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2 August 2014
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You are here: BBC Science > Human Body & Mind > The Body > Puberty

Boys growing up

Kickboxer
teenage boys grow taller and stronger

In four years, the average teenage boy will grow a foot taller, put on a stone of muscle, drop an octave in the pitch of his voice and develop 40% more heart muscle.

Late developers?

Boys start growing later than girls, but they are not entering puberty later. Rather, their growth spurt comes at the end of puberty, not the beginning. This delay gives boys the advantage of an extra two years of normal childhood growth before their final growth spurt. This is one of the reasons why adult men are on average 13cm taller than women.

Another reason for their height is that boys grow faster than girls at their peak rate. They grow faster because they have higher levels of testosterone in their bloodstream than girls. The testicles release more and more testosterone into the blood stream as they mature. During puberty an average boy's production of testosterone will increase tenfold.

Gaining strength

Testosterone triggers cells all over the body to grow. The skeleton grows in much the same way as in girls, with the arms and legs growing ahead of the torso. But the bones become much denser and heavier than in girls. The final phase of skeletal growth in boys is a broadening of the chest and shoulders. This generally finishes around the age of 20.

 
Testosterone triggers cells all over the body to grow

Boys' muscles go through a phase of rapid development which girls miss out on. Our muscles are made up of fibres. At puberty in boys, these fibres don't increase in number, but the length and width of existing muscle fibres increases.

Testosterone increases the amount of haemoglobin carried in red blood cells. Haemoglobin is the oxygen-carrying red pigment in red blood cells. Higher levels of haemoglobin enable red blood cells to deliver more oxygen to muscles. This oxygen is used to create energy for muscles to exercise. As a result, teenage boys become fitter and stronger than they have ever been before.

When testosterone levels start to surge inside a boy's body, other things also begin to grow. Find out more about erections.


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