Insulin lowers your blood sugar levels by causing cells to take up more glucose. Glucose is the fuel used in metabolism, so it seems logical that you can work your muscles harder if they have more glucose. This is the idea that is used to sell insulin as a bodybuilding supplement. However, even though this is logical, muscles don't fatigue during bodybuilding because they run out of glucose - they fatigue because lactic acid builds up. The rate lactic acid builds up is not affected by insulin. The insulin supplements are a scam.