Abstract
Introduction: Neck pain often arises without any evident trauma suggesting that everyday loading may cause fatigue damage to spinal tissues. However, little is known about the forces acting on the cervical spine in everyday life. The purpose of this study was to determine spinal compressive forces using an electromyo-graphic (EMG) technique.
Methods: Eight subjects performed a number of tasks while cervical flexion/extension and surface EMG activity of upper trapezius and sternocleidomastoid were measured. Dynamic EMG signals were corrected for contraction speed, using a correction factor obtained from lumbar muscles, and were then compared with isometric calibrations in order to predict moment generation. Calibrations were performed in different amounts of cervical flexion/extension by each subject to account for changes in the EMG-moment relationship with muscle length. Compressive force on the C7-T1 intervertebral disc was determined by dividing the generated moments by the resultant lever arm of flexor or extensor muscles obtained from MRI scans on the same subjects.
Results: Peak values (mean ± SD) of extensor and flexor moments increased from 1.9±1.6Nm and 1.4±1.0Nm respectively in standing to 52.7±32.2Nm and 4.2±1.8Nm when lifting above the head. Resultant muscle lever arms ranged between 3.0–5.2cm and 1.6–3.5cm for extensor and flexor muscles respectively. Therefore, peak compressive forces on the C7–T1 disc were 110±74N in standing and 1570±940N during overhead lifting.
Conclusion: Neck muscles generate high forces in activities such as overhead lifting. If applied on a repetitive basis, such forces could lead to the accumulation of fatigue damage in life.
Correspondence should be addressed to Ms Alison McGregor, c/o BOA, SBPR at the Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE.
