1. Groundforce

I will not spend time on this much understood and explained source of power.

However top players use this effectively to import power when late or to whip up extra power and spin on the stretch.

3. Rotational

Here you can clearly see how long Roger has held the right leg behind the left. 

This activation of the right glute as a brake (counter force) results in the arm swinging through the ball and rotating around the body faster.

4. Linear

Linear power is the body moving behind the line of the ball in the direction of the shot.

Not only is power gained by the body following the line of the ball but it can also be gained by the racket flattening out behind the ball.

5. Rebound

Another source of power that is not often talked about is the use of one’s opponents pace as it rebounds off the racket.

Using an opponent’s pace effectively is one of the attributes that separate top level players.

Agassi on this return of serve has almost no backswing and is relying on linear arm and rebound power – using the pace of the serve against his opponent.

It is interesting how you can use the understanding of the forces and directions of power to help modify technique.

My first example shows how the importance of the midcourt ball can be explained.  It is the one area of the court where a player can bring nearly every power into play and maximise most of them.

The player now explodes off the ground following the line of the ball for maximum use of groundforce and linear body power. 

He can flatten out the shot for maximum linear racket/arm power and also explode the hips and arm with powerful rotation. 

You would also expect the player to take the ball on at the top of the bounce using whatever speed the opponent has imported for some rebound power as well. 

You can quickly deduce what needs to change if, for instance, the player is imparting too much vertical racket power and ‘fizzing’ the ball short with excessive topspin.

Ask the player to deliver more linear racket force and less vertical.  Seeing the different powers as a graphic equaliser, which can tune up or tune down the different powers is an excellent aid to learning technique.

A second example of a player running laterally along the baseline clearly defines which force will be the most dominant. 

ANSWER: UPWARD ROTATIONAL POWER

Clearly there is no linear body force, no groundforce, possibly one ball length of linear arm/racket, some rebound help so the major power source is vertical power in the form of upward rotational power.

This is an additional tool to help learn and modify technique by deciding which powers you can utilise and how they change the way you hit the ball by increasing or decreasing the different powers.

The following graphs show how we can use graphics in a way to explain modifying techniques by changing different power sources
 

The above graphic shows an optimum midcourt ball being struck with maximum groundforce, almost no upward rotational power, good rotational yet not maximum, maximum linear body and linear arm/racket power and because the ball will be slow limited rebound power.

Highlighting differences, eg not getting off the ground, will offer a great visual to a player of what needs to be done.

The only limits to how much you can use the directions of power is your imagination.  Young players in particular easily relate to the idea of increasing or decreasing certain powers.