An injury epidemic from an unexpected source.
The century-old tennis coaching buzzwords are: turn sideways, get the racquet back early, step into the ball. Those commands, combined with the "modern" windshield wiper motion, twist the body in unnatural ways. That mix of linear and rotational movements is predominant in the two-handed backhand, and has quite a following on the forehand side as well. It is causing profound repercussions for the body's alignment and health (as evidenced by increasing injuries in both ATP and WTA pros).
The forward foot anchors the leg and the rotational torque travels up to the hip, stressing the sacroiliac, which gets out of place. The most common indicator: a leg shorter than the other.
Most pro tennis players are coached with and exhibit this technique in practice, while in matches they play more naturally, using more an open stance. Unfortunately, continued repetition of the step-into-the-ball technique not only by playing professionals but also by the general public has created an epidemic that affects young and old.
Further, the sacroiliac out of place affects joints such as the hips, spine, knees, and also nerves that lead into male and female organs.
From Dr. Carl Barniak (Evansville, Indiana):
"The fatigue of repetition of an unnatural twist is likely to subluxate (slip slightly out of joint) the sacroiliac joint causing a series of normal but painful bio-mechanical compensations,"
"In the dysfunctional anatomical model that I have described, the ilium on one side of the patient would have rotated posterior. To correct this, a chiropractor would have to drive that portion of the ilium in an anterior and superior angle."
Oscar's story: I had my sacroiliac corrected (after 50 years of problems) and my legs are now even length as a result. Neck and back problems and scoliosis are also gone. And more!
Oscar's advice to coaches: make up your mind! You are coaching hybrid tennis, bio-mechanically unnatural, causing injury with the mix. Either coach linear sideways like old times or go totally modern rotational with the open stance.
The closed stance is ideal for the one-handed backhand. You step onto your front foot to "find" the ball then lift your arm while shifting backwards onto the back foot, squeezing your shoulder blades. This permits you to rotate the body and turn back to cover the court efficiently. But the 2-handed backhand should always be taught primarily open stance, like a left-handed forehand, from day one!