Neck pain and shoulder pain commonly arises from irritation of the C5 and C6 nerve roots. Both roots supply the deltoid muscle.

Deltoid has three portions namely the portion at the front of the shoulder (anterior deltoid), middle of the shoulder (middle deltoid) and a portion at the back of the shoulder (posterior deltoid). It arises from the front of the outer third of the collar bone, the acromion and the spine of the shoulder blade. It attaches to deltoid tuberosity on the arm bone (humerus). It is supplied by the axillary nerve which carries the C5 and C6 spinal nerve root fibers.

The anterior deltoid brings the arm up and forward (flexion), the middle deltoid moves the arm away from the trunk (abduction) and the posterior deltoid brings the arm backward (extension).

It is a powerful muscle in lifting type activities and also works constantly when the arm is outstretched to the front as in writing, typing, driving etc..

These forward motions of the arm known as shoulder flexion is performed by the contraction of many muscles which are large and powerful such as the pectoralis major, long head of the biceps, coracobrachialis and the anterior deltoid. To stabilize the flexion movement, the muscles that bring the arm backward (shoulder extension) have to undergo lengthening contraction. Shoulder extension is performed by three major muscles, mainly the latissimus dorsi, teres major and the posterior deltoid.

Pain, spasm and weakness of the poster deltoid is thus very common especially since C5 and C6 nerve root involvement is also very common. When the deltoid does not function properly, lifting type movements are performed by shrugging the shoulder, an action of the upper trapezius comes into play by . Since trapezius is not strong enough to perform lifting activities, the trapezius muscle goes into spasm and aggravating the neck and shoulder pain.

Therefore pain in the deltoid region cannot be localized to treating only the deltoid muscle. Selective activation using eToims Twitch Relief method must include all muscles involved in lengthening contraction supplied by the C5-C7 nerve roots and the entire length of the paraspinal muscles from the neck to the base of the spine.