Motion capture technology generally requires that recordings be
performed in a laboratory or closed stage setting with controlled
lighting. This restriction precludes the capture of motions that re-
quire an outdoor setting or the traversal of large areas. In this paper,
we present the theory and practice of using body-mounted cameras
to reconstruct the motion of a subject. Outward-looking cameras
are attached to the limbs of the subject, and the joint angles and root
pose are estimated through non-linear optimization. The optimiza-
tion objective function incorporates terms for image matching error
and temporal continuity of motion. Structure-from-motion is used
to estimate the skeleton structure and to provide initialization for
the non-linear optimization procedure. Global motion is estimated
and drift is controlled by matching the captured set of videos to ref-
erence imagery. We show results in settings where capture would be
difficult or impossible with traditional motion capture systems, in-
cluding walking outside and swinging on monkey bars. The quality
of the motion reconstruction is evaluated by comparing our results
against motion capture data produced by a commercially available
optical system.