A comparison of lower-extremity skeletal kinematics measured using skin- 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 markers to measure the three-dimensional kinematics of the underlying bone. Kinematic data for marker arrays mounted on skeletal pins screwed directly 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 millimeters was measured; (2) The accuracy of segmental rigid body velocity estimates was inadequate for determining instantaneous helical axis (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 from the relative rotation of skin-mounted arrays had significant differences compared to the expected values due to soft tissue effects; and (5) Skin-mounted marker data exhibited a transient response to heel strike in gait, but for low-mass markers the transient was well-damped and could be removed with optimal smoothing.