Sweet Taste of Liberty - S1-E3
Revealing mistake: Barney tells the cab driver to lock the doors, and he does. We then see Ted trying to open the door and you can hear the taxi is still driving by the engine noise. However, the scenery outside the windows has stopped moving. (00:01:50)
Answer: After running the marathon for Marshall with no training, he's allowed to ride the subway for free. While sat on the train, the muscles in his legs start to seize up after the marathon to the point it is painful to move his legs. This is why he can't move them. It's the same reason your body aches after a good session at the gym the day before.
Ssiscool ★