There are a lot of things that contribute to a tennis elbow injury. While proper technique goes a long way towards protecting the elbow from tennis elbow injury, some of the recent technological innovations in tennis racquet design are actually causing even more stress than the older technology.
The biggest problem stems from the fact that tennis racquets have gotten so unbelieveable light. This reduces the swing weight of the racquet, unfortunately this also dramatically increases the amount of force that the elbow has to resist to send the ball back over the net.
Determining the weight of a racquet is straightforward, most manufacturers list them. Racquet balance is also easy to determine. Most modern racquets have a open space between the head and the grip. Use a finger to attempt to balance the racquet. If the racquet is head heavy, the head will pitch forward. A head light racquet will have the handle be heavier.
Light racquet lovers argue that a light racquet allows you to swing faster. This is true, right up the the point where you hit the ball. A lighter racquet slows down dramatically when it impacts the ball. This slow down happens because there is less stored kinetic energy in the racquet. As the racquet slows down, the elbow adsorbs a massive amount of stress because it's the weakest link between the racquet and the powerful muscles of the hips and torso.
If you look at the racquets favored by the pros, you'll notice that most of them are using much heavier racquets than the more popular racquets selected by the general public. Andre Agassi uses a 13.2 oz. racquet that is 5/8 inch (5 points) head-light. Pete Sampras uses a 14 oz. racquet used a racquet with an even balance, Mark Philippoussis uses a 13.5 oz. racquet that is 3/4 inch (6 points) head-light. These tennis professionals use racquets no heavier than the old wood racquets, and even children can swing them.
To make things more complicated, many racquet manufacturers don't sell to the public the same racquet that they see their favorite player using. The pro tennis players are often using a racquet customized to their preference that is painted to look like the racquet they are paid to endorse. This is unfortunate, but something the consumer should be aware of.
If you are still inclined to buy a light racquet, consider this: in a car accident, which would you rather be driving a compact Smart Car or a tractor trailer truck? The answer of course is a no brainer, the light car would get crushed like a can. While the collision of a racquet and a ball is less dramatic, the principles are the same. A heavy racquet will keep going after impact, giving the ball better pace and spin AND creating less force in the elbow.
Grip size also contributes the tennis elbow. Rackets with too large a grip or too small decreases control and promotes excessive wrist movement. To measure an appropriate grip size for your hand see image below.