A strong lower body is essential for many tasks, including most sports and many daily activities. Both the hip and thigh muscle groups are important for lower-body strength. As with any muscle groups, use makes the muscles larger, while disuse will cause them to shrink. In order to strengthen the hips and thighs, different exercises are indicated.
Hip and thigh muscles grow via the same process as all muscles. When muscles are exercised, the muscle fibers receive a small amount of trauma, causing muscle injury, the University of New Mexico explains. This disrupts small structures within the muscle fibers called organelles. Organelle disruption leads to the activation of other nearby cells called satellite cells, which try to help repair the damaged muscle fibers. These satellite cells fuse to the muscle fibers and many of them are used to form new myofibrils, which are long protein strands that help muscles produce contractile force. An increase in the number of myofibrils allows the muscles to grow larger, which is why exercise increases muscle size.
Muscle atrophy, in which the muscles are reduced in size, actually has two different causes. Less common is neurogenic atrophy, Medline Plus notes, and is caused by damage to the nerves that connect to the muscles. More common is disuse atrophy, in which the muscles are not used enough, causing them to waste away. This form of atrophy is often seen in people with jobs that don't involve much physical activity or who have medical conditions that limit their ability to exercise. Disuse atrophy can be reversed through exercise and proper nutrition.
Exercises that increase the size of the hips are exercises that work the gluteus maximus, the hip abductor muscles (gluteus medius and minimus), the hip flexors (iliopsoas, sartorius, pectineus, rectus femoris and tensor fasciae latae) and the deep hip external rotators. The gluteus maximus muscles are perhaps the easiest of these to exercise, as many basic weightlifting exercises, such as deadlifts, lunges and squats, will strengthen this hip muscle, ExRx.net explains. Strengthening the other muscle groups requires the use of weights or other forms of resistance to oppose hip abduction, flexion and external rotation, Exrx.net notes.
The main muscles of the thighs are the quadriceps, hamstrings and hip adductors. Some of the same exercises that work the gluteus maximus muscles, such as lunges and squats, will also exercise the quadriceps. To exercise the hamstrings, exercises such as hamstring raises, leg curls and straight-leg raises will allow you to increase your hamstring strength. As is the case for the hip abductors, exercising the hip adductors requires adducting (pulling the hip in) against resistance.