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It is important to try and use your non-racket leg to help you cover the longer distances, particularly on movement to the forecourt.
A common fault is for players not use their non-racket leg and then overextend the lunge and subsequently collapse on the shot.
Your racket leg is generally considered as your lunging leg, but there also are times when the shuttle is at the side of the body that it is suitable to use the non-racket leg to reach the shot.