Read the following context and answer the question.
Context: I forgot . It 's kind of tricky to learn a scary new thing when your heart rate is n't supposed to get much over 140 . I hopped my way sideways off the bike and sat down , practicing the emergency edition of my old biofeedback exercises , until my heart slowed to something more reasonable ; then I tried it again . My leg seemed too heavy to lift a foot onto the pedals , mysteriously heavy the way a reluctant cat can make itself heavier than it should possibly be .
Question: What may be the reason why they hopped off of their bike ?
Answer:


Their heart was racing .