Simplify the following text for a Grade 4 native English-speaking person.
Original: About 75 % of people contracting the disease experience `` limb onset '' ALS i.e. first symptoms in the arms ( `` upper limb '' , not to be confused with `` upper motor neuron '' ) or legs ( `` lower limb '' , not to be confused with `` lower motor neuron '' ) .
Simplified: About 75 % of people have limb onset ALS .