Hormonal changes

It has been seen that performing cardio exercises increases the level of the hormone cortisol in the body that breaks down the muscle to develop new energy. Along with this, the level of testosterone also increases in the body that is good for the muscle development. But this happens only when you are performing long duration cardio exercises such as marathon or cycling. But if it is just a cardio exercise before the session of weight lifting, then it can cause adverse effects. It can make the cortisol break muscle energy, but the testosterone may not increase its level at this short time. Hence, while you are lifting the weight, you will be not able to use all your energy.