Elsevier

Journal of Biomechanics

Volume 31, Issue 8, August 1998, Pages 713-721
Journal of Biomechanics

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

https://doi.org/10.1016/S0021-9290(98)00086-4Get rights and content

Abstract

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.

Introduction

Repeated bending and lifting activities greatly increase the risk of developing low back disorders (Kelsey et al., 1984; Magora, 1973; Marras et al., 1993). Bending forwards generates a high bending moment on the osteo-ligamentous lumbar spine (Adams and Dolan, 1991) and the effort of lifting generates a high compressive force at the same time (Dolan et al., 1994a). This combination of bending and compression can cause lumbar discs to prolapse posteriorly, especially if the forces are applied repetitively (Adams and Hutton, 1982; Adams and Hutton, 1985). According to these cadaveric experiments, disc prolapse occurs under a wide range of compressive forces, but only when the bending moment is high, suggesting that the amount of lumbar flexion is critical in determining the likelihood of disc injury.

Lumbar flexion increases markedly if objects are lifted with straight legs, or if the object to be lifted is bulky or far from the body (Dolan et al., 1994a). However, the particularly high risks associated with repetitive lifting suggests that there may be another cause of excessive lumbar flexion, and that is muscle fatigue. The erector spinae muscles protect the spine from excessive flexion, but in so doing impose a high compressive force on it. If these muscles become fatigued and therefore less able to generate high forces quickly, then the bending moment acting on the lumbar spine may increase, and the compressive force decrease.

Previously, we developed and validated techniques for quantifying (a) the bending moment acting on the osteo-ligamentous lumbar spine, from continuous measurements of lumbar curvature (Adams and Dolan, 1991) (b) spine compressive loading, from measurements of the electromyographic (EMG) activity of the back muscles (Dolan and Adams, 1993a) (c) back muscle fatigue, from frequency shifts in the EMG signal (Dolan et al., 1995; Mannion and Dolan, 1994). In the present experiment, we combine these techniques in order to test the hypothesis that repetitive bending and lifting movements lead to fatigue in the back muscles and altered spinal loading.

Section snippets

Overview of experiments

Several measurements were made on healthy subjects before, during and immediately after a repetitive lifting task. Unfortunately, not all measurements could be made simultaneously, so subjects were assessed on two separate days (days A and B) with at least three days rest in-between. Subjects were randomly assigned to perform the tests of day A before or after those of day B. During the repetitive lifting task, continuous measurements were made of lumbar flexion, and of EMG activity in the

Frequency of lifting

The average time taken to complete the 100 lifts was 12.1 min (STD 1.9) on day A, indicating a mean lifting rate of 8.4 lifts per minute. A similar rate was observed on day B (Table 1). The frequency of lifting was consistent except for a slight increase during lifts 91-5 which reached significance on day A (Table 1).

Lumbar flexion and bending moment

Range of lumbar flexion (i.e. lumbar mobility) showed a small but significant increase on day A from 54.9° (8.7) to 55.7° (8.9) following the 100 lifts (p=0.05). A similar small

Evaluation of techniques

Movement artefacts have long been a problem with skin-surface EMG, but in preliminary tests we found that the most vigorous flicking of the leads generated a spurious signal less than 10% of the true signal. This is probably because the impedance between reference and recording electrodes was brought below 10 kΩ (and often below 5 kΩ) by means of careful skin preparation. Setting the high pass filter at 5 Hz also helped to minimise low-frequency noise. A high-frequency cut-off of 300 Hz was

Conclusions

  • 1.

    Repetitive forward bending and lifting movements substantially increase the peak bending moment acting on the osteoligamentous lumbar spine.

  • 2.

    Repetitive lifting also causes measurable fatigue in the erector spinae muscles.

  • 3.

    Poor correlation between muscle fatigue and increased spinal bending may be attributable to deficiencies in the currently-used EMG indices of fatigue. New methods may be required to measure fatigue in dynamically contracting muscle.

Acknowledgements

This work was funded by the arthritis research campaign (UK). Patricia Dolan and Michael Adams are ARC Research Fellows.

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