One of the biggest problems beginning/intermediate players have is standing up too straight. Watch the top players and you'll see how they stay low - feet relatively wide and pointing slightly outward, weight toward the front inside balls of their feet, knees bent, and leaning slightly forward from the waist. This allows much stronger play than standing up straight - you'll move quicker, have better balance, recovery more quickly after shots, and your shots will be more natural and more powerful. And you'll even feel more like an athlete because you'll be playing like one!
Here's a good example - here are highlights of the Men's Singles Final at the 2013 World Championships between Zhang Jike and Wang Hao, both of China. Or pull up just about any video between two top players. It may not be easy to play with a lower stance at first. Older players and those with knee problems might have particular problems, and might need to adjust - but even they should focus on staying at least somewhat low.
Why is staying low important?
- It lowers your center of gravity. This increases your leverage in movement, giving you a quicker start.
- The bent knees give you a quicker start. If your knees are straight, you'll have to bend them before you can move.
- The wider stance gives you stability and balance during rallies. It's hard to play effectively if you are off balance. Players who stand up too straight tend to make up for this by reaching for the ball instead of moving to it, leading to awkward and inconsistent shots.
- The wider and lower stance increases power. The extra power comes from a greater weight exchange as you rotate about from a wider stance (especially on forehand shots), and from the extra power from the legs from staying low, allowing you to push off into your shots.
- Quicker recovery. This comes because the wider and lower stance allows better balance. Players who stand up too straight will go off balance after a fast movement or powerful shot, and will be slower in recovering, especially on the forehand.
- It makes your shots more natural. If you stand up straight, your natural stroking movement will be up, but your target is ahead of you. Staying low gives you a more natural shot in the direction of the far side of the table, and allows you to more easily put your body weight into it. It might not be natural at first, but it will once you get used to it.
- It makes it easier to loop heavy backspin with power. While standing up straight gives you a natural lifting stroke, it doesn't give much power for great topspin and speed. For that, you need to use your legs, and to use your legs you have to get down with the knees at least slightly bent.
Besides table tennis players, you can find examples of athletes staying low in many sports - for example, an infielder in baseball, a goalie in soccer, and basketball players when they are dribbling or covering someone.
You might have to build up your leg strength to stay low - but staying low in itself builds up those muscles, and the more you do it, the stronger your legs will be, and the easier it becomes. Truly serious players should hit the weight room and focus on lower body strength training.
You can make the change to a lower stance in stages, focusing on staying a bit lower for a week, and a bit more the following week, and so on until you find a comfortable low stance. If this doesn't work, try going for the other extreme, and practice staying too low. (Careful if you have knee problems!) It may feel silly, but after playing that way for a short time you may find it easier to compromise between the too-low stance and your previous too-high stance.
And when you develop the habit of staying lower, you'll begin to feel the benefits as your game improves. A lower stance will allow you to stand tall on the winner's podium.