Source:mirv camexport
From HLAEwiki
Supported engines: Counter-Strike Source
Exporting camera motion data
Set your fps with host_framerate.
Start recording as usual.
To start exporting (replace fps with the same value you set for host_framerate):
mirv_camexport start test.bvh fps
In case no path is specified the .bvh file will be created in the folder where the hl.exe of your game is located.
To stop exporting:
mirv_camexport stop
Stop recording.