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Muscle forces and spinal loads at C4/5 level during isometric voluntary efforts


Medicine & Science in Sports & Exercise: April 2000 - Volume 32 - Issue 4 - p 830-838
Applied Sciences: Biodynamics

CHOI, H. and RAY VANDERBY, JR. Muscle forces and spinal loads at C4/5 level during isometric voluntary efforts. Med. Sci. Sports Exerc., Vol. 32, No. 4, pp. 830–838, 2000.

Purpose: The goal of this study was to determine neck muscle forces and spinal loads that result from isometric muscle contractions.

Methods: Electromyographic (EMG) activity of the neck musculature and a three-dimensional biomechanical model of the neck were used. The model was EMG-based and estimated muscle forces and spinal loads at the C4/5 level. EMG signals were collected from eight sites at the C4/5 level of the neck using Ag-AgCl surface electrodes from 10 adult male subjects. The subjects performed isometric contractions gradually developing to maximum efforts in flexion, extension, left lateral bending, and right lateral bending.

Results: During maximum voluntary contraction (MVC) trials most muscles generated high levels of EMG signal during cervical rotation. The posterior surface of the neck (trapezius) was the only electrode site at which maximum activity EMG consistently occurred by the same method (rotation) in all subjects. Variations in the EMG patterns were observed in different experiments that produced overall neck moments of equal magnitudes. With these data the model computed variations in load distribution among the agonist muscles. Consistent also with EMG distributions, the model also computed co-contractions of antagonist muscles. The average (± SD) magnitudes of peak moments were 28.3 (± 3.3) Nm in extension, 17.7 (± 3.1) Nm in flexion, 16.9 (± 2.8) Nm in left lateral bending, and 17.0 (± 2.9) Nm in right lateral bending. The model predicted C4/5 joint compressive forces during peak moments were 1372 (± 140) N in extension, 1654 (± 308) N in flexion, 956 (± 169) N in left lateral bending, and 1065 (± 207) N in right lateral bending.

Conclusions: Results suggest that higher C4/5 joint loads than previously reported are possible during maximum isometric muscle contractions.

Division of Orthopedic Surgery and Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI

Submitted for publication March 1998.

Accepted for publication June 1999.

Address for correspondence: Ray Vanderby Jr., Ph.D., University of Wisconsin, Division of Orthopedic Surgery, G5/330 Clinical Science Center, 600 Highland Avenue, Madison, WI 53792-3228. E-mail:

© 2000 Lippincott Williams & Wilkins, Inc.