16 rabbits received either 50 min of uni-lateral, cyclic eccentric (n = 8) or concentric (n = 8) knee loading. Muscle activation for these dynamic conditions was equivalent to an activation level that produced 20%of maximum isometric force. Contralateral joints served as unloaded controls. Cell viability was assessed using confocal microscopy.
Eccentric contractions produced greater knee loading than concentric contractions. Sub-maximal contractions caused a significant increase in cell death in the loaded knees compared to the unloaded controls, and eccentric loading caused significantly more cell death than concentric loading.
Cyclic sub-maximal muscle loading of the knee caused increased chondrocyte death in rabbits. These findings suggest that low levels of joint loading for prolonged periods, as occurs in endurance exercise or physical labour, may cause chondrocyte death, thereby predisposing joints to degeneration.