ELI5: Why is dead weight (unconscious person) so heavy when the person's weight doesn't change?

When a person is alive and with-it, they “participate” in being held by making sure weight is distributed to your core. Getting a floppy body to have its weight distributed to your core muscles is hard and this makes balancing difficult and causes you to engage relatively weak muscles.

Even young children will do this instinctively, making them much easier to carry.

Explain Like I`m Five: good questions, best answers.