REDUCTIONS IN EXERCISE CAPACITY in patients with chronic heart failure (CHF) were originally linked solely to central limitations associated with a malfunctioning cardiac pump. However, over the last 20 years, peripheral factors related to skeletal muscle and the vascular/skeletal muscle interface have been implicated in this phenomenon. Specifically, muscle atrophy, fiber-type alterations, reduced mitochondrial enzymes, decreased mitochondrial volume density, and decreased capillarity (3,4,9,(12)(13)(14)22) have gained attention since these changes have been associated with CHF. Most certainly, the central and peripheral alterations are intimately linked, requiring an integrative approach to understanding the limitations and mechanisms responsible for the diminished work capacity in this patient population.In this issue of the American Journal of Physiology-Heart and Circulatory Physiology, Copp et al.(1) employed an integrative approach to examine peripheral dysfunction in CHF as measured by the off-transient or recovery kinetics of microvascular O 2 pressure (Pmv O 2 ) and coupled this peripheral measure with central hemodynamic and morphological indexes of heart failure. The most novel and important findings of this study were that 1) progressive CHF increasingly slowed the recovery of Pmv O 2 in a mixed fiber-type muscle, 2) mean response time of the off-transient (MRT off ) and the MRT off-on differences (a measure of the asymmetry between on-and off-transients) correlated with central hemodynamic and morphological indexes of heart failure, and 3) off-transient differences may be present despite a lack of on-transient alterations. A recognized limitation of this study is the finding that the correlations of off-transient and central indexes of cardiac failure were likely driven by the inclusion of four animals with the most severe heart failure; this group could be increased in number to solidify these conclusions. Despite this concern regarding the relatively small cohort of severe CHF in the study of Copp et al. (1), these findings have potential importance in terms of functional capacity and may provide insight into why work and exercise tolerance as well as the ability to perform activities of daily living are reduced in patients with CHF.The technique of phosphorescence quenching (20), as used by Copp et al. (1), allowed the direct determination of Pmv O 2 during the transition from exercise to rest in the muscle of mixed fiber type (spinotrapezius). The spinotrapezius was chosen since the muscle fiber-type composition and oxidative capacity are similar to the human quadriceps (21), thereby making this model a relevant and potentially translational animal model. Pmv O 2 is determined by the relationship of delivery and uptake of O 2 (Q o 2 to V O 2 ), which in turn facilitates blood-to-tissue O 2 flux and has an impact on the regulation of cellular metabolism (6,15,17 (1), in the muscle of rats with CHF likely interferes with the ability to recover from muscular contractions, thereby accelerating fati...