Simplify the following text for a Grade 5 native English-speaking person.
Original: The movement has grown dramatically since its early days â `` for example the number of athletes participating in the Summer Paralympic Games has increased from 400 athletes in Rome in 1960 to over 3,900 athletes from 146 countries in Beijing in 2008 .
Simplified: 400 athletes from 23 countries competed at the 1960 Games . The Games were open to athletes in wheelchairs .