The effectiveness of the NASA SPRINT exercise countermeasures program in preserving myocellular health in the vastus lateralis during long duration spaceflight was investigated. Single muscle fiber size and MHC distribution were assessed in six astronauts before and after 166±18 days in space. The SPRINT exercise protocol consisting of sprint interval training with concurrent, high intensity aerobic and resistance exercise offered complete protection of group mean MHC I and MHC IIa fiber CSA (P>0.05). However, heterogeneous responses were present within individuals for both MHC I (-15% to +27%) and MHC IIa (-19% to +33%) fiber CSA change. Additionally, the SPRINT countermeasure protected the muscle from the fiber type shift consistently seen with spaceflight. Surprisingly, the percentage of hybrid muscle fibers decreased (-5%, P<0.05) suggesting improved skeletal muscle health following spaceflight. The SPRINT exercise program is the first countermeasure to provide full protection of single muscle fiber size and MHC distribution during spaceflight.