By Sarah Holt

Meet the man charged with flexing Andy Murray's muscles - fitness guru Mark Grabow.

Grabow is an expert in setting strength and conditioning regimes

The American is Murray's physical trainer and has masterminded training programmes for the likes of Andy Roddick, Murray's coach Brad Gilbert, Mary Pierce and Jennifer Capriati over the last 20 years.

While Murray remains on the comeback trail after a wrist injury, he is stepping up Grabow's fitness plan, which aims to turn him into the ultimate tennis player within two to three years.

Grabow says: "The body of a player works as a kinetic chain, moving up from the feet, through the legs, into the core, and through the back and shoulders."

BBC Sport asked Grabow, who also works with NBA team Golden State Warriors, to share the secrets of Murray's fitness plan.

"Whether it is basketball, football or tennis, all these skill sports start right at the ground with footwork.

"The ability to move off a dime and to change direction quickly is essential for any elite tennis player.

"When you watch Andy, he never appears to be moving very fast but he has great anticipation and can read the ball off the strings so well.

"Andy has a natural ability to move. He always seems to have time. A lot of balls you think he isn't going to get, not only does he get there but he is able to do something with it.

"What we are working on now is building up his strength so he can keep moving from the first ball to the last in a three or four-hour match."

"A player's ability to apply force into the ground and to move quickly is dependent on hip strength.

"We are improving Andy's power in his glutes, hamstrings and quadriceps because these are the important areas to focus on if you want a real explosion of power on court.

"These parts of the body are developed with a variety of single leg and split leg squatting movements.

"We also build up Andy's lower extremities by using dumbbells in the weight room rather than focusing on work with the bar."

"Building up strength in the core of the body - the lower back and abdominal muscles - is very important.

Murray is working on building up his "six-pack" in his rotator zone

"This is the area that rotates and what Andy and I work on in the gym is trying to mimic the same movements that you make when you're hitting the ball.

"Andy will hold a four or five kilogram medicine ball and copy the movement of playing a forehand or backhand.

"We also work on power movements where he will hurl the medicine ball to replicate the concentric and eccentric forces that are produced through the muscles during a match."

"Tennis is in many ways a throwing sport, especially when players toss up a serve, and so it is really important to work on the muscles in the back and shoulder areas.

Read more on Murray's tactics

"Having huge, rippling muscles is a huge weapon for three-time French Open champion Rafael Nadal, but world number one Roger Federer has a totally different body type and he can put just as much pace on the ball.

"It is all about genetics and every player is different.

"Andy is still growing into his body. He put on 10lb in the last year and will probably put on another 5-10lb through natural maturation and work with weights.

"He is learning how to lift weights correctly and up top we are working with both dumbbells and lifting weights on the bar."

"The best players like Federer and Nadal, Bjorn Borg and John McEnroe all have the ability to slug it out for four or five hours at the highest level - and that is where we are trying to get Andy.

"Fitness levels do start to drop after two or three hours on court but you have to go beyond that. If you don't, your technical skills start to drop.

"There were some concerns about Andy's stamina but that will be resolved in time.

"He is spending more time in the weights room, more time on court and he has also been doing work on the track to improve his endurance.

"During the winter, Andy had a real test when he was put through his paces by Olympic sprinter Michael Johnson, running over 100m, 200m and 400m.

"If you want to get fit then you have to do the work all year round - and that's not easy to do with the long tennis season - but you have to find ways of getting those sessions in."

"A stronger player is a more confident player. When they have more physicality and can dominate a point, then that builds confidence.

Grabow says Murray's wrist injury will only make him stronger

"Andy took Nadal to five sets at the Australian Open, that was territory he had never been in before, but slugging it out with the world number two has made him believe in himself a little more.

"Andy has had to cope with coming back from a wrist injury but injuries make you tougher, you realise how much you miss the game and that makes you hungry.

"I'm sure there are times when Andy walks off court when he has lost, but thinks in his heart 'I could have beat that guy' - and that's the attitude you want.

"Andy may not hit his physical peak until he is 22 but every athlete has their own body clock and so we just have to be patient."