In healthy individuals, maximum vertical ground reaction force (MVGRF) remains close to constant during constant velocity running, despite variation in stiffness of the surface underfoot. Because the anterior cruciate ligament (ACL) possesses mechanoreceptors that influence recruitment of knee muscles, it may play a role in regulation of lower limb force output. This study was designed to examine the effect of recent ACL reconstruction on MVGRF in running. Seven patients who were 513 weeks post-ACL reconstruction and 7 healthy participants ran for 60 seconds in shoes modified with hard and with soft 1-cm outsoles. The MVGRF during running was measured for the ACL reconstructed and nonsurgical limbs of patients and limbs of healthy participants. The difference in MVGRF between running in hard and soft shoes was significantly greater in ACL reconstructed limbs than nonsurgical limbs (p 0.003) and compared to limbs of healthy participants (p 0.001). In contrast, a difference in MVGRF between shoes was not found between patients' nonsurgical limbs and those of healthy participants. A lack of mechanoreceptive feedback from the ACL graft may be among the factors explaining the difference between the ACL reconstructed limbs and the other two limb conditions.