The original post reminds me of guys I used to play basketball with. Some guys looked pretty and worked out in the weight room a lot. However, they didn't spend a whole lot of time doing cardio or just playing basketball. They would dominate for the first few minutes. However, after that, they were basically worthless; they couldn't shoot, jump or rebound with any kind of force.
Accordingly, I'm a fan of working out for having practical strength, and not for looks. If I look good, that's great, but getting pretty muscles is not my purpose for working out.
I exercise regularly and am in pretty good shape. Each workout includes cardio and/or weight training. For cardio, I typically run the hills around my house. This serves also to build leg muscles. At my age (35) and for my body, running hills is better than a gym workout of squats. Running hills is like a combination of cardio and strength training at the same time, which better simulates real world activities. For weight training, I like to do mostly body weight exercises like push-ups, dips, pull-ups, chin-ups, etc. I do free weights in the gym no more than once a week.
9mm+ said:
Exercise (if done properly) helps out with all shooting disciplines. Be careful going to the range after a heavy arms/chest workout. If your muscles are too fatigued and filled with lactic acid, you may have difficulty even holding your weapon properly. Not only will you have trouble sighting in, you may be weaving all over the place. If you're not used to upper body exercise at first, then alternate activity days (work out/shoot/work out/shoot). Let your muscles recover before grabbing a weapon.
What you described may be the case initially. However, your body can become accustomed to shooting after a heavy workout. That's the ultimate in knowing you can shoot when your body is fatigued.
I'll go back to the basketball analogy. When training for basketball in high school and college, the best training trick was to go play basketball immediately after weight training. The first few weeks of this routine, it's nearly impossible to shoot a basketball. However, after awhile, your body becomes accustomed. At that point, you know that during a game you'll be able to shoot even when your muscles are fatigued. Your confidence goes up.
Think about NBA players. By the time the playoffs come around, these guys' bodies are basically fatigued. However, they've trained themselves to be able to play while fatigued. So, they can still shoot, jump, grab rebounds, etc.
That would include any form of marksmanship or firearms training. 
I jokingly told them that if this old guy can handle it you all shouldn't have a problem.
