To control force accurately under a wide range of behavioral conditions, the central nervous system would either require a detailed, continuously updated representation of the state of each muscle (and the load against which each is acting) or else force feedback with sufficient gain to cope with variations in the properties of the muscles and loads. The evidence for force feedback with adequate gain or for an appropriate central representation is not sufficient to conclude that force is the major controlled variable in normal limb movements.

Morton's hypothesis, that length is controlled by a follow-up servo, has a number of difficulties related to the delays, gains, variability, and specificity in feedback pathways comprising potential servo loops. However, experimental evidence is consistent with these pathways providing servo assistance for some movements produced by coactivation of α- and static γ-motoneurons. Dynamic γ-motoneurons may provide an additional input for adaptive control of different types of movements.

The idea that feedback is used to compensate for changes in muscle stiffness has received experimental support under static postural conditions. However, reflexes tend to increase rather than decrease the range of variation in muscle stiffness during some cyclic movements. Theoretical problems associated with the regulation of stiffness are also discussed. The possibilities of separate control systems for velocity or viscosity are considered, but the evidence is either negative or lacking. I conclude that different physical variables can be controlled depending on the type of limb movement required. The concept of stiffness regulation is also useful under some conditions, but should probably be extended to the regulation of the visco-elastic properties (i.e., the mechanical impedance) of a muscle or joint.

The book describes three elementary units of action – the reflex, the oscillator, and the servomechanism – and the principles by which they are combined to make complex units. The combining of elementary units to make complex units gives behavior and the neural circuitry underlying behavior a hierarchical structure. Circuits at higher levels govern the operation of lower circuits by selective potentiation and depotentiation: by regulating the potential for operation in lower circuits – raising the potential for some and lowering it for others – a higher unit establishes the overall pattern to be exhibited in the combined operations of the lower units, while leaving it to the lower units to determine the details of the implementation of this pattern. A theory of motivation of the kind long championed by ethologists and physiological psychologists grows out of the notion of the selective potentiation and depotentiation of hierarchically structured units of behavior. The question then becomes how this conception of motivation may be integrated with a conception of a learned representation of the world to yield a theory of how animals produce novel motivated behavioral sequences based on learned representations. The representation of space, which is central to the behavior of organisms at least as lowly as the digger wasp, is a promising domain for the investigation of this question. Another promising area is the representation of skilled movements, such as handwriting. A number of issues in cognitive and developmental psychology are illumined by this theory about the organization of action. Among these are the degrees-of-freedom problem, the role of practice in development and in the learning of skilled action, and the role of mental rotation of spatial representations.