The normal pattern of surface strain produced in the distal third of sheep tibiae was determined by attaching rosette strain gauges to the cranial (anterior), medial, and caudal (posterior) cortices of the tibia. The strain data showed that during locomo tion the cranial (longitudinally concave) cortex was the tension surface and the caudal (longitudinally convex) cortex was the compression surface. (This relationship between bending and curvature is the reverse of that which occurs in the radius of the same species.) The principal strain directions remained almost constant throughout the main strain period of each stride and were practically unaffected by the speed of locomotion. On neither the cranial nor the caudal surface of the tibia was the larger principal strain aligned along the long axis of the bone. On the cranial surface it formed a proximal and lateral angle of 29 degrees with this axis and on the caudal surface, a similar angle of 23 degrees. This strain pattern is consistent with a loading regimen of craniocaudal bending and torque. The mean peak principal strain on the cranial and caudal surfaces of the bone during walking in five sheep was +709 and - 666 microstrain, respectively (ratio, 1.07 to one). In eight sheep the mean thickness of the cranial cortex was 3.0 millimeters and of the caudal cortex, 2.87 millimeters (ratio, 1.03 to one).

The directions of secondary osteons in the cranial and caudal cortices of the tibia in these eight sheep were shown to lie between the direction of the long axis of each bone and that of the larger principal strain in that cortex during locomotion. Determinations of the transverse axes of the proximal and distal joint surfaces of the tibia in adult sheep and in sheep in late fetal life showed that during postnatal development the dis tal end of the bone rotates internally some 14 degrees with respect to the proximal end. The limited strain data available from the tibia of a human suggest that the tibial torque during locomotion is in the same di rection in both sheep and humans. The developmental rotation of the tibia in these species, however, is in the opposite direction.