Symphony of Illumination - S7-E12
Revealing mistake: While Marshall is on the roof putting up Christmas lights he accidentally drops his cell phone, which supposedly then falls to the ground where Scott picks it up and holds him hostage on the roof. But the cell phone can be seen to fall harmlessly into the gutter and can be seen behind the ladder, despite the added sound of it hitting the ground. (00:10:45)
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 ★