As we become more fit, we get better at tuning in to our body's signals. I've been training with weights (OK, off and on) for 30 years, and I know in an instant the different between good pain (the "burn" you feel while training) and bad pain (injury). You develop that sense with training experience. Novices can't always tell the difference, but once they learn to distinguish between discomfort and injury, they gain confidence and their training can make huge leaps forward.
Then there's the difference between not training because you just don't feel like it and not training because your body is worn to a nub and needs time off. Strength coach Bill Starr reminds us that the ingredients for gaining strength and size are three–workout, nutrition, and recovery–and can be visualized as an equilateral triangle. The more intense the training (the longer that side of the triangle), the greater the other two had better be.
When you're just unmotivated or bored by your training, go to the gym anyway, and give it 10 minutes. Grant yourself permission to quit and go home if you don't get into it within 10 minutes. If your problem isn't physical, the odds are pretty good that you'll get into the spirit of the workout and want to continue. Some of my very best workouts have taken place on days when I didn't particularly want to go to the gym.
But when your body is depleted–you're shaky from low blood sugar, you're wasted from lack of sleep, you're spent from too many workouts, you have a bad cold–you cannot make progress in the gym. Training is a stress, and when your body is already pushed past its limit, you can't recover from your workout.
Learn to read the signals, and don't feel guilty about skipping a workout when you have nothing to give in the weight room. Get some rest, carb up, and see how you feel the next day or the day after that. Do something light in the meantime: walk, ride your bike, rake the leaves. Then go back to the gym and tear the roof off.
