According to an article from The New York Times, even the control group that had lightly exercised grew tired more quickly than the sedentary group that had been given epicatechin. The combination of epicatechin and exercise resulted in further improvements
Other findings showed that epicatechin administration also led to higher mitochondrial activity, and there were significant increases in capillaries in the skeletal muscle.
However, according to the New York Times' interview, the study’s author said only a very small amount of chocolate consumption was enough to intensify the effects of a workout. Extrapolating the dose in this mouse study to humans, the researcher claims just 5 g of dark chocolate daily -- about half of one square of a typical chocolate bar -- is probably a reasonable human dose (in which case, I'm WAY overdoing it, according to this statement).
Source: Epicatechin enhances fatigue resistance and oxidative capacity in mouse muscle