OBJECTIVE The purpose of this study was to understand the detailed architectural properties of the human flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU) muscles and their implications for tendon transfer surgery. METHODS Muscle fiber length was measured in 6 separate regions of the FCU and FCR from 10 cadaveric specimens. Sarcomere length was measured by laser diffraction for normalization. Moment arms were estimated by measuring tendon excursion with respect to joint angle. The position of entry of the motor nerve branches into each muscle also was measured to establish limits for the safe length of muscle mobilization. RESULTS Muscle fiber length varied significantly along both the FCU and FCR. Fiber length variability in the FCU was twice that of the FCR. Although the average fiber length for both muscles across all regions was similar (62.6 +/- 2.1 mm for the FCR and 63.1 +/- 4.0 mm for the FCU), the proximal fibers of the FCU were longer compared with the proximal fibers of the FCR and the distal fibers of the FCU were shorter compared with the distal fibers of the FCR. The 99% confidence interval for the second nerve branch entry into the muscles was located approximately 69 mm distal to the medial epicondyle for the FCU and approximately 73 mm distal for the FCR. CONCLUSIONS These data show different designs of both the FCU and the FCR. The functional significance of fiber length variability is not clear but imply that, when used in tendon transfer, the properly mobilized FCU has a much greater excursion.