Even though Clarke wasn't particularly interested in learning how to fight, she did realize that it was important to stay in shape. During her year of isolation in the Skybox, she had developed something of a routine to keep herself from going completely crazy and that had served her well when she ended up on the Ground and had to fight to survive. She saw the message on the network that the self defense class was canceled for the evening and decided to head down to the gym to get in a workout, expecting it to be empty.
She was surprised to find another girl there, already using the equipment, but the place was big enough that they shouldn't get in each other's way. Besides, Clarke needed to make an effort to be friendlier with the other folks here at camp. So far, she really only talked to Joey, who was great, but they were all in this together.