Currently I restarted lifting (weights) after a 2 month hiatus to get stronger. Back when I was lifting almost daily, like 5 times a week, there was no soreness in my arms. But when I started, I remember there was a lot of soreness and you just adapt. Is there a phrase or even a single word that describes how it starts off hard, then it only gets easier? I realize that you can "adapt" to the situation, but that is not the phrase/word I am looking for. The best analogy I come up with is in chemistry, where some things have a high activation energy but then the chart gets lower. The phrase does not have to relate to weights/lifting/fitness.
In business, one might call these high startup costs a barrier to entry, which is defined by Wikipedia as "an obstacle that makes it difficult to enter a given market".
growing pains: The difficulties experienced in the early stages of an enterprise.