It is not known what causes the well-established inverse relationship between whole-body exercise economy and exercise intensity. The purpose of this study was to: (1) evaluate muscle exercise economy at 45%, 70%, and maximum isometric strength using 31P magnetic resonance spectroscopy (31P-MRS); and (2) determine the relationship between percent type II muscle fiber cross-section, whole-body exercise economy, and muscle exercise economy. Subjects included 32 premenopausal women. Muscle exercise economy was significantly different across the three exercise intensities (28.1 ± 10.4, 24.8 ± 8.2, and 20.2 ± 7.5 N/cm2 · mmol/L adenosine triphosphate [ATP] for the 45%, 70%, and maximum intensities, respectively). Percent type II muscle area was significantly related to whole-body metabolic economy during activities of daily living (r = -0.68) and 31P-MRS muscle metabolic economy during isometric plantar flexion (r = -0.53). These data suggest that skeletal muscle becomes less economical as force production increases, and that these decreases in metabolic economy may be related to increased dependence on inefficient type II muscle. © 2001 John Wiley & Sons, Inc.