The following reflect the general areas of interest of J. H. Challis (staff member of Biomechics Lab at Pennsylvania State University USA): Measurement and modeling of the complex parameters associated with a direct dynamics model of the human musculo-skeletal system, with the aim of examining the role, function, and co-ordination of muscle in vivo. (abstract 1) Investigation of human impacts, with emphasis on quantifying the effect of the "wobbling" mass on joint loads. Development of improved measurement protocols (three-dimensional photogrammetry, determination of segment position and orientation, data smoothing and differentiation). (abstract 2) Investigation of procedures for determining body segment inertial parameters. (abstract 5) Representative publications include Challis, J.H., and Kerwin, D.G. (1994) Determining individual muscle forces during maximal activity: Model development, parameter determination, and validation. Human Movement Science 13:29-61. (abstract 1) Challis, J.H. (1995) An examination of procedures for determining body segment attitude from noisy biomechanical data. Medical Engineering and Physics 17:2;83-90. (abstract 2) Challis, J.H. (1995) A procedure for determining rigid body transformation parameters. Journal of Biomechanics 28:5;733-737. (abstract 3) Challis, J.H., and Kerwin, D.G. (1996) Quantification of the "uncertainties" in resultant joint moments computed in a dynamic activity. Journal of Sports Sciences 14:219-231. (abstract 4) Challis, J.H. (1996) Accuracy of human limb moment of inertia estimations and their influence on resultant joint moments. Journal of Applied Biomechanics 12:517-530. (abstract 5)