Figure 4. Cumulative muscle force vs. number of recruited motor units. Based on Kernell (1992). The recruitment of motor units follows the size principle. According to this principle, the smallest (slow, fatigue-resistant) motor units are recruited first; these units produce relatively small forces. Therefore, for a small muscle force to be produced, a relatively large number of motor units is recruited. For muscle force to be increased, the larger (faster, less fatigable) units are recruited; these units produce larger forces, and thus, a relatively small number of motor units is recruited. As a consequence of the size principle, a finer modulation of muscle force is possible at lower than at higher muscle forces.