FUNCTIONAL SUBDIVISION OF THE UPPER TRAPEZIUS MUSCLE DURING MAXIMAL ISOMETRIC CONTRACTIONS

The electromyographic (EMG), activity pattern across the upper trapezi us of 22 healthy subjects was investigated during maximal isometric co ntractions. Eight bipolar surface electrodes with 10 mm distance betwe en adjacent electrode pairs were placed on a line from the clavicle to the scapula. At the region near the clavicle the highest EMG amplitud es were recorded during 90 degrees arm abduction. At the more posterio r parts the highest amplitudes were found both during arm abduction an d shoulder elevation. A double differential recording technique which reduced the EMG cross-talk contribution supported the finding that the upper trapezius was differently activated when the arm posture was ch anged. The normalized EMG amplitude-force relationship during the shou lder elevation showed a curvilinear relationship on the anterior part of the upper trapezius with a slower increase in ER;IG amplitude than force at low force. The slope of the curve, at low force, increased gr adually in the posterior direction on the upper trapezius. The EMG act ivity patterns across the upper trapezius indicate a flexibility in mo tor activation which maybe reflects a functional optimization of the c ontractions performed by this muscle.