My doc advises against weight work and body building. He likes to say there are no old body builders. When I started with him four years ago after a precipitous weight loss and a diagnosis of Lyme's, I was in shit for shape. Pretty much weak as a kitten. Under his regime of QiGong and a very restrictive diet, I am feeling better than in decades now. The QiGong series I do has many elements of isometrics, where tension of muscle to muscle is in play. The doc likes this series because, as he says, it brings stimulation to all the acupuncture meridians and brings healing forces to all the organs. It's a full body tuneup.
I have literally been able to see my body changing. My upper body and arms are really muscling up, but these look like swimmer muscles, not weight lifter muscles. Because the forms include a conscious abdominal breathing (changing my regular breathing habits), I can see I am developing six pack abs.
When I first got my rep count up, I found I couldn't do the reps just before bed, as I would be totally wired. That effect has passed, as I have been at my maximum of 49 reps per individual form for some time now. With weights, one usually does one session, three times per week, to let the overworked muscles repair, and that's how weight work builds muscles. With the muscles working against each other, you control the level of tension. Doing it every day, I can tell when I have been overdoing it on particular muscles, and will reduce the tension on those parts until they are again up to the task.
Anyway, feeling great has become my meter, and though getting into both the exercise regime and the diet was tough for me, I have become a true believer. My doc told me one day that he will not go on facebook and look at the pictures of his family because he can't stand to see them looking so unhealthy. At my three year assessment last year, where my doc measured strength and mobility in about a zillion ways, he said my body was "getting younger." My massage lady who I see every two weeks told me that she is telling her other clients about the changes she's seen in my musculature, and recommending they look into what I am doing.