A COMPARISON OF LOWER-EXTREMITY SKELETAL KINEMATICS MEASURED USING SKIN-MOUNTED AND PIN-MOUNTED MARKERS

Measurement of three-dimensional, skeletal kinematics is important for clinicians and engineers alike. Most in vivo motion data are acquired using skin-mounted markers or marker arrays, Experiments were carried out to quantitatively evaluate the validity of using skin-mounted mar kers to measure the three-dimensional kinematics of the underlying bon e. Kinematic data for marker arrays mounted on skeletal pins screwed d irectly into the bone were compared with data for markers, and arrays of markers, mounted on the skin. Findings included: (1) Task-dependent soft tissue motion relative to the underlying bone of up to twenty mi llimeters was measured; (2) The accuracy of segmental rigid body veloc ity estimates was inadequate for determining instantaneous helical axi s (IHA) parameters; (3) Power spectra for skin- and pin-mounted arrays cover similar frequency bands and there was no evidence of a distinct , frequency domain soft tissue artifact; (4) Joint angles calculated f rom the relative rotation of skin-mounted arrays had significant diffe rences compared to the expected values due to soft tissue effects; and (5) Skin-mounted marker data exhibited a transient response to heel s trike in gait, but for low-mass markers the transient was well-damped and could be removed with optimal smoothing.