I also wondered about this "don't strain yourself." Seems impossibly vague. STrain is what makes muscles grow, eh? On one hand, you're supposed to be exercising. On the other hand, you're not supposed to do anything fun. At first I was content to work on the cardio--it's the heart that needs to get better, after all... The disappearing gut I'd earned during a year of doing NOTHING was also starting to go, as well. But I couldn't get over the gnawing desire--I. Want. My. BODY! BACK! I made my cardio explain, as best he could, why "no weights." It's mostly the pressure put on your aorta by heavy lifting and, worse, holding your breath (bearing down) during heavy lifts. He gave me guidelines--no static exertion. All effort must be accompanied by movement--no breath-holding. No straining. No squats or deadlifts. But chest, arms, back, abs, etc., are free to work out as long as Commandment Number One is obeyed--absolutely NO breath-holding during exertion. Finally started working out like that this week. Feels pretty light compared to what I'm used to, but it's better than nothing, and my work-starved muscles are already responding to these "light" workouts. Maybe improvement is possible after all, even with all the "NO!'s" from my doctor....
Don't strain yourself, to me, means--if I can complete the last rep in my set without pausing to HEAVE it home--if the weight is light enough to finish the set comfortably. Working like this, I usually do one extra set of whatever it is, just for good measure. But yeah, no more red-in-the-face, vein-popping, grunting exertion against a bar that won't move. Used to love that, now it's off limits.
But I must say, after a year off, even my little baby "light" workouts these days feel like they're doing a world of good.
Scott