duration. Results Median frequency decreased steadily during the contractions, whereas total power and peak amplitude increased. The most repeatable and linear index of change was the increase in the EMG signal in the 5–30 Hz frequency band. The middle-to-high frequency component of the EMG signal increased during the early stages of the contractions, but decreased as the endurance limit was approached. Conclusions Changes in the 5–30 Hz band of the EMG power spectrum provide a more reliable and linear index of fatigue in the erector spinae muscles than do changes in median frequency. In the erector spinae, the early effects of fatigue appear to be delayed by the recruitment of additional motor units.

Diurnal changes in spinal mechanics and their clinical significance

Diurnal changes in the loads acting on the spine affect the water content and height of the intervertebral discs. We have reviewed the effects of these changes on spinal mechanics, and their possible clinical significance. Cadaveric lumbar spines subjected to periods of creep loading show a disc height change similar to the physiological change. As a result intervertebral discs bulge more, become stiffer in compression and more flexible in bending. Disc tissue becomes more elastic as its water content falls, and its affinity for water increases. Disc prolapse becomes more difficult. The neural arch and associated ligaments resist an increasing proportion of the compressive and bending stresses acting on the spine. Observations on living people show that these changes are not fully compensated for by modified muscle activity. We conclude that different spinal structures are more heavily loaded at different times of the day. Therefore, the time of onset of symptoms and signs, and any diurnal variation in their severity, may help us understand more about the pathophysiology of low back pain and sciatica.

Discogenic Origins of Spinal Instability

Study Design. Cadaveric motion segment experiment. Objective. To show how two physical aspects of disc degeneration (dehydration and endplate disruption) contribute to spinal instability. Summary of Background Data. The origins of spinal instability and its associations with back pain are uncertain. Methods. Twenty-one cadaveric thoracolumbar motion segments aged 48 to 90 years were secured in cups of dental plaster and loaded simultaneously in bending and compression to simulate full flexion, extension, and lateral bending movements. Vertebral movements, recorded using a two-dimensional “MacReflex” motion analysis system, were analyzed to calculate neutral zone (NZ), range of motion (ROM), bending stiffness (BS), horizontal translational movements, and the location of the center of rotation (COR). Intradiscal “stresses” were measured by pulling a miniature pressure transducer through the disc along its midsagittal diameter. All experiments were repeated after each of two treatments, which simulated physical aspects of disc degeneration: creep loading to dehydrate the disc and compressive overload to disrupt the endplate. Results were analyzed using ANOVA and linear regression. Results. Motion segment height was reduced by 1.0 (SD 0.3) mm during creep and by a further 1.7 (0.6) mm after endplate disruption. In flexion and lateral bending, the combined treatments increased NZ and ROM by 89% to 298%, and increased the “instability index” (NZ/ROM) by 43% to 61%. Translational movements increased by 58% to 86%, whereas BS decreased by 42% to 48%. In extension, ROM and NZ were little affected, although the COR moved closer to the apophyseal joints. Measures of instability increased most in lateral bending, and following endplate disruption. Stress concentrations in the posterior anulus fibrosus increased markedly after endplate disruption. Conclusions. Two physical aspects of disc degeneration (dehydration and endplate disruption) cause markedsegmental instability. Back pain associated with instability may be attributable to stress concentrations in degenerated discs.