In the first study of its kind, scientists investigated just how much an increased testosterone levels impacted athletic performance. They randomly assigned 48 physically active and healthy 18-35-year-old old women to 10 weeks of daily treatment with either 10mg of testosterone cream or 10mg of an inactive (placebo) substance. Hormone levels and body composition were measured at the beginning and end of the 10-week trial period, along with aerobic performance, which involved measuring how long the women could run on a treadmill before reaching the point of exhaustion.
In the group of women that were given the testosterone cream, testosterone levels rose from 0.9nmol/litre of blood to 4.3nmol/l. This had the effect of increasing their running time to exhaustion by a significant by 21.17secs (8.5%). Their lean muscle mass increased from 135g to 923g overall; and in their legs 91g to 398g.