Kinematics is a branch of mechanics that studies the motion of objects without concern for the forces that cause the motion. Kinematics can be applied to understand the movement of orbiting planets. These may be modeled as a point mass and described by a position, velocity, and acceleration in three-dimensional space. Baton twirling can also be described using kinematics. In addition to having position, velocity, and acceleration of the center of mass, a baton also has an angle, angular velocity, and angular acceleration. Often these do not align with standard Cartesian coordinates (x, y, z) so the movement contains components in multiple dimensions. Multi-component objects such as the human body can also be analyzed using kinematics. In this analysis, each body segment can be described by the position, velocity, and acceleration of the center of mass, and the angle, angular velocity, and angular acceleration within three-dimensional space. Body segments are connected to other body segments, which form constraints in the system. For example, one side of the forearm is connected to the upper arm at the elbow and the other side is connected to the hand at the wrist. Because of these constraints, all kinematic equations must be solved simultaneously in order to ensure that the constraints are not violated. These complex problems are usually solved using local coordinate systems and transformation matrices to convert the local coordinates to generalized coordinates.
Kinematics provides important information about a patient that can be used to guide surgeries and develop treatment plans. It can also be used to evaluate outcomes of treatment either from surgery, physical therapy, or a combination of both. Advanced 3D motion capture camera systems are capable of collecting movement data and automatically calculating kinematics using software programmed in the system.
