Those athletes fortunate enough to get video of themselves training or competing usually use the footage to check their technique. If you are one of those lucky athletes, there's something even more important than technique to examine. Use video to check how you "hold" your body when training or competing.
Is one hand held higher than the other? Are your shoulders stiff and hunched up? If you play a team sport like basketball, does one arm remain passive, rarely getting in on the action? Do you take larger steps with your dominant leg?
If the answer to any of those questions is "yes," then you should think about changing the way you hold your body. In fact, you don't even need video to gauge whether one limb, or one side of the body, is doing more work than the other. It will be reflected not only in activities or training, but in little everyday tasks as well.
More questions to ask yourself: Do you always open a door with the same hand? If you're on a bike and coasting, does the same foot always take the front pedal spot? Is it harder to look over one shoulder than the other?
One reason these self-questions are important is because your body easily adapts to the way you use it. If one hand and arm always reaches out to open or close a door and do most other tasks, that limb will be stronger and more dexterous than the opposite one. The fine motor skills developed in the more dominant limb will be lacking in the weaker, less-used limb.
It also follows that the weakness spreads. If one leg does less work, the glute on that side will be weaker than the other. If one arm does less work, the rotator cuff and shoulder muscles on that side of the body will be less developed than those on the other side of the body.
You don't necessarily need video to see if a limb dominates too strongly. You can feel it. You can even prove it to yourself. One method of proof for arms is to walk around the courtyard of a parking lot while bouncing a basketball with one hand. Count the number of times you can bounce the ball without missing a bounce.
Wait about five minutes, then do the same with the other hand, also counting the bounces. How much weaker is the less dominant arm and hand? To make sure the results of this informal test are accurate, do the same test over the following two days. If the results are consistent, it's time to start paying more attention to using your weaker arm more than you do now.
The test for legs is equally simple. Use any flat surface. Bend one leg back, and hop on the other leg, counting the hops. Rest five minutes, then switch legs and test again. There is usually a much larger difference between legs than between arms.
The workout required to strengthen the less dominant limb requires devout concentration. Step out on the weak leg first when you rise from a seat to walk. Step up stairs with only the weak leg taking each step, and the dominant leg coming behind.
It will take many months to change the habit of using one side of the body to do more work. It will also take a lot of focus. But the result of having a balanced body where the dominance of one side is reduced is worth the mental concentration. Not only will you be a better athlete, your body will also be more balanced in strength, and you'll be far less likely to get injured and also less likely to fall.