1. Ph.D. Student, Depts. of Civil Engineering and Mechanical Engineering, Univ. of Bristol, Bristol BS8 1TR, U.K. (corresponding author).
2. Reader, Dept. of Civil Engineering, Univ. of Bristol, Bristol BS8 1TR, U.K.
3. Senior Lecturer, Depts. of Mechanical Engineering and Anatomy, Univ. of Bristol, Bristol BS8 1TR, U.K.