The aim of this short review is to describe the role of myosin isoforms during the adaptation of skeletal muscle to prolonged physical activity (for example endurance exercise) and to show the coordination between changes in muscle oxidative capacity and myofibrillar apparatus in slow-twitch and fast-twitch muscles. Adaptational changes in myosin isoforms during long lasting muscle activity (decrease of MyHC IIb isoforms relative content and increase of that MyHC IIa and decrease of MyLC 1 fast isoforms in fast-twitch muscles) are in good coordination with changes of muscle oxidative capacity. These changes show that during regular endurance exercise fast-twitch muscle fibers (type IIA) are also recruited and create the potential source of increase in endurance capacity during the process of adaptation to the prolonged physical activity.