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

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 fifth cervical vertebra (left) along the muscle fibers.