In recruitment, the strength of contraction of a muscle is increased by increasing the number of motor units stimulated. When only a few motor units are stimulated, a small force of contraction is produced, because only a small number of muscle fibers are contracting. As the number of motor units stimulated increases, more muscle fibers are stimulated to contract, and the force of contraction increases. Maximum force of contraction is produced in a given muscle when all the motor units of that muscle are stimulated.