Of course, the article typically refers to athletes as those people who spend a lot of time running, swimming or in the saddle (ie. aerobic activities/metabolic conditioning) rather than strength/power athletes. They are not (usually) trying to grow muscle and are not training their muscles at high intensity (% 1RM).
This sentence struck me as interesting as it is not in line with the thought that muscles can continue to grow even in the event of constant loading:
<div></div><div id="QUOTEHEAD">QUOTE</div><div id="QUOTE">During exercise, muscles stop the biochemical reactions used to maintain themselves such as replacing and resynthesizing the proteins needed for day to day activities. It’s not that exercise is damaging your muscles; it’s that they halt the maintenance process until exercise is over.</div>
Of course, constant loading does not necessarily imply constant work. If someone was to wear a heavy rucksack all day long then they would still be able to rest their leg muscles any time they sat down or stood still, effectively transferring the load to their skeletal components.
I can quite believe that when working hard, muscle tissue is in a different modus operandi to when it is at rest. For strength sports such as weight lifting and weight training there is actually plenty of rest time during a workout, so biochemical repair and maintenance within a muscle could be occurring during a workout. However, if it were the case that certain levels of hormones in the body that were elevated during exercise also shut down the repair mechanisms, then that would be a different matter. I haven't read about this happening though.