Standing up too straight
Many players stand up way too straight when they play. The result is they are unable to move as quickly as they could if they kept their legs father apart (which also adds stability and power) and bent their knees slightly. It also tends to mess up some strokes, especially on the backhand, where you lose leverage if you stand up straight.
Some players do this because they are getting old and have knee problems, or are overweight, but even then you can get in the habit of bending the knees slightly, as well as keeping the legs a little farther apart. And very young players (or short players) don't want to get down too low because they are already rather short and if they get down any lower they'll have problems on their backhand.
There's a rather easy cure. Rather than think of getting down, imagine you are covering someone in basketball, or playing shortstop in baseball, or you're the goalie in soccer. As soon as I tell a player to imagine this, they immediately get lower. It's almost impossible not too - you can't do these things in basketball, baseball, or soccer without getting down, and so players instinctively get down. They don't have this same instinct in table tennis, so the habit needs to be learned.
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