I have been quite interested in the recent BIOMCH-L postings about
joint kinematics and kinetics, in particular about the planar ICR
(instant center of rotation) and the spatial ISA (instant screw axis)
- also denoted as the spatial IHA (instant helical axis). I have two
points which I would like to discuss in this regard.

1) We have developed linear least-squares methods to compute both
two- and three-dimensional INSTANTANEOUS angular velocity and
angular acceleration - and consequently the ICR/ISA and the AAA
(angular acceleration axis) - from positon, velocity, and
acceleration of multiple point landmarks on a rigid body. If using
video/photo methods, the velocity and acceleration of the landmarks
may be determined by smoothing and differentiating landmark
position trajectories. These methods have been combined to also
determine the instantaneous central point of the screw axode ruled
surface (the point on the ISA with minimum acceleration about
which the ISA instantaneously changes direction with time) denoted
as the spatial ICR by Herman Woltring in recent BIOMCH-L postings.

Mathematical development of these methods has been presented and
published through ASME. Application of these methods to
biomechanics will be presented in July at the Int. Symp. on 3D
Analysis of Human Movement in Montreal.

2) Although Herman Woltring and I may be the only two people in the
world interested, these methods may be easily extended to
determine two- and three-dimensional angular jerk from third
derivatives of landmark position trajectories.

H.J Sommer, Professor of Mechanical Engineering
The Pennsylvania State University
University Park, PA 16802 USA
phone (814)865-9214 FAX (814)863-4848
bitnet hjs@psuecl internet hjs@ecl.psu.edu