A complex movement is a multi-joint movement that recruits large portions of the body to complete the exercise. Let's compare a simple movement leg exercise with a complex movement leg exercise:
The leg extension machine uses a simple, isolated movement to work the quadriceps. You're in a seated position moving only your knee joint. There isn't much involvement, if any, from other muscles and it doesn't burn very many calories.
Now let's look at a free weight walking lunge. You start by standing with your feet together and a dumbbell in each hand at your sides (or a barbell across your shoulders, or a medicine ball held at your chest, or even with no weight at all). ou take a large step forward and lower your back knee, keeping your front knee at a 90 degree angle. Now you push off your front foot and pull your back leg forward, repeating the movement.
How many muscles did you utilize while performing the lunge? Probably too many to count. You certainly worked your quadriceps, gluteus, hamstrings, calves, abdominals, supporting muscles in your shoulders, arms and back- just to name a few. You also raised your heart rate and really kicked your metabolism into high gear. That's what I call a great exercise.