Joint Reaction Forces Muscle Moments Joint Power Link Segment Model Joint Reaction Forces Muscle Moments Joint Power

Newton’s Laws of Motion Law of Inertia: A body at rest stays at rest and a body in motion stays in motion unless acted upon by a force or torque. Law of Acceleration: The acceleration a body experiences is directly proportional to the force or torque, inversely proportional to the mass or moment of inertia and occurs in the direction of the force or torque. Law of Reaction: For every action there is an opposite reaction.

Newton’s First Law: Law of Inertia Law of Inertia: A body at rest stays at rest and a body in motion stays in motion unless acted upon by a force or torque. Mass is the measure of an object’s resistance to linear change. Moment of inertia is the measure of an object’s resistance to angular change. ΣF = m a ΣT = Iα To solve for forces and torques, we need both mass and moment of inertia.

Moment of Inertia (I) ICG= 3.5 kg·m2 The Moment of Inertia (I) represents an objects resistance to angular change about some axis. Moment of inertia is a sum of the product of mass times the squared distance of the mass about the axis. When body parts are moved closer to the axis of rotation the moment of inertia (resistance to spin) is reduced. ICG= 15 kg·m2 ICG= 6.5 kg·m2 ICG= 83 kg·m2

Link Segment Model Assumptions Each segment has a fixed mass located at its center of mass The joints are considered as hinge joints The moment of inertia is fixed during movement The length of each segment is constant

Input Information to Solve for Joint Forces and Torques Mass & moment of inertia of each segment. Location of each segments Center of Mass (CM). Position, velocity, acceleration of each segments CM (x, y, Vx, Vy, Ax, Ay). Angular position, velocity, acceleration about each segments CM (Θ, ω, α). External forces for all contact points. Center of pressure or location of external forces acting on the body. Center of Pressure gives the location of external forces acting on the body.

Inverse Dynamics: Finding Forces & Torques from Position Data Inverse Dynamics: Finding Forces & Torques from Position Data. [working backward to solve for forces & torques]

Anatomical Relationship

Forces & Torques Acting On Each Segment

Joint Moments