Objective
To determine the function of the normal upper trapezius and scalenus medius muscles during neck motion by quantitative eletromyographic analysis.

Method
Nine subjects were evaluated electromyographically with monopolar fine wire electrodes. The isometric tilting and rotation of neck with manual resistance were performed in the sitting position. The Root Mean Square (RMS) and Mean Rectified Voltage (MRV) were recorded.

Results
The RMS and MRV of upper trapezius muscle were significantly higher at each degree of ipsilateral rotation than contralateral rotation. But the RMS and MRV of scalenus medius muscle were significantly higher at each degree of contralateral rotation than ipsilateral rotation. The RMS and MRV of upper trapezius and scalenus medius muscles were significantly higher at ipsilateral tilting than at contralateral tilting.

Conclusion
Based on these results, we concluded that the upper trapezius muscle acts more in ispilateral rotation, while scalenus medius muscle acts in contralateral rotation, and upper trapezius and scalenus medius muscles act in ipsilateral tilting.