There can be a number of reasons you’re losing energy mid-workout,” says Carwyn Sharp, Ph.D., C.S.C.S., chief science officer for the National Strength and Conditioning Association. Start with fuel: Muscle requires energy, predominantly from fats and carbs.

 “If you start with insufficient carb stores, you will feel growing fatigue as you continue to exercise.” Dehydration can also be a factor. Unless it’s very hot or you sweat a lot, you probably don’t need mid-workout fluids or electrolytes if you’re training less than one hour. But if you’re already dehydrated at the start, you may not have the fluids available to sweat effectively. Finally, watch your intensity. “Training at too high an intensity than your body is prepared for can result in fatigue, soreness, or injury.”