Objective To make some revisions on the planimetric model of unipennate skeletal muscle,as well as Otten's mechanical analysis since Otten only considered the equilibrium of the aponeurosis(local equilibrium)and made too much simplification on the intramuscular pressure.Method The new method in this paper is based on the mechanical equilibrium of whole muscle and a volume-related force is introduced to keep muscle volume constant. Mechanical equilibrium of whole muscle yields a different relation between fiber and muscle force,compared to relations derived when only equilibrium of aponeurosis is considered.Results The newly derived relation improves prediction of the rat gastrocnemius muscle force-length characteristics.Conclusion The prediction of muscle force-length characteristics is very good with a simple model such as a planimetric model,which suggests that the influence of properties neglected in such a simple model is either small or internally compensated in the net effects.