People often confuse kinetics and kinematics. What are they, and which one can you derive from motion capture data?
Kinematics
Kinematic analysis concerns the geometric aspects of motion, including position, velocity, and acceleration. When dealing with motion capture, you typically get either position (from a camera-based system) or something similar to acceleration (from accelerometers). These are related, and since I have now figured out how to make equations here on the blog, here you get a summary of high school physics:
The position vector is $\mathbf{r}=(x,y,z)=x\mathbf{i}+y\mathbf{j}+z\mathbf{k}$, where $\mathbf{i},\mathbf{j},\mathbf{k}$ are unit vectors.
The path is $\mathbf{r}(t)=x(t)\mathbf{i}+y(t)\mathbf{j}+z(t)\mathbf{k}$.
Displacement is the vector change $\Delta\mathbf{r}$.
The instantaneous velocity is $\mathbf{v}=\dfrac{d\mathbf{r}}{dt}$.
The instantaneous acceleration is $\mathbf{a}=\dfrac{d\mathbf{v}}{dt}=\dfrac{d^2\mathbf{r}}{dt^2}$.
These definitions assume a non-rotating frame, so unit directions are constant. The beautiful thing about these equations is that they work both ways. You can derive the position to get velocity (and acceleration). Similarly, you can integrate acceleration to get velocity (and position), though there will be some drift.
Kinetics
Things are not equally easy when it comes to kinetics, which deals with forces and torques that cause motion.
In short, the kinetic energy can be calculated as $KE=\tfrac{1}{2}m \mathbf{v}^2$.
From motion capture data, you can easily obtain velocity $\mathbf{v}$ (either by deriving or integrating). However, you typically don’t know the mass $m$ and therefore cannot compute absolute kinetic energy (in joules).
There are some tricks, though. I remember discussing this with Petri Toiviainen a long time ago, around the time he implemented the mckinenergy function in the Mocap Toolbox for Matlab. There, he calculates kinetic variables from MoCap data using Dempster’s body-segment model. This model converts measured marker kinematics into segments, uses scaled anthropometric mass and inertia parameters, and sums translational and rotational energies per segment to produce an estimated total kinetic energy.
Conclusion
Taking some shortcuts, it is possible to calculate kinetic features from motion capture data, at least if you have a balanced full-body marker setup. In general, though, it is better to rely on kinematics. In my experience, there are plenty of things you can derive from position, velocity, and acceleration.
CoPilot helped me with formatting the equations correctly and Grammarly improved the grammar.