The role of muscles in joint adaptation and degeneration.
Muscles are the primary contributors to joint loading. Loading is typically associated with the onset and progression of joint degeneration, and in turn, joint degeneration is known to affect negatively the control of muscle forces and co-ordination patterns. Nevertheless, the role of muscles in joint adaptation and degeneration has been largely ignored. Here, we review some of our research on the in vivo changes in muscular forces and joint loading in animal models of osteoarthritis and in patients with joint injury and disease. We attempt to emphasize the close dependence of muscle forces, joint loading and degeneration and, vice versa, try to point out how joint degeneration affects muscle forces and joint loading. We measured the forces and electromyographic signals in normal and anterior cruciate ligament transected feline knees and measured (1) a consistent decrease in the knee extensor and ankle extensor muscle forces for weeks following intervention; (2) a corresponding decrease in the static and dynamic external ground reaction forces; and (3) a change in the electromyographic signals (in terms of the firing patterns of individual muscles and of the co-ordination of extensors and flexors during locomotion). We introduced results on the biosynthetic response of articular cartilage to controlled, in vivo, loading and discuss preliminary results from an experimental animal model of muscle weakness. In contrast to much of the published literature, loading, in our case, is introduced by controlled nerve stimulation and the corresponding muscular forces that load the joint in its in vivo configuration. We found that short-term loading (30-60 min) in the cat knee produces distinct up-regulation of mRNA of specific metalloproteinases (MMPs) and some of the MMP inhibitors. In our newly developed muscle-weakness model, we confirmed that controlled Botox injections in the rabbit knee extensor muscles cause a 60-80% decrease in muscle force, and that these changes in muscle force are associated with changes in the external ground reaction forces, and most importantly, that muscle weakness seems to be associated with degeneration of the knee in the absence of joint instability or any other intervention. From the results of our research, we conclude that muscle health and muscle rehabilitation are key components for the successful prevention of, and recovery from, joint injury and disease.