Vertebrate muscle fibers vary considerably in their contractile, energetic, morphological and biochemical properties. For instance, some fibers contain many myofibrils but relatively few mitochondria; these fibers tend to produce a lot of force but rely on glycolytic (anaerobic) pathways for energy production. Other fibers have relatively high densities of mitochondria (at the expense of myofibril content) and thus can remain active for long periods without fatiguing.