Flexion angle and attachment site to attachment site ACL length are shown as a function of % gait cycle. Increased attachment site to attachment site ACL length and strain were observed during midstance (length = 28.5±2.6 mm, strain = 5±4%, mean ± standard deviation), and during heel strike (length = 30.5±3.0 mm, strain = 12±5%) when the knee was positioned at low flexion angles (−2±8° and −4±10°, respectively). Attachment site to attachment site ACL strain is defined as the change in length of the ACL in each knee position during the motion cycle normalized to the length of the ACL in its minimally loaded state during MR imaging.