Imagine what it must be like to be Madonna's fitness trainer. The 51-year-old star is famously demanding and is obsessed with keeping her body in shape - so much so that her workouts have become legendary for their length and their intensity.
James D'Silva trained her for four years. He studied as a dancer at the Royal Ballet before becoming a teacher specialising in disciplines including yoga, dance and pilates. He has evolved pilates into Garuda, his own hybrid workout system. Madonna, and others celebrities including Gwyneth Paltrow and Trudie Styler, have worked out under his guidance.
The St John's Wood-based fitness instructor says that they, and all his other students, heard of his techniques by word of mouth. Madonna, as you might imagine, took the sessions very seriously. "She has to look good. Her body is a big part of her image. She doesn't do anything by halves. If she was here for two hours to work, she would do exactly that, there is no messing about. That is something I really appreciate - sometimes you have time wasters and it just doesn't work. So it was a great working relationship. Unfortunately she's no longer here - she's based in New York but I totally enjoyed it."
With Styler he has taken the relationship a step further by releasing a series of DVDs, filmed at the Italian villa she shares with husband Sting, featuring yoga, pilates and dance techniques. "I've been working out with her for a few years, she is a dear friend but I work her very hard - I don't take any nonsense."
However, D'Silva is at pains to emphasise that his system is not just about celebrities - he works with athletes, with people recuperating from injuries and with all ages - his oldest student is an 83-year-old who suffers from Parkinson's, and there are teenage classes too.
So what is the advantage of D'Silva's workout over a regular gym? He says: "The classes can be aerobic - we work in such a way that we can get your heart rate up. It also strengthens and tones the muscles but the muscle texture is completely different from what you might develop in the gym. It is softer and longer, much more like an athlete's or a swimmer's body. In a gym you either pull or push - here, you have a far greater range of movement. As you get older you lose flexibility. We work really hard on the spine to give it strength and fluidity.
"The new system takes pilates to a different level. Over the years I have found pilates quite stunted, so I have come up with a contraption which amalgamates pilates machines into one and adds some features. I found the whole repertoire quite boring after so many years and I thought I could take it somewhere else. It has worked really well."
D'Silva takes me through a session to demonstrate what he is talking about. He is aware that he working with a fairly inflexible middle-aged man with a history of knee trouble, so he gives me what he calls the "baby workout".
It starts with gentle but sustained stretches before we move on to he equipment which combines more stretches to encourage suppleness with resistance work to give enhanced core strength.
Baby workout or no baby workout, my muscles are singing by the end of the 20-minute session, but it does serve to illustrate how limited is my range of movement, particularly compared to Styler, several years my senior, whose body shape and flexibility would put most of us to shame.
Of course, she is a celebrity with plenty of time to concentrate on her physique. So how would it work for the rest of us?
D'Silva says: "The important thing is to give yourself a some time every day to work out. It can be a swim or a walk but it is so important. This workout has everything in it but I encourage people to do a variety of exercises."
The programme is catching on. D'Silva has been spending plenty of time in Israel, training teachers who are reproducing his system over there. D'Silva, who spent his formative years in Goa, feels very at home there. "It's like being in India - total chaos. People make sure you understand exactly where you stand. It's what I'm used to," he laughs.