gameboy, sorry you are very wrong and your post is bad advice for parents. It is not true at all that a shorter racquet is better or safer when all factors are taken into account.. In tennis racquets, shorter means lighter and in many cases stiffer. What you are saying is very bad advice. Imagine some parent reading your quote and going out to get a 23-25 inch frame for their smaller but hard hitting child now....not good.
Honestly, you should remove that post.
You are confused as to what "moment" is. Moment is not based on just racquet length. Moment is also comprised of weight and balance point. Both weight and balance point can be customized using lead. Different off the shelf racquets can have very different balance points. Some are head light, others head heavy. Moment is important regarding volleys mostly. But in this discussion is not as big a contributor to kid's arm stress compared to other factors.
From Racquet Research:
"Moment is the turning force pivoting the racquet head down when you hold the racquet parallel to the ground. Moment, in Newton.meters, is a measure of how heavy the racquet feels to hold up parallel to the ground (not merely the weight of the racquet, but this weight multiplied by its lever arm).
Moment should be especially important for juniors and ladies. A light racquet having a balance point far from the hand may have a larger Moment than a heavy racquet with a head-light balance, so merely knowing the weight of a racquet is not enough, and may be misleading.
It is deplorable that ignorant consumers are being enticed to buy dangerous racquets by misleading sales ploys like inviting them to pick up the racquet by the wrong end to see how light it is. This "pick up appeal" pitch is causing tennis to lose players and popularity at an alarming rate. What counts is Moment, not weight. Moment is the racquet's weight times its lever arm, which is the distance to the balance point from the axis of rotation (this axis is at the middle of the hand, 7 cm from the handle end). Weight in the metric system is the mass of the racquet, in kilograms, times the acceleration due to gravity (9.81 meters/s2), and the distance to the balance point is in meters. The unit of measure of torque is the Newton.meter (1 Newton.meter = 0.7375 foot pounds of torque = the same force you would feel holding a 3.6 ounce weight at the end of a meter stick). The lower the Moment, the better."
So you have no idea if that short and lighter racquet has less or more moment than a properly designed full size frame. A 26 may actually have higher moment than a 27 depending on balance point, weight, etc. How about a short racquet that is head heavy vs a 27 that is head light?? So shorter may be worse on all counts...including moment. But again, not as important on all the other factors.
You go hit balls with the full and hard strokes of any well coached kid. Go hit with a 23 and a 25. Then come give us your "anecdotal evidence"
Anecdotal? We have talked to 2 sports doctors in their 60s who are tennis nuts. They remember all types of racquets. Back when kids would mostly pick up wooden sticks, full size, 14-15 ounce....they never saw any arm injuries in kids. As the lighter racquets came about, more. As the shorter and even lighter frames came out...still more.
Anecdotal? How come players we have TRACKED, players who have hit as many or more balls from age 5 than anyone on the planet with full size frames using good technique, proper stiffness frames, proper warm ups....have no arm injuries?
Sounds like anecdotal is way more important than you guessing that shorter vs longer physics principal overrides all other factors.
Heavy racquets do prevent twisting somewhat. Twisting is caused by several factors....bad technique and the sweet spot. Off center hits cause twisting. Of course head size is important in how much a racquet will twist with off center hits.
But where you are really wrong is what factors are big contributors to arm strain. You cite a physics principal of short vs long objects which is a meaningless contributing factor. The difference in a 26 vs 27 is not going to matter at all compared to things like frame stiffness, head heavy or head light, technique, string tension and type, proper warm up, etc. And go hit with a 25 vs a 27 and the "moment' factor will pale in comparison to all the other factors that increase stress.
Technique, proper warm up, string type, tension, racquet weight, stiffness, head heavy or light.....these things matter much, much more.
For you to put up a post telling people to go out and let their kids use short racquets to be safer....with no mention of frame stiffness, balance point, string type and tension, warm up, technique, is not very responsible.
Here is more information, from a MD and PhD, concerning racquet characteristics and strain on the arm.
http://tennis.about.com/od/tenniselbow/a/racquets_strings_to_prevent_tennis_elbow.htm