Answer: His arms muscles work in pairs so that one contracts while the other relaxes as he climbs up.
Explanation: when we exercise one muscle, there is usually another muscle that isn't the target of the exercise but it helps us maintain the form by doing the opposite movement. For example, when we do a bicep curl, the muscle that we are working is the biceps, and it contracts, but at the same time, the triceps relaxes, this is called antagonist muscles. The same thing happens when a rock climber climbs up the rock wall, the biceps contract as he pulls up and the triceps relax at the same time.
