Inverse Dynamic Analysis
Kinetics is the study of the forces and moments that cause motion of a body. For human movement, biomechanists attempt to determine the forces that result from muscle contractions and the moments that are produced. These forces are within the body and cannot be measured directly (without using invasive procedures). One way to estimate these forces and moments is using inverse dynamic analysis.
Inverse dynamic analysis uses link-segment models to represent the mechanical behavior of connected limbs. Biomechanists first measure the kinematics and externally applied forces such as ground reaction forces. Form this information net joint reaction forces and net moments can be computed through a series of differential equations based on the equations of motion.
For inverse dynamic analysis of walking, internal forces and moments at the ankle can be calculated using knowledge of the ground reaction forces and solving for equilibrium. The internal forces and moments at the knee can then be calculated by taking into account the forces and moments at the ankle and the linear and angular acceleration of the shank. Internal forces and moments at the hip can be calcualted in a similar manner.
It is common to make certain assumptions within inverse dynamics calculations. Some of these assumptions include:
- The segments are rigid
- The distribution of each segments mass is uniform
- All the forces produced by the joint elements pass through the joint centers
- There is no co-contraction of agonist and antagonist muscles
- Air friction is negligible
