first of all let me say, to anyone who finds a k90 too heavy etc, my comments are not for you. there is no reason anyone should ever play with a racket they do no like. but the original poster said he CANNOT sustain a rally with this racket. so i am going to address that point. not being able to sustain a rally is different from saying something like 'it is a bit too heavy for my game.' etc
you asked about the k90 being demanding. well let me rephrase it in a way that might prove more helpful: any smaller head racket with a higher weight will TEND TO reveal your stroke problems. the original poster was being very honest by saying he cannot sustain a groundstroke rally with a k90. that's the key statement, i feel. and i say to the original poster, if you cannot do that, then your stroke/footwork is off.
i play with a k90 and vantage 90s every day. a hitting buddy (hey ryan!) took my advice last year when he was just starting out in the game and bought a dunlop maxply macenroe, a fine racket that in my opinion would not be too demanding for an athletic male beginner while still having specs that either he could stick with forever or transition to heavier metal later. over the year we played about 200 times, including hours with a Playmate ball machine, him getting better and better. last month we went shopping and got him a k90. he IMMEDIATELY played 15% better by my estimation on the receiving end of his shots. same mistakes he was making before but now with more plowthrough and control. so he went from an evenly balanced good tweener racket strung with syngut to a 'demanding' racket strung with a soft poly with no learning curve and no degradation in play whatsoever. from the first hit (because i was there).
so if you can rally with your racket, but not with a 'player's racket,' i would check the following first:
1. do you have a fluid and sound stroke with a full followthrough? you have to have a smooth and well-grooved stroke. the heavier and smaller the racket the less BS you can get away with. wristiness and that sort of thing, you had better have the hand-eye coordination of a neurosurgeon or you're going to shankytown in a hurry. last minute flicks are tougher with an extra ounce of swingweight slowing things down.
2. do your body/arm/racket go through the same motions every time or are you improvising on every ball? you have to pay extra attention to how you 'address the ball.' MOVE YOUR FEET. a bigger sweet spot will allow you to get away with more laziness there.
3. keep that wrist in check! get a wrist-assist if you have developed bad habits here and want to correct them FAST.
how can you accomplish all this efficiently? an hour on a ball machine, maybe 5 - 10 sessions, depending on how bad/serious you are. ge a wrist-assist if you are unconsciously wristing shots and want to keep your arm/racket unit in the hitting position longer (shanks often come from bailing out of the stroke too soon. the wrist assist helped to daydenko-ize my forehand lol every forehand i hit now looks identical to the one i hit before it and i maintain the arm unit in the hitting position until well after the ball is off the strings, in otherwords full extension, whereas before i was bailing out way too soon but didn't realize it.)
listen, you should be able to pick up ANY racket, including a wooden one, and sustain a rally. PERIOD.
if you shank on the 'kill shot,' please believe me when i say it is not the racket; it is you.
upload a video of your groundstrokes to the technique forum, and those guys will help you. they did me. bungalow bill remade my forehand, then i switched to heavier/smaller frames and soft poly, spent hours and ours on a ball machine grooving it in and now i am a solid 4.0.
but repeating what i said before, you should be able to sustain an easy rally with any racket that TW sells, including junior rackets! i am not saying you will play your best, but you should be able to rally with some confidence with anything. andy roddick can sustain a rally with a frying pan. current rackets are not night-and-day different.
again, this does not conflict with the good players here who feel the k90 is a bit too heavy for their taste, a bit too small-headed, a bit too stiff or whatever the complaint is. i am just responding to someone who seems to have had their stroke problems revealed by a player's frame.
a final thought: when i was off for a month due to an injury, on my return to the courts i did notice that my frames felt a bit 'demanding' and i was wishing for lighter/bigger. that went away after 3 to 5 hitting hours. so if you do not play regularly and often, i think using player's frames could be a tall order. the benefits of these types of rackets only come after hitting many many balls.