That troublesome area under the butt, located on the hamstrings, is one of the hardest places for women to lose fat. You cannot spot-reduce or specifically target the fat from one area of the body. Where your fat is used is largely based on genetics. The fat on your hamstrings is primarily hormonal, so it's usually the place you will lose fat. The best way to target hormonal fat is with high-intensity sprints, because they will train your muscles, sculpt your legs and burn a lot of calories.
Unlike distance running, sprints are intense and help build your muscles. It requires muscular effort to run extremely fast. According to the National Strength and Conditioning Association, your quads activate during the forward leg extension and during the foot-first strike on the ground. Your hamstrings activate during the push off from the ground. Your glutes drive your leg up and behind you, and your hamstrings once again activate to bend your knee as it swings the leg back under your body to prepare it for the initial foot strike again.
An article published in the "European Journal of Applied Physiology" examined hormonal response in sprint-trained athletes versus endurance-trained athletes and concluded that sprint-trained athletes have higher occurrences of growth hormone immediately after sprinting and one hour post sprinting. Their resting growth hormone levels are also higher than those of endurance-trained athletes. Growth hormone is a fat-burning hormone that works to preserve muscle mass during extremely intense exercise, where endurance exercises may actually deplete your hard-earned muscle, and is not as efficient at burning calories. In women, growth hormone helps burn that stubborn hormonal fat that rests under your butt. In short, sprints are better at burning calories, using fat and building muscle, making them the perfect leg-sculpting exercise.
High-intensity exercise, like sprints, raises your resting metabolism through a process called excess post-exercise oxygen consumption, or EPOC, according to the IDEA Health and Fitness Association. Sprints break down muscles and take your body out of its comfort zone. Your body has to work extra hard after your workout to restore your muscles and return your body back to normal resting levels. This requires more energy than usual, which leads to a higher calorie burn at rest. A higher metabolism leads to better fat use while you are resting, meaning you burn fat better than the average person.
The faster and more intense your sprint is, the longer you need to recover between sets. The metabolic efect suggests doing sprints that are 10 seconds or less for optimum hormonal response and muscle activation. For proper recovery, do a 10-second sprint followed by a 50-second walking recovery, then repeat five to 10 times. If you're new at exercise, start with five sprints twice per week on nonconsecutive days. Increase to 10 sprints three days per week as you get stronger.