AbstractThis study describes the effect of active joint motion on the maintenance of ligament and meniscus mass in a non–weight‐bearing model of disuse. Denervation and fixation models of immobilization have shown that resorption of isotope and atrophy of mass occured for hard tissue (bone) and soft tissues (ligament, tendon, or meniscus). A unilateral ankle disarticulation model of disuse that maintains active knee motion without weight bearing was studied for 8 weeks in dogs that were chronically prelabeled with three different isotopes. The effects of non–weight‐bearing without denervation or fixation were analyzed for the resorption of isotopes, and net atrophy of bone mass (femur or tibia) and soft‐tissue mass (collateral or cruciate ligaments, menisci). A large and similar loss of all three isotopes, as well as collagen and calcium mass occurred for whole femur and tibia; this indicated that mass loss was equivalent to bone resorption and suggests little replacement with new bone. No loss of isotope or mass per whole tissue occurred for the collateral and cruciate ligaments or menisci. The strength of the femur–anterior cruciate ligament–tibia complex was analyzed by a tensile failure test when a fast rate of deformation was applied; the results did not differ qualitatively or quantitatively between control and experimental limbs. The absence of weight bearing for 8 weeks resulted in marked bone atrophy without resorption or atrophy of soft tissues, or decrease of the mechanical strength for the femurligament–tibia complex. With this model the amount of active joint motion remaining appears able to prevent ligament and meniscus atrophy even though marked bone atrophy occurred dur