Sickle cell anemia results in vascularopathy, organ damage, and impaired cardiorespiratory function and exercise tolerance likely due to a combination of central and peripheral abnormalities stemming from deranged hemoglobin (Hb). A transgenic mouse model of sickle cell anemia has been developed to help elucidate the mechanisms of vascular and organ damage, but a valid and reproducible measurement of exercise capacity and the severity of impaired physical function have yet to be determined in this model. Therefore, the purpose of this investigation was to measure the speed/duration relationship, known as critical speed (CS), and the anaerobic work capacity (AWC, the finite work capacity available above CS) in healthy wild type mice (WT) and mice expressing human HbSS (BERK). Following ethical approval from the institutional animal care and use committee (University of Colorado, Denver), six young-adult female mice (WT, n=3 and BERK, n=3) performed 5 constant-speed treadmill tests that resulted in fatigue within the range of 1.5 to 20 min. Additionally, the speed at lactate threshold (Tlac) was determined (~5 µl blood sample, Lactate Pro Analyzer) during an incremental exercise test consisting of 9×2 min runs separated by 20 second intervals at speeds corresponding to 70-110% of CS (5% increments). Speed and time to exhaustion for both groups conformed to a hyperbolic relationship (WT: r2 = 0.98 ± 0.01, BERK: r2 = 0.98 ± 0.02, p>0.05) which corresponded to a linear 1/time function (WT: r2 = 0.97 ± 0.02, BERK: r2 = 0.94 ± 0.03, P>0.05). CS was significantly lower in BERK mice when compared to the WT control (WT: 34.8 ± 1.3, BERK: 23.2 ± 1.5 m/min, p<0.05) with no differences between linear and hyperbolic models (p>0.05 for both). Additionally, AWC was significantly higher (WT: 1456.2 ± 237.2, BERK: 2639.2 ± 106.8 m), and Tlac significantly lower (WT: 27.5 ± 1.3, BERK: 17.1 ± 0.6 m/min) in BERK relative to WT (p<0.05 for both). CS, the hyperbolic relationship between speed and time, was lower in the BERK mouse model of sickle cell disease when compared to healthy WT mice. Furthermore, the lactate threshold was obtained at a lower speed in BERK mice relative to WT with BERK mice showing a higher AWC. Considering that CS represents the highest sustainable rate of aerobic metabolism and the lower CS and earlier onset of Tlac in BERK mice, these data suggest that sickle cell disease impacts aerobic capacity and may disrupt the tight matching between oxygen delivery and utilization within the skeletal muscle. In this regard, these results also demonstrate that sickle cell disease reduces exercise capacity in this model of the disease and provides compelling evidence to support future investigations into therapies aimed at restoring aerobic function.