Three-dimensional constraint forces of the finger tendons and joints in isometric hand functions were determined. The joint and tendon orientations were defined from biplanar X-ray films. Coordinate systems at each joint were used to define the constraint forces and moments. Four index fingers, one long finger, and one little finger were analyzed. Through free-body analysis, a statically indeterminate problem was derived. The method of systematic elimination of redundant unknown tendon forces was applied. Constraint conditions based on EMG and physiologic assessments were used to obtain admissible solutions. The findings are important in understanding the functional anatomy and pathologic deformities involving the hand and the basic requirements on prosthetic design of finger joints.