(A) Schematic showing the HIIT protocol using a treadmill. During the initial 4 weeks (w) treadmill incline was increased gradually each week while speed remained constant. In the final 4 weeks, incline was set at 25° but speed increased gradually each week. See Materials and methods for further details. (B) Muscle weight to tibia length ratios for multiple muscles in sedentary and HIIT-trained (8 weeks) mice. (C–G) In vivo isometric tetanic force measurements of plantar flexor muscles demonstrates functional improvements in muscle strength after 8 weeks of HIIT. (C) Force-frequency curve, (D) peak force, (E) rate of force generation, (F) rate of contraction relaxation, and (G) impulse. Data are presented as mean ± SD. Statistical analyses: (B), (D), (E) unpaired two-tailed Student’s t-test, (G) Mann-Whitney test. *p<0.05, **p<0.01.