From the article:
To investigate whether testosterone supplementation improves measures of aerobic function ― the peak oxygen uptake and the gas exchange lactate threshold ― Dr. Storer and his colleagues analayzed data from subjects in a larger randomized controlled study of men over age 65 who had low testosterone levels and difficulty performing the usual physical activities of daily living. For 6 months, 28 men in one group received 10 milligrams of testosterone gel and 36 men in a second group received a placebo gel. All subjects completed a cycle exercise test to measure their peak aerobic fitness before and after the 6 month study.
The men taking testosterone displayed a slight improvement in aerobic fitness while those taking placebo showed a slight decline. This small increase in aerobic capacity in the testosterone group eliminated the expected decrease that men generally experience with natural aging.
Among the men taking testosterone, the age-related decline in the peak oxygen uptake was 3.4 times less than expected, while the rate of decline among the men taking placebo accelerated to nearly twice the expected rate. The decrease in gas exchange lactate threshold was significantly smaller in the testosterone group than in the placebo group. Longer term studies are needed to evaluate safety and durability of effect.