I think it was David Leadbetter who popularized the saying in golf of letting the dog wag the tail, not have the tail wag the dog. The idea behind the saying was to let the larger muscles in the body control the movement of the smaller muscles, not react to their motion. In other words, the hands and arms do not start the backswing in golf; they react in unison to movment in the larger back muschles. The so-called ‘one-piece’ takeaway was thus popularized.
The tennis swing bears some similarity. So many swings I study are dominated by arm motion. The hips, back, and legs seem to come along for the ride. This is very evident in the motion at and through impact. I see people trying to put ‘body english’ on the ball to complete the stroke.
The two most common moves are standing up at impact and throwing the body around (particularly on the forehand side) just before impact. The former move often comes from a psychological desire to add more height to the ball. The latter move is often an attempt to direct the ball crosscourt or add more power. In golf, we would call this coming ‘over the top.’ The downswing is started by ‘throwing’ the arms from the top, then the body tries to compenste by pulling around and to the left, which only throws the clubhead even further outside the swing path, causing an outside-to-in motion. With an open clubface, this produces a slice; a pull results from a closed clubface.
In either case, we should not be using the upper body to ‘add’ to arm motion at impact. The upper body should be positioned prior to impact. Its role in the swing is one of stabilization and being part of the total kinematic chain. If the body is rapidly rotated back around during impact, the racquet face is either closed or starting to close, so the result is a flatter swing path that has a tendecy to direct the ball downward. With perfect timing, the ball can be hit low and hard, but net balls are a common result. The compensation is additional body motion by raising and over-rotating at impact. This causes the ball to often miss wide crosscourt or long.
Stroke inconsistency can often be traced to short-circuiting the kinematic chain, moving the arms too much and trying to compensate with body motion at impact. With sufficient time to setup and hit a ball in the strike zone, the body should appear to rotate around the spine at impact with a relatively still head. We see this so well in Federer’s strokes. The body posture is formed prior to impact and remains relatively constant through impact as the kinematic chain is unwound.
Getting rid of body compensations takes some effort and I suggest you work with a professional instructor. Identifying specific compensations and setting up drills to alleviate the issue is important. Two people that have apparently similar compensations may need completely different remedial action. For example, I have a tendency to shift the body laterally back towards midcourt. My instructor identified this as an attempt to recover too quickly and get into position for the next shot. So, we do lots of crosscourt drills where the goal of the drill is to maintain strict posture and consistently hit the ball back hard and deep (we usually try for 8-10 straight).
In golf, we used to say that you ought to be able to take a practice swing and see your left shoulder go under the chin on the backswing and see the right shoulder go under the chin after impact. Take a practice swing with your racquet and look at your shoulders. Do you really get the feel of pivoting around your spine? Do the arms act in response to motion in the hips and shoulders or does the upper body react to motion in the arms? Is the dog wagging the tail or the tail wagging the dog?
Good luck with the game!
The Algorithmist 