Background: Why some but not all patients with severe AS (SevAS) develop otherwise unexplained reduced systolic function is unclear. We investigate the hypothesis that reduced creatine kinase (CK) capacity and/or flux is associated with this transition. Methods: 102 participants were recruited to five groups: moderate AS (ModAS, n=13), severe AS, LVEF ≥55% (SevAS-pEF, n=37), severe AS, LVEF<55% (SevAS-rEF, n=15), healthy volunteers with non-hypertrophied hearts with normal systolic function (NHv, n=30), and patients with non-hypertrophied, non-pressure loaded hearts with normal systolic function undergoing cardiac surgery and donating LV biopsy (NHbx, n=7). All underwent CMR imaging and 31P magnetic resonance spectroscopy (MRS) for myocardial energetics. LV biopsies (AS and NHBx) were analysed for; CK total activity, CK isoforms, citrate synthase (CS) activity and total creatine. Using serial block-face scanning electron microscopy, mitochondria-sarcomere diffusion distances were calculated. Results: In the absence of failure, CK flux was lower in the presence of AS (by 32%, p=0.04), driven primarily by reduction in PCr/ATP (by 17%, p <0.001), with CK kf unchanged (p=0.46),and is present in ModAS. Although lowest in the SevAS-rEF group, CK flux was not different to the SevAS-pEF group (p>0.99). Accompanying the fall in CK flux, total CK and CS activities, and absolute activities of MtCK and CK-MM were also lower (p<0.02, all analyses). Median mitochondria-sarcomere diffusion distances correlated well with CK total activity (r=0.86, p=.003). Conclusions: Total CK capacity is reduced in SevAS, with median values lowest in those with systolic failure, consistent with reduced energy supply reserve. Despite this, in vivo MRS measures of resting CK flux suggest that ATP delivery is reduced earlier, at the moderate AS stage, but where LV function remains preserved. These findings show that significant energetic impairment is already established in moderate AS, and suggest a fall in CK flux is not per se the cause of transition to systolic failure. However, as ATP demands increase with AS severity this could increase susceptibility to systolic failure. As such, targeting CK capacity and/or flux may be a therapeutic strategy to prevent/treat systolic failure in AS.