If it's not hurting, it's not working. At least this saying is true when it comes to exercise. While low-intensity workouts like walking or doing standard crunches are good for your body, if you're looking to lose weight and tone up quickly, you must commit to doing some killer exercises.
Check out this video to learn how to do a one-legged dumbell squat using a step plank. This exercise will help strengthen your glutes and your quads and will also improve your balance.
This exercise is very good for leg development. Most of you will benefit from doing single leg exercises. Most people who do regular squats or leg presses have an imbalance in their hips, glutes, or hamstrings.
This imbalance throws and uneven stress into the spine, especially when you do two legs at a time and go too heavy. You can still build mass with this one legged exercise and it is safer than most two legged.