Sir Alex Ferguson has ridiculed the theories over Cristiano Ronaldo's free-kick technique and claimed the Manchester United star's success at set pieces proves the old adage: 'practice makes perfect'.

Ronaldo's free-kick during United's 4-0 win against Sunderland on Boxing Day led to a number of suggestions regarding how Ronaldo manages to get such swerve and dip on the ball.

One theory is that he tries to make contact with the valve on the ball, which then moves around violently making it almost impossible for a goalkeeper to stop, but Ferguson said: "It is rubbish. He takes about 30 free-kicks after training every day. The most important thing is practice. Practice makes perfect."

He added: "All I have heard about over the last day or two is about the ball and how Cristiano shapes up.

"Not only are the balls not the same ones we use in matches, I have never once seen him pay any attention to where the valve is."