system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why do some exercises (eg. wall sits) hurt intensely in the moment but cause minimal soreness, whereas others (eg. deadlifts) feel effortless in the moment then hurt terribly the days after?
gpt
I kinda see where you're going with this. But, if deadlifts feel effortless, you may need to increase the weight. 

To simplify, there is central, and peripheral muscle fatigue. Central refers to the motor neurons of the nervous system. This tells the muscles to contract. The more force you need, the higher the frequency. 

For wall sits, it's a much lower amount of force. But, since you're holding it for longer, the lactic acid produced by muscles when contracting has longer to build up, causing that initial soreness. 

The delayed onset muscle soreness (DOMS) you're referring to is still not totally understood, but believed to be a result of the damage we do to our muscles when we use them in ways they aren't used to yet. But, some people still get DOMS even after months or years of regular training, but it's usually much easier to manage.