A lunge works all of the major muscles in the lower body. This one basic exercise strengthens the glutes (butt), quadriceps (front of the thigh) hamstrings (back of the thigh) and the calves. It also stretches the hip flexors and promotes balance and stability in the ankle and knee joints. Like the push up, lunges are an extremely efficient exercise, providing major benefits in a short period of time.
Beginning exercisers should start by learning the reverse lunge. Intermediate exercisers can progress to a forward lunge and advanced exercisers should try the walking lunge. Weights can be added for an additional challenge, or to increase stability try putting a balance board or Bosu under one or both feet. These YouTube videos provide instruction for each variation of the lunge.