I am aware of the other question on feeling the burn, but that question seems to be about feeling the burn after a workout.
My question is, should I feel the burn during the workout.
At the moment when I do my reps, I feel..almost nothing. I'm lifting a 15 pound dumbbell and it's heavy, but I don't feel any sort of burn.
Is this an indication I should try a heavier weight?
"Feeling a burn" during a workout just means that you're working endurance instead of strength or power. From my layman's understanding, which may be wrong, it's because you're no longer producing force anaerobically. Notice that you'll rarely experience that sensation on the first few reps, which is when (if you're using a challenging weight) you're doing the work of stimulating strength gains. So focus on lifting heavier weights only a few times, instead of lifting lighter weights enough times to feel a burn.
If your goal is strength, then gauge the effectiveness of your workouts by measuring strength, not by how much "burn" you feel. Can you lift a heavier weight this workout than last workout? Great, you're stronger, forget the burn.
As you get used to exercise, your body gets adapted to that level of work. For that reason, "feeling the burn" is a remarkably inaccurate way of deciding when to increase.
Burn sensation in muscles is general indicator, that you've been working our beyond your Anaerobic Threshold for some time. It means, that your body can't eliminate the exertion of lactic acid. It usually happens with runners, cyclist, etc.
The anaerobic threshold (AT), the point at which lactic acid starts to accumulates in the muscles, is considered to be somewhere between 85% and 90% of your maximum heart rate. [1]
But why pain or burn is felt ? It's because of the acidity of lactic acid, which is made of (non-acidic) lactate and (acidic) hydrogen ions:
The breakdown of glucose or glycogen produces lactate and hydrogen ions (H+) - for each lactate molecule, one hydrogen ion is formed. The presence of hydrogen ions, not lactate, makes the muscle acidic that will eventually halt muscle function. As hydrogen ion concentrations increase the blood and muscle become acidic. This acidic environment will slow down enzyme activity and ultimately the breakdown of glucose itself. Acidic muscles will aggravate associated nerve endings causing pain and increase irritation of the central nervous system. The athlete may become disorientated and feel nauseous [2]
Very high levels of H+ can damage cells, so, long term training beyond the anaerobic threshold is not an good idea :)
Burn sensation can be used as indicator in endurance trainings or during high performance interval training -- but anyway, it means that you've reached you limit and you need to lower the intensity to 40-50% to eliminate the extra acid.
You didn't specified your goal, but I assume that it's strength or common fitness. So, lower the intensity when you feel that your muscles are burning and do not use it as indicator in strength training.
