"arduous exercise" is correct and usable in written English.
You can use it when you want to describe a very difficult or tiring exercise. For example: I worked out for two hours every day, doing an arduous exercise regimen.
"We rarely find a musician who doesn't follow a very arduous exercise and rehabilitation program".
Negotiations for a new collective bargaining agreement will have to be held in the next two years, and that arduous exercise will most likely have to be completed before such a significant new venture is undertaken.
The well-fed, well-swaddled beneficiaries of corporate entertainment leaving Covent Garden after a performance... do not look as if they have been subjected to arduous exercise, mental or physical".
He ends the year having changed policy more than politics.Austin: On balance, I see 2009 as an arduous exercise that has put lots of people on deck for a major 2010.
It's been an arduous exercise in willpower".
This is also observed in fitness gains: a recent study actually showed that one minute of arduous exercise is comparable to 45 minutes of gentler exercise, requiring a relentless focus on performance metrics (e.g. heart rate) rather than time (article).
Friendly regards such arduous exercises as a natural and unavoidable part of his everyday existence.
When he was at home, he was punished with arduous exercises and locked in a tiny room with no food or sanitation.
A representative study of collegiate rowers, for instance, found that after eight weeks of an arduous core-exercise regimen — added to their normal workout routines — the rowers had great-looking abs but weren't better rowers; their performance was unchanged in a rowing-machine time trial, compared with measurements before they'd undertaken the extensive core routines.
Morris decided to motivate his black comrades by having them mimic the arduous training exercises done by white paratroopers.
Inside the centers, they are often denied evidence-based drug treatment and other basic health services and instead are forced to perform arduous physical exercises, military drills, or forced labor.
