Slow Oxidative Fibers

Slow oxidative fibers are skeletal muscle fibers characterized by relatively slow contraction and a high capacity for oxidative metabolism. In humans, the term generally corresponds to Type I muscle fibers, which express the slow Type I myosin heavy chain isoform. Their myosin has relatively low ATPase activity compared with that of faster Type II fibers, contributing to slower shortening and sustained force production .^1,2

Slow oxidative fibers contain abundant mitochondria and oxidative enzymes, allowing them to rely extensively on oxidative ATP production. They also tend to have a dense capillary supply and substantial amounts of myoglobin, an oxygen-binding protein within muscle cells. Together, these features support the delivery and use of oxygen during sustained muscular activity .^1,2

Because their contractile machinery generally turns over ATP more slowly and their capacity for oxidative ATP production is high, slow oxidative fibers are highly resistant to fatigue. They are therefore well suited to prolonged, relatively low-intensity muscular activity, including that involved in maintaining posture and endurance-type activity. This contrasts with faster Type II fibers, particularly Type IIx fibers, which have faster contractile properties but generally lower oxidative capacity and fatigue more rapidly .^1,2

The term slow oxidative combines two characteristics: slow contractile behavior and high oxidative capacity. These characteristics are strongly associated in Type I fibers, but muscle fiber properties exist along a continuum, and individual metabolic characteristics cannot always be predicted perfectly from fiber type alone .¹