Read the following context and choose the best option to answer the question.
Context: The run in the gym on Friday was a rather tough one . I ' m not so sure why , but my legs just felt sooo heavy!But I pushed on at 8.6 KPH all the way through . I was going to increase the speed later but could n't , so it was a slow run . After 20 - 25 mins , I think then I thought I will try run 5 K ... so I did ! I was quite glad I did it in the end .
Question: Why would the narrator 's legs be feeling so heavy ?
Options: 
- They have been injured .
 - They are a paraplegic .
 - They have been running from cars .
 - They have been overtraining .


They have been overtraining .