Keep Lifting Heavy While Getting Shredded
You've been training hard and heavy on basic movements while trying to gain as much muscle as possible. Now that you're on a diet you must give your body a reason to hold on to this new muscle tissue.
High-intensity strength exercises (in the 70-100% range) are better than low-intensity strength exercises (in the 40-70% range) while dieting. The higher training loads help you preserve strength and muscle while on a reduced calorie diet much better than super-high volume/low-intensity workouts.
Believe it or not, the human body is more interested in survival than being a hulking hunk of manhood (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 mass goes away – it's broken down into amino acids and then transformed into glucose for energy.
To keep your muscle mass, you must give the body a reason to do so. Will lifting light weights do it? No. You need to continue to lift heavy, otherwise some muscle will go to waste!
You've Been Lied to
We've been brainwashed by the muscle magazines to believe that you should do high-rep training for definition. This is absolutely ridiculous!
Sure, you use a little more energy during your workout, but think about it: the higher the training volume you perform, the more energy you need to recover from your workout. 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 that you've lowered your carb intake quite a bit. So you need more carbs, but you're actually giving less to your body!
Furthermore, while on a hypocaloric diet your body has a lowered anabolic drive, meaning that 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. A lot of microtrauma 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. Not exactly good news.
Perhaps one of the greatest benefits of high-rep training is an increase in blood and nutrient flow to the muscles, but if you have a reduced amount of nutrients available in your body, this benefit is pretty much wasted.
Repeat after me: I will use my diet and cardio/metcon work to stimulate fat loss. I will use strength training to maintain or gain muscle.
That's the bottom line.