Everything powerful in the human spirit can be seen as a foundation. Maintaining, and better-still building-upon and strengthening, those important foundations, takes constant care, nurturing, and attention. Self confidence begins with a foundation of respect and needs constant attention and care to maintain and build one's unique self. Similarly, strong, loving relationships need continual nurturing and respect to honor that unique someone in your life.
Why would our bodies be any different? We need to apply these same principles to our unique physical bodies as well. Some readers may laugh at this statement but we truly have to respect, admire and attend to our physique before it can develop in the powerful way we want it to. Watching TV, reading the newspaper, or talking to others while working out, (specifically while strength training) does not develop this foundation. Yes, it makes the workout pass by more quickly and distracts you from something you may not absolutely feel like doing at that moment, but the truth is why do it if you're not going to do it properly?
It's analogous to going to a lecture to learn a subject of interest and wearing your headphones with music blaring. You may learn "something" just like in the distracted workout, you may get "something" out of it, but just as in the lecture, wouldn't it be more beneficial to fully commit and pay attention to your body for that 45-minutes or so you are working out?
Buddhist culture suggests whatever you do, whether it's reading, watching TV, or engaging in conversation, that you are best served if you remain fully present in that moment. Buddhists believe that it's important to do the thing you are doing with purpose. Multi-tasking is not something Buddhists believe gets us very far at all in the long-run. If you think about it, our most satisfying and deeply fulfilling relationships are the ones we are completely engaged in and not just going through the motions with.
It's the same with a relatively simple thing like your workout. How can you ever get in the shape you desire, if exercise is trying to get it over-and-done-with as quickly as possible. If you are simply going through the motions, at best you'll get results to match. At worst you can do real harm to yourself and put back your progress substantially.
Commit attention, respect, and admiration to your physical body and just like any other satisfying relationship, you will reap all the wonderful rewards it has to offer in return.