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The kinetic
chain (link System) is an important principal in understanding
how the human body performs movements in sports. There are 5
basic links in the chain during the tennis serve. They are, in
order: leg extension, hip rotation, shoulder girdle rotation,
rotation of the shoulder joint (the subject of this coach’s
corner), and pronation of the forearm. These movements move
the body’s segments to produce a serve. There are other
movements within the kinetic chain, but they play a more minor
roll in producing a powerful serve.
Research on
the kinetic chain has shown that the 4th link (outward to inward
rotation or lateral to medial rotation) of the shoulder joint
can produce 40 to 50% of the resultant speed of the ball on the
serve. This movement begins with the angle between the forearm
and upper arm equaling 90 degrees (see photo 1). When the
forearm rotates back (see Randy Johnson photo and photo 2) this
is the outward or lateral rotation of the shoulder. When it
moves forward again, this is the inward or medial rotation of
the shoulder joint. When this movement is performed quickly,
the rotator cuff muscles in the shoulder are utilized and a very
powerful serve is produced. With the 90-degree angle and relaxed
arm, the rotation of the shoulder joint can be optimized, and
that serve could start popping.
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Photo 1
Photo 2
Randy Johnson |
