Upper/middle trapezius muscle: instantaneous signal. Slight shoulder elevation.

Electrode positioning

Upper trapezius muscle: instantaneous signal. Differential acquisition along the fiber direction.

Map of the differential voltage distribution on the surface of the upper trapezius muscle. Grid of 5x13 electrodes with diameter of 3 mm spaced by 8 mm (32x96 mm).

Acquisition rate: 2048 frames per second.

Reproduction rate: 2 frames per second (1000 times slower) with image interpolation.

The surface motor unit action potentials, each comprising a positive peak (red) and a negative peak (blue), are generated in the center of the image (innervation zone) and propagate toward the acromion (right) and the and the fifth cervical vertebra (left) along the muscle fibers.