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Hello, welcome to Morales on the BBC World Service.

I'm Wesley Stevenson.

The Tour de France is in the mountains and there's a change of personnel at the front.

The sprinters like Marcel Kittel and Andre Greipel are taking a back seat.

This is the territory of climbers like Vincenzo Nibboli and Joachim Rodriguez.

But why is it that the riders in the Tour de France are physically so different?

The sprinters are stocky and packed full of muscles but the climbers are live and almost skeletal figures.

Paola Manasper is a physiologist and cycling coach who studies that the Australian Institute of Sport.

Climbers are thinner and usually lighter.

The constant thing is the BMI, so the body mass index which is a sort of a ratio between body weight and their height.

And this BMI is usually around 20 for a climber.

On the other side, the sprinters, they need much more muscle.

There are studies showing the ability to produce a lot of power which translate into speed,

is correlated with the volume of the lower limb muscle.

What makes the difference is the ratio between their height and their body weight.

And usually sprinters going back to that body mass index, they are around 22-23,

which is the number of kilogram for meter square.

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