Further analysis of a simple prototypal muscle model near to and far from equilibrium.

The same prototypal model used in a previous paper to illustrate proper construction of a muscle model is modified here with the much more realistic choice e(Deltap) = 10(8) rather than e(Deltap) = 100, where e(Deltap) is the ratio of physiological ATP activity to equilibrium ATP activity. For steady isotonic contractions, the range 1 </= e(Delta) </= 10(4) can be approximated quite well by use of linear terms only in expansions of F (force) and J (ATP flux) in powers of e(Delta) - 1 and v (velocity). This will presumably also be true in most cases of much more complicated models. However, this region is of theoretical interest only (irreversible thermodynamics, etc.) because F and J are very small. In addition, numerical calculations of F and J were made in the region 10(4) </= e(Delta) </= 10(8). The optimal efficiency eta(*) is larger under physiological conditions (about 1%) than at equilibrium by a factor of 2.1 x 10(4). The rate of entropy production is discussed in this connection.