Cardiac energetics and performance depend on the expression level of the fast (α-) and slow (β-) myosin heavy chain (MHC) isoform. In ventricular tissue, the β-MHC isoform predominates, whereas in atrial tissue a variable mixture of α- and β-MHC is found. In several cardiac diseases, the slow isoform is upregulated; however, the functional implications of this transition in human myocardium are largely unknown. The aim of this study was to determine the relation between contractile properties and MHC isoform composition in healthy human myocardium using the diversity in atrial tissue. Isometric force production and ATP consumption were measured in chemically skinned atrial trabeculae and ventricular muscle strips, and rate of force redevelopment was studied using single cardiomyocytes. MHC isoform composition was determined by one-dimensional SDS-gel electrophoresis. Force development in ventricular tissue was about 5-fold more economical, but nine times slower, than in atrial tissue. Significant linear correlations were found between MHC isoform composition, ATP consumption and rate of force redevelopment. These results clearly indicate that even a minor shift in MHC isoform expression has considerable impact on cardiac performance in human tissue.