High Intensity Training

High-intensity strength exercises (in the 70-100% range) are better than low-intensity strength exercises (40-70%) while dieting. The higher training loads help you preserve strength and muscle while on a low calorie diet much better than super-high volume/low-intensity workouts.
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Your body is more interested in survival than being a hulking hunk (or a chiseled Wonder Woman). So energy reserves such as body fat are more precious than muscle tissue since the latter actually consumes energy. When calories are dropped, we enter a survival mode and the energy-costly muscle goes away – it’s broken down into amino acids and then transformed into glucose for energy. To keep your muscle, you must give the body a reason to do so. Will lifting light weights do it? No.
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We’ve been brainwashed to believe that you should do high-rep training for definition. This is ridiculous! Sure, you use a little more energy during workouts, but the higher the training volume you perform, the more energy you need to recover. The more glycogen you burn while strength training, the more carbs you’ll need to recover and progress. If you’re on any kind of cutting diet, chances are you’ve lowered your carb intake. So you need more carbs, but you’re actually giving less to your body.
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Also, while on a hypocaloric diet your body has a lowered anabolic drive: it can’t synthesize as much protein into muscle as it does when you’re eating a ton. A super-high volume of work leads to a lot of microtrauma to the muscle structures. That requires a great protein synthesis increase, which your body can’t do at this point. So if you use high-volume/low-intensity training while dieting, you’ll break down more muscle and build up less.