Figure 1.

(a–c) The bite force F_b is a function of muscle stress σ, physiological cross-sectional area A_phys, pennation angle ϕ, apodeme angle γ and the mechanical advantage determined by inlever L_i and outlever L_o, the axis of rotation $\hat{\mathbf{R}}$ and the apodeme main axis $\hat{\mathbf{A}}$. The mechanical advantage and muscle stress, as well as the apodeme and pennation angle, vary with the mandibular opening angle θ, so affecting the magnitude of the maximum bite force that can be generated at different opening angles. γ and θ are defined with respect to the vector projections of $\hat{\mathbf{A}}$, L_i and L_o onto the rotational plane (rp). The position of this plane with respect to the head coordinate system, defined by the horizontal (hp), transversal (tp) and sagittal (sp) plane, can often be inferred from joint morphology [85]. (d) Muscle fibres attach either directly or via thin filaments to the apodeme, the functional equivalent of a vertebrate tendon. When muscle fibres shorten (L_f,0 to L_f), the pennation angles increase from ϕ₀ to ϕ, and the apodeme is displaced by Δ; the filament length remains approximately constant (see text). (e) As a result of the apodeme displacement, the inlever rotates about $\hat{\mathbf{R}}$ and γ changes. Notably, the apodeme appears to move in pure translation, i.e. its main axis has a constant orientation. The lateral displacement of the attachment point which must accompany mandible rotation is likely accommodated by rotation of the putatively soft apodeme ligament, which connects the apodeme to the mandible base.