The role of the hamstrings in extension of the hip was determined in eight normal subjects by measuring maximum isometric hip extension torque in different positions of hip flexion before and after sciatic-nerve block. Total hip extensor strength increased more than twofold as the hip was flexed from zero to 90 degrees. Following sciatic-nerve block, hip extension strength was reduced approximately 50 per cent in all positions of hip flexion. The hamstrings accounted for approximately one-third of total measured extensor torque.