It is thought by most tennis scientists and engineers that grip tightness has almost no effect on the power you get from a racket. This conclusion is based on both theoretical analysis and from data obtained in laboratory testing. Having stated a conclusion, I must now justify that position.
When a ball strikes the strings of a racket, a wave propagates down the frame toward the handle. When the wave reaches the butt end of the free racket or the hand holding the racket or a clamp holding the handle in place, part of that wave is reflected back toward the ball. The type of reflection depends on how the handle is held (free, clamped, hand-held). It is not until that reflected wave gets back to the ball that the ball can tell how the butt end of the racket is held. If the ball has left the strings before the reflected wave gets back to it, the ball CANNOT know whether the racket was free or clamped or hand-held.
For typical ball dwell times on the strings (5 milliseconds), the reflected wave does not return in time to influence the ball. Therefore, the ball will come off the strings in the same way for a clamped handle, free handle or hand-held handle.
Experiments and Observations
When a free racket is struck near the tip or the throat, it vibrates at 125 to 200 times per second (125 to 200 Hz- the stiffer the fame, the higher the frequency). When a racket with its handle clamped is struck in a similar manner, it vibrates at 25 to 50 Hz. When a hand-held racket is struck near the tip or throat it vibrates at almost the same frequency as the free racket, even when held as tightly as possible. This is strong evidence that a hand-held frame behaves in the same manner as a free racket. This result is probably due to the fact that the human palm, hand, etc are very soft and flexible compared to the stiff, graphite reinforced composite rackets in use today.
When rackets are tested in the lab for their rebound power, the freely suspended racket responds almost in the same way as the clamped racket, leading us to believe that a hand-held racket will behave in the same manner as a free racket will. For some rackets there is a slight difference in racket response (free vs clamped) when the ball impacts close to the throat where the round-trip of the propagating then reflected wave might arrive just as the ball is departing.
It is sometimes stated that a tight grip is necessary to keep the racket from twisting relative to you hand on off-axis impacts. This is not true. Just watch a 5 year-old play tennis. The kid has a very weak grip, hits the ball all over the racket head, and yet you do not see the racket twisting relative to the youngster’s hand. The racket does twist on an off-axis impact, but the hand twists along with it.
As a final test of the independence of racket power vs grip tightness, I took an old wood racket and installed a completely flexible hinge in the shaft. There was no way that the ball/racket head could know about the grip tightness since they were connected to the handle via the flexible hinge. I was able to play tennis with this (illegal by ITF rules) racket and get a great deal of power on groundstrokes and serves. I could not volley with it. I gave this racket to many players and they had little trouble playing tennis with it. (Some of this is documented in the Braden-Brody video "The Science and Myths of Tennis.")
One needs to hold the racket only tight enough to be able to control it during the swing and to keep it from being knocked out of your hand.
Also by Dr. Brody:
The Physics and Technology of Tennis by Howard Brody, Rod Cross, and Crawford Lindsey.