This study investigated the effects of endurance exercise on matrix metalloproteinases (MMPs) in human skeletal muscle. The study found that: 1) MMP-2 expression and activity were elevated after a single bout of exercise and remained elevated after 5 weeks of training. 2) MMP-9 expression and activity significantly increased after a single bout of exercise and remained elevated after 10 days of training, though levels varied between individuals. 3) Circulating levels of MMP-9 and type IV collagen increased significantly in the bloodstream after exercise, suggesting MMPs may play a role in exercise-induced remodeling of skeletal muscle and blood vessels.