duty cycles" of motoneurons, while other motor unit types may be involved in "lower duty cycles." However, they state that the evidence is not conclusive "to support or deny the intuitively appealing notion that there is a correlation between metabolic activity, motoneuron size, and motor unit type." Under some circumstances, the normal order of motor unit recruitment may be altered, such that small motor units cease to fire and larger ones may be recruited. This is thought to be due to the interaction of excitatory and inhibitory motoneuronal inputs. Recruitment of motor unit types Another topic of controversy resides in the way in which Burke and colleagues categorized motor unit types. They designated three general groups by which motor units could be categorized: S (slow – slow twitch), FR (fast, resistant – fast twitch, fatigue-resistant), and FF (fast, fatigable – fast twitch, fatigable). These designations have served as the basis for motor unit categorization since their conception, but modern research indicates that human motor units are more complex and possibly do not directly fit this model. However, it is important to note that Burke himself recognized the risk in classifying motor units:My friend the late Elwood Henneman told me several times in conversation that he thought classifying motor units into distinct categories was probably a bad idea because, unless used with care, classifications tend to distort reality. I agreed, and still do, that taxonomies can lead to overly rigid thinking (and sometimes even lack of thinking) but they are necessary for communication, which requires that things be named; and scientific communication demands that things be named precisely, according to their attributes. If a correlation were to be drawn between Henneman's size principle and the motor unit categorization of Burke regarding the order of motor unit recruitment, it would resemble the following order: the smallest units, S (slow), would be recruited first, followed by larger FR (fast, resistant) units, and lastly the largest FF (fast, fatigable) units, reserved for high-energy tasks that require additional motor unit recruitment. Rate coding of muscle force The force produced by a single motor unit is determined in part by the number of muscle fibers in the unit. Another important determinant of force is the frequency with which the muscle fibers are stimulated by their innervating axon. The rate at which the nerve impulses arrive is known as the motor unit firing rate and may vary