One of the worst stresses upon a player's arm comes from torsion, the force that makes the racquet twist around its long axis. Racquet Research measures torsion using a formula, "torque x moment = torsion," where torque results from the force of the ball's impact driving the head of the racquet back, and moment (which Mr. McCutchen treats as another torque) results from the downward pull of the racquet due to its weight and balance.
For his torsion formula, Mr. McCutchen assumes that the ball impact occurs exactly on the long axis (centerline) of the racquet. He states, "Torsion or Longitudinal Torque is the screwdriver twisting force around an axis running up the handle. Such a force arises even from impacts on the centerline. . . . For a right-handed forehand, Torsion would be a twist in the clockwise direction."
If any torsion occurs for hits on the racquet's centerline, I can't feel it when I hit a ball or even when I hold a racquet by my fingertips and hit the centerline with my other hand. This crude test result alone shouldn't disprove Mr. McCutchen's conclusion, but there's also a logical flaw in his reasoning:
If the ball is to your right, hitting below the centerline creates a counterclockwise torsion that is quite easy to verify and probably familiar to most players. If hitting on the centerline always creates a clockwise torsion, then hitting at a certain distance slightly below the centerline should counterbalance that torsion so that there's a line of zero torsion. Hitting slightly below the centerline would then negate the excess torsion allegedly resulting from more weight in the racquet head.
But, why not have less weight in the racquet head, just to be safe?
Off-centerline impacts cause quite a lot of torsion. Mr. McCutchen acknowledges this, but he omits this factor from his rankings. His rankings explicitly favor racquets that are head light, but a racquet with less mass in the head has less polar moment to resist off-center hits. He addresses this problem briefly by arguing that one should learn to hit on the long axis centerline. He also points out, correctly, that adding weight at the 9:00 and 3:00 positions on the racquet head would help. This would make the racquet head-heavy, though, so one would need to add tail weight to restore head-light balance. Unmodified, the small-headed, head-light racquets that he rates best for elbow and shoulder safety may have many desirable characteristics, but for a ball impact that occurs at any given distance off the long-axis centerline, they will suffer the worst torsion.
