SIZE PRINCIPLE AND INFORMATION-THEORY

The motor units of a skeletal muscle may be recruited according to dif ferent strategies. From all possible recruitment strategies nature sel ected the simplest one: in most actions of vertebrate skeletal muscles the recruitment of its motor units is by increasing size. This so-cal led size principle permits a high precision in muscle force generation since small muscle forces are produced exclusively by small motor uni ts. Larger motor units are activated only if the total muscle force ha s already reached certain critical levers. We show that this recruitme nt by size is not only optimal in precision but also optimal in an inf ormation theoretical sense. We consider the motoneuron poor as an enco der generating a parallel binary code from a common input to that pool . The generated motoneuron code is sent down through the motoneuron ax ons to the muscle. We establish that an optimization of this motoneuro n code with respect to its information content is equivalent to the re cruitment of motor units by size. Moreover, maximal information conten t of the motoneuron code is equivalent to a minimal expected error in muscle force generation.