Many teenagers play more than one sport at competitive level. In fact they can be as young as 9, 10, 11 and 12 years old.
With this intense repetition of movement and usually on hard surfaces, repetitive strain injuries and tightening of structures leads to pain in many of these young sports people which will sideline them and cause them great anxiety.
The joints have important structures called epiphysis near them and it is from there that bone is laid down. These do not become solid until 18 years of age.
If a young person plays too much sport they can sadly develop injuries at these sites which are sometimes called growing pains. Large muscles can be attached to or near the epiphysis, the knee being the best example.