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SWAIN, D. P., and B. A. FRANKLIN. V̇O2 reserve and the minimal intensity for improving cardiorespiratory fitness. Med. Sci. Sports Exerc., Vol. 34, No. 1, 2002, pp. 152–157.The American College of Sports Medicine has stated that aerobic training needs to occur at a minimum threshold intensity of 50% V̇O2max for most healthy adults and at 40% V̇O2max for those with a very low initial fitness. Recently, the concept of V̇O2 reserve (%V̇O2R, i.e., a percentage of the difference between maximum and resting V̇O2) has been introduced for prescribing exercise intensity. This analysis was designed to determine the threshold intensity for improving cardiorespiratory fitness expressed as %V̇O2R units.Previous studies in healthy subjects (N = 18) that evaluated the results of training at low-to-moderate intensities (i.e., ≤ 60% V̇O2max) were identified. The original studies described the intensity of exercise variously as %V̇O2max, %HRR, %HRmax, or as a specific HR value. In each case, the intensity was translated into %V̇O2R units.Exercise training intensities below approximately 45% V̇O2R were consistently ineffective at increasing V̇O2max in studies that used subjects with mean initial V̇O2max values > 40 mL·min−1·kg−1. In studies using subjects with mean initial V̇O2max values < 40 mL·min−1·kg−1, no intensity was found to be ineffective. For this latter group of subjects, the lowest intensities examined were approximately 30% V̇O2R.Although evidence for a threshold intensity was not strong, this analysis of training studies supports the use of 45% V̇O2R as a minimal effective training intensity for higher fit subjects and 30% V̇O2R for lower fit subjects.