The pop star unveiled biceps of comic-book

Madonna has been working out for two hours, six days a week - and boy does it show.

The pop star unveiled biceps of comic-book proportions as she left the gym following her latest vigorous fitness session in London today.

Dressed in adidas tracksuit pants, a tight Nike vest and a hooded jacket hanging over her head, it was her bulging muscles that really had heads turning.

Bulky: the pop star flexed her muscle after yet another two hour work out in London today

Madonna, who is famous for her punishing exercise regime, was joined by her pint-sized personal trainer and constant gym companion Tracy Anderson, whom she was referred to by best friend Gwyneth Paltrow.

Of the star, Tracy will say only: "It comes down to whether or not you are a hard worker and someone who puts in 100 per cent.

"I prefer to train with the dedicated and these women aren't afraid to work."

Coach: Madonna was joined by her personal trainer Tracy Anderson

With her 50th birthday less than five months away, Madonna seems more determined than ever to be in great shape, but her increasingly muscular appearance suggests she may be trying too hard.

Last month there was even speculation the image-concious star had undergone plastic surgery after she was seen with a bruised face and eyes.

The mother of three is currently preparing to embark on a promotional tour for of the release of her new album Hard Candy, which sees her duetting with Justin Timberlake.