Imagine the following situation:
When you are cycling or skating, you want to push hard to get to your maximum speed. In the beginning this goes well, but if it takes longer you will feel a heaviness in your legs after 30-120 seconds. Your effort continues because you still want to maintain speed but your legs are getting heavier and even more painful. They don't want more and you feel that you can deliver less power with your legs. The complaints can occur in both legs, but they are often more pronounced in one leg. Due to the complaints, you will eventually have to adjust your pace. When you stop the (heavy) effort and continue cycling/skating calmly, the heavy and/or painful and/or powerless feeling in the legs will disappear within 5 minutes. If you then try again to turn on vigorously, everything repeats itself and the complaints are noticeable again after a while.
These complaints actually come back every time with (almost) maximum effort and go away again at rest. Over the years, the symptoms have become increasingly apparent and worsened.