The Masseter Muscle The masseter muscle, the principal...

The Masseter Muscle The masseter muscle, the principal muscle for chewing, is one of the strongest muscles for its size in the human body. It originates on the lower edge of the zygomatic arch (checkbone) and inserts in the angle of the mandible. Referring to the lower diagram in Figure $11-39,$ where $d=7.60 \mathrm{cm}$ and $D=10.85 \mathrm{cm},$ (a) find the torque produced about the axis of rotation by the masseter muscle. The force exerted by the masseter muscle is $F_{\mathrm{M}}=455 \mathrm{N}$. (b) Find the biting force, $F_{\mathrm{B}}$ exerted on the mandible by the upper teeth. Find $(\mathrm{c})$ the horizontal and $(\mathrm{d})$ the vertical component of the force $F_{\mathrm{j}}$ exerted on the mandible at the joint where it attaches to the skull. Assume that the mandible is in static equilibrium, and that upward is the positive vertical direction.

The Masseter Muscle The masseter muscle, the principal muscle for chewing, is one of the strongest muscles for its size in the human body. It originates on the lower edge of the zygomatic arch (checkbone) and inserts in the angle of the mandible. Referring to the lower diagram in Figure $11-39,$ where $d=7.60 \mathrm{cm}$ and $D=10.85 \mathrm{cm},$ (a) find the torque produced about the axis of rotation by the masseter muscle. The force exerted by the masseter muscle is $F_{\mathrm{M}}=455 \mathrm{N}$.
(b) Find the biting force, $F_{\mathrm{B}}$ exerted on the mandible by the upper teeth. Find $(\mathrm{c})$ the horizontal and $(\mathrm{d})$ the vertical component of the force $F_{\mathrm{j}}$ exerted on the mandible at the joint where it attaches to the skull. Assume that the mandible is in static equilibrium, and that upward is the positive vertical direction.

Key Concepts

Torque

Torque is the rotational equivalent of force, representing how much a force acting on an object causes that object to rotate. It is defined as the product of the force and the perpendicular distance from the axis of rotation (the moment arm). This concept is not only key in mechanical systems like levers but also in anatomical contexts, such as the forces acting on the jaw during chewing.

Moment Arm

The moment arm is the distance between the axis of rotation and the line of action of the force. It is crucial in calculating torque, as the effectiveness of a force in producing rotation depends on the length of this perpendicular distance. In biomechanical systems such as the human jaw, the moment arm determines how different muscle forces result in rotational movements.

Static Equilibrium

Static equilibrium refers to the condition in which an object remains at rest because the net force and the net torque acting on it are zero. This principle is fundamental when analyzing systems like the mandible during biting, where all forces (muscular, biting, and joint reactions) must balance each other to maintain stability.

Force Decomposition

Force decomposition involves breaking a force vector into its horizontal and vertical components using trigonometric functions. This process is essential in mechanics to analyze the effect of forces in different directions, especially when considering how combined forces contribute to the overall equilibrium of a system.

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