It's normal that in a series, especially in long series, the art evolves little by little as the artist gets used to constantly draw the same characters, improves his style or apply new techniques. But this series is something like no other.
The artwork was by no means bad at the beginning, it was quite good actually, but after just a few chapters you could notice great changes and in just 10 or so chapters the art was peak.
Something about the art evolution that I might not have liked that much is how the girls changed. At first they were slim girls with slightly big breasts and tight butts, but as the art changed they kept getting more and more curvy, finally turning into voluptuous women. It's not that I don't like voluptuous women, because I freaking do, but nowadays almost every pornhwa is about women with those kind of bodies, so reading one with slim girls (especially in this series' gym context) would have been nice for a change of pace.
And when you think it can't get any better... After the hiatus on chapter 30 there is a notification that there is a change in the artist. To put it simple, the new author is nowhere as good as the previous one, not even close, so the downgrade is really noticeable.
I guess something happened between the team working on the series an the publisher, because in the next chapter there is a notification that the author and artist will be removed from the credits and not long after the series ends. It was a shame since it was a really good series. Even though the MC can be frustrating till you get used to him, each of 3 girls is a good character in its own way and the story was also quite enjoyable, so I would have liked if it lasted longer, but it is what it is.