Kicking a soccer ball requires orchestrating everything just so: feet, legs, hips, torso, head and even the arms out to provide balance. The hip joint, which connects the femur or thighbone to the pelvis, serves as the crossroads for a kinetic chain that transmits power to the soccer ball. Some of the body's most powerful muscles allow the elegantly designed hip joint to move forward and backward, and to rotate when thwacking the ball.
Anatomy
Fortunately for soccer players, the hip joint is an inspired production for the purposes of standing, walking, running and kicking. The femoral head, with its convex shape, fits perfectly in other half of the joint: the socket or acetabulum, a concave depression in the pelvis. The femur can thus rotate around an axis. Cartilage coatings in the acetabulum permit smooth motion in the joint during a soccer kick. The hip joint enjoys a wide range of motion in all directions, with flexion limited by the hamstrings, extension by ligaments around the joint, abduction by the adductor muscles and adduction by the tensor muscle on the outside of the hip.
Muscles
The largest muscle in the body, the gluteus maximus in the buttocks, extends the hip and rotates it. The hamstrings flex the knee and rotate and extend the leg. The abductors enable the leg to move sideways in the hip joint, and adductors permit you to swing one leg across the front of another. During normal gait, the muscles of the hip flex, sending one leg backwards, and then extend, swinging the same leg forward through 60 degrees of motion until the heel again strikes the ground. A soccer kick entails more dramatic use of the hip joint and its muscles.
Biomechanics
Kicking is the most widely studied skill in soccer, writes Adrian Lees, an exercise sciences professor at Liverpool John Moores University. Lees contributed a chapter on how biomechanics apply to soccer skills in the landmark text "Science and Soccer." Kicking begins with placement of the supporting foot beside the ball. The hip joint enjoys constant involvement in all phases of the kick; the movements can be divided into four stages.
Stages
The kick begins with a priming of the leg in a backswing, The second stage, the forward motion of the kicking leg, begins with the rotation of the leg around the hip joint and bringing the thigh forward. In the third stage, the upper leg decelerates until it is motionless at ball contact, Lees explains. At this point the hip joint takes a back seat to the powerful involvement of the knee, as the lower leg snaps forward and the foot, toes pointed down, smacks into the lower middle of the ball. During follow-through, the fourth stage, the foot may reach above the level of the hip. A kinetogram looks like a series of stick figures freeze-framing each hip and leg position during a soccer kick. The hip joint smoothly rotates forward in an arc to help the player kick the ball and perhaps achieve a well-placed pass or shot.
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