Two reasons why the mind will attempt to shut off muscle from moving. Two extremes that cause confusion in the new strength trainer. Very often, newbies are worried about overtraining. I have one boiler-plate response to questions about overtraining: if you're asking, then you're not overtraining.
Overtraining isn't what happens when you've worked out really, really hard and you're still tired two hours later. Its something that happens either over a period of several days or after a workout that's way past insane (think: triathlon horror story). It's not unlike having the flu: headaches, nausea, extreme fatigue, etc and all you want to do is sleep! Not enough sleep and/or not enough food are keys. In other words, it's a period of complete disregard for your body and a complete lack of caution. By asking the question, chances are that you're showing too much caution to overtrain in the first place.
Something that plays into overtraining and training injuries alike is mistaking the notion that you can measure the merits of your workout by how much pain you experience. Pain is a side-effect of hard training. So, the harder you work, and the more pain you're in, the better your workout is... to a point. Frankly, most newbies don't know where this measure becomes null and void. Muscular pain from fatigue can be good: it's the side effect of good work. Achy joints or pulled tendons/ligaments are not! Even worse is feeling your workout hours after it's completed. A good workout shouldn't hurt for much more than two hours afterwards. An exception would be a new exercise. The body takes more time to adjust. That's why it's a good idea to ease into a new exercise gently, on an easy day.
Let's be frank though: Too many people undertrain. They use pain as an excuse to pussy out of working hard. They pull the overtraining card out when they're tired afterwards. There's another reason to go easier the next day. It's all bullshit. As long as we get the proper amount of healthy foods and a good night's sleep, most of us are capable of pushing ourselves much harder than we do. That's why some exPURTS insist on training with other people: someone needs to deliver that kick in the ass that you need! As long as you're not insane or being stupid, you probably don't need to worry about injury or overtraining.
Pushing through pain also a great spiritual and mental exercise too. Embracing pain in life is key to getting what you want. You have to work hard to get that thing that you want. Hard work always brings on the pain. The only way that you get what you want is to accept that pain is necessary. Pain will always be there. If you don't get that through your thick-ass skull, then life will always be harder than it needs to be. It's kind of funny but in trying to make life too easy, it actually becomes much harder.
Here's a good example, relevant to the spirit of this blog...
Let's say that person x just didn't like to work out. It's painful and tiring, so person x stops reading The Bodyweight Files. It's too much to eat right too. It's cheaper to buy potato chips, ice cream, fast food, soda, hot dogs and hamburger helper than it is to buy fruits, veggies, nuts, eggs, fish and lean steak. Besides, person x enjoys the former more than the latter. So, this goes on for a while with a few, sporadic and fleeting efforts at getting in shape just to say that person x tried to. Exercise is painful, after all.
By age 35, person x shops at Causal Male XL (or maybe Lane Byrant). By age 45, person x starts having problems with blood sugar, causing the feet to ache and feel cold all the time. Then, person x need cholesterol medication. All the while, the aches and pains of carrying around 35% Bodyfat keeps taking its toll and by 55, person x has arthritis in one of the knees. That blood sugar problem warps into type 2 diabetes. So, now person x is on insulin, cholesterol medication, and a pain killer... plus something for depression. Shopping at the fat clothing store takes a toll...
Now, by 65 person x has a heart attack. The doctors manage to patch that up, for now. The effects of all of diabetes has taken it's toll on the arteries going to the feet and now simple sores on the feet turn into septic infections requiring treatment. That arthritic knee is now bone-on-bone and needs to be replaced. Then cancer...
How does that compare to the pain of working out every day for 20-40 minutes?
Be smart when you're working out, but for crying out loud... DO SOMETHING! It's really not that painful in the bigger picture.