Knowledge Base

Article Number: 11 | Post Date: August 16, 2016 | Last Updated: August 19, 2016

## I need to conduct a biomechanical study involving head impacts. How can I access data for the multibody head segment?

This feature is available as of the 2016-08-16 update.

To access this feature, go to the report dynamics menu in the left side control panel. Select "multibody elements." Let's suppose we have an accident case involving a person falling backward off of a high platform (shown below).

Here is a video of the simulation:

You'll notice in the report dynamics window, data for all segments is now visible under the multibody data:

For our analysis, let's paste our head segment data into a spreadsheet tool:

Suppose our analysis required an estimate of head Delta-V upon ground impact; in a fairly straight-forward analysis, we can simply use the change in Earth frame position data to estimate Earth frame velocity of the head segment. Simply take (CG-position(time+time_step) - CG-position(time))/time_step. Do this for the cg x,y,z positions separately. Once you have the velocity estimates in Earth frame, you can then calculate the Delta-V at each time step. Since the multibody physics model effectively models the impulse exchange as being instantaneous, taking the maximum instantaneous Delta-V is sufficient to estimate the "full"  Delta-V.

In this case, the head segment underwent a 9.55 mph Delta-V at ground contact (14 fps). Neglecting restitution, the equivalent drop height for this Delta-V would be about 3 ft.

Note the Diagram tool can also be used to analyze segment velocity versus time. In this case, we note the right hand and arm make ground contact prior to the head segment, thereby causing some loss in velocity before head impact.

With the 2016-08-19 auto update, one has the ability to report segment velocity data in either the Global (Earth) frame or the local segment frame. This can be very useful for multibody studies, as it makes estimate velocity changes to individual segments much easier.

In another fall scenario shown below, we see a multibody undergoing ground contact with the head segment. In the report dynamics menu in the left side control panel, select both the "multibody elements" and "use global space" options. "use global space" will cause the report table to show velocity direction data as defined in the Global (Earth) frame. With this option enabled, "vni" corresponds to the velocity vector angle about the Global z-axis, and vnz corresponds to the velocity vector angle with respect to the Global x-y plane.

With the velocity magnitude and direction defined in Global space, it is now a matter of simple trigonometry to estimate the Head Delta-V as a function of time.

Tags: Segment data, accessing multibody data, how to print multibody data.