Motor units (MUs) are recruited according to the size principle. At low force output, macro-motor unit potentials (macro-MUPs) with small amplitude and area are recorded; MUs with large electrical size fire at high force output. Our objective was to elucidate whether this difference is caused by a higher fiber density (FD) of the MUs recruited at high force output. We measured the FD and respectively the amplitude and the area of the macro-MUPs via single-fiber macro electromyography (EMG) recorded at low and high force output in 48 healthy subjects. The macro-MUPs were significantly larger at high force output than at low force. The FD did not differ at the two force levels. Our findings suggest that the larger electrical size of the high recruitment threshold MUs is due not to a higher FD, but to a larger territory. FD is a robust EMG parameter, independent of the level of the force output.