This study evaluated intramuscular movement of hamstrings muscle after reconstruction of the ACL with a hamstrings tendon graft. The movement of the muscles during knee flexion was measured using a novel MRI technique called the "tagging snapshot" technique, which labels multiple tagging bands within the musculature. Eleven patients who underwent ACL reconstruction using autogenous semitendinosus and gracilis tendons were studied. The difference in the maximum active knee flexion angle between the ACL-reconstructed and the intact knee was calculated as knee flexion lag. Knee flexion strength was measured in 8 of the 11 patients. The semitendinosus muscle of the ACL-reconstructed knee exhibited three different morphological patterns; group I, similar shape to the intact knee with distal tendonlike structures; group II, smaller proximally than the intact knee with distal tendonlike structures; and group III, considerably smaller proximally than the intact knee without distal tendonlike structures. Each group was associated with a different knee flexion lag and different knee flexion strength. Our results indicated that the effect of hamstrings tendon harvest on knee function is not uniform.