Bite and joint force analysis in Caiman crocodilus.

The equilibrium of forces acting on the lower jaw during biting can be assessed by static modelling. Usually, no account is taken for the actual recruitment level of the involved jaw adductors and only one fixed orientation of the food reaction forces is considered. These conditions conflict with reality. Therefore, recruitment levels of eight muscles of Caiman crocodilus are determined by means of quantitative EMG. For 12 crushing bites and one holding bite, these levels were normalized (per muscle) to the maximal activity level ever observed in a total of 72 bites. These activity levels were used as input for a static bite model. A large numbers of simulations are run in which the orientation of the food reaction forces varies over a large range. Several bite points are considered. The model calculates the magnitude of the bite forces and the orientation and magnitude of the joint forces. The results for the individual bites are compared to a model simulation where all muscles are fully active (100\%) and to an averaged bite representing a generalized crushing bite of Caiman. This allows to assess the biological meaning of such simulations. It turns out that (apart from the absolute size of the forces) both the 100\% model and the generalized bite simulation result in an equilibrium condition which closely approximates the actual in vivo equilibria. Some functional and morphological implications are discussed.