I know this topic has been discussed for some time (as I have been reading this thread and from
http://tt.tennis-warehouse.com/showthread.php?t=112708 and
http://tt.tennis-warehouse.com/showthread.php?t=173305, plus
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2577481/ and other articles in this thread), but as I am now truly realizing how to get more topspin on the ball (I have a few threads on how I changed the balance of my sticks to increase whippiness and therefore spin) via mechanics, I would like to inquire about it again in a slightly different way.
I hope most everyone can agree with at least this one chunk of my post's thinking - with slow motion video, it is easy to see that almost every pro (male pro at least) has their wrist lag behind the rest of their arm starting from the end of the racquet take-back through accelerating to contact (or for people that know high-low-high, when you are at low and going to high) (by the way, this topic is specifically for the forehand, even though I believe the principle is the same for backhands. If anyone wants to chime in about the BH please do so, but again, all of what I'm writing is about the FH). One of the (main?) reasons I believe this happens is because the pros get their whole body into the shot, and the turning of the hips and shoulders first drags the arm along which then drags the wrist along. Essentially, whip lash that travels through your arm, or the wave-motion of a towel when you give someone a rat tail (and the tip of the towel making contact is the wrist). I have not seen any video of Fed/Nadal/Djo locking their wrist back at a 90ish degree extension angle through the back swing, as it seems to happen spontaneously when the arm moves from the end of the racquet take-back to contact (low to high). While I've been focusing on the top male pros, I did read and see that Sharapova does lock the wrist back, just putting that out there.
Now, the question I see people disagreeing on I believe can be summed up as asking "around contact, does the wrist catch up with the rest of the arm naturally or forcibly?" I think this is being asked since people want to know how much, if any, wrist action they should voluntarily put into their forehand. When you watch the slow motion video and see the lag and bending-back (natural extension) of the wrist, it is possible to equate the wrist's movement up to and through contact as a "flick of the wrist" or "brushing up with the wrist" (which between naturally and forcibly, would qualify as forcibly). To add to this, when watching the same strokes in real time you can see that there's general acceleration through contact, and utilizing the wrist's properties as a joint to increase swing speed is certainly a feasible reason in why acceleration occurs (again, forcibly).
Personally, when I see the slow motion video and the very cool frame-by-frame pictures (starting on page 2 here), I can't help but notice that the wrist seems to move faster than the arm when they both are nearing contact. This to me highly suggests that there's a level of forced or voluntary wrist movement since the arm is not being yanked back or decelerating (if that were the case, it would mean the forehand would physically be like a rat tail, since us expert rat-tailers out there know that pulling back on the towel at the right time increases the whip/flick at the end, and leads to a more satisfying crack and "OUCH!" from the victim :twisted: ).
I am trying to see the argument that it's forearm pronation, but I'm really struggling and it may be that I don't fully understand it outside of the serve. I know that pronation for the serve accelerates the racquet through contact, so that could explain the wrist catching up with the arm at forehand contact. It can explain why the wrist turns over on almost every shot of a modern forehand (like it does for the serve). Also, it goes hand in hand with the slow motion video/pictures showing the angle between the racquet and arm to be 90ish at contact, so the motion of pronation in that case would look like someone waving "hi" sideways (fingers going from 3o'clock to 12o'clock (going past 12o'clock would requite shoulder movement) with their palm facing their opponent across the net) and would effectively brush up on the ball. But can you really have forearm pronation without any wrist involvement? I'm trying myself and watching PT videos on wrist and forearm pronation and to me they feel the same and look the same. I know I involve my wrist when I pronate on serve.
Toly, when you say Monfils and other pros have wrist ulnar deviation and pronation, you are implying that there is also wrist radial deviation through/just after the ball contact, correct (since as far as I know, wrist ulnar deviation is just the dropping of the outstretched hand sideways toward the ulnar styloid)? Like I said above, this would look like someone waving "hi" sideways around contact and would effectively brush up on the ball, right? Then after contact the wrist turns over (you point out that the racquet is closed) so the arm can fully extend/follow through safely and naturally?