This will be a two part post. In Part 1, I explain the relationship between intensity and duration. In Part 2, I will go over the physiological adaptations associated with high, moderate, and low rep training.
Part 1: Intensity vs. Duration
The primary thing to understand is the relationship of intensity and duration. Intensity and duration will always be inversely related. When the intensity is higher, the duration will be lower. And when you are working at a lower intensity, you should be able to endure the activity for longer. It is your job as a trainee to apply the correct amount of intensity to the prescribed durations found within your workouts. You can associate intensity with weight, and duration with reps.
If I gave you one minute to push your car as far as you can, and then gave you one minute to push a shopping cart as far as you can, which would you push further? You would be able to push the shopping cart much further than your car.
You can also lift 20lbs. more times than you can lift 100lbs.
You can also sustain a light jog for much longer than you can maintain an all-out sprint.
To train effectively, you must acknowledge the relationship of intensity vs. duration when selecting weight. I see far too many people disrespect this law by using too low of intensity for a prescribed duration.
Here is a real life example...
Your workout calls for you to do a set of 10 squats...you choose to use 50lbs. to do your set of 10 squats...after rep 10 you stop, but your body actually had the capacity to reach 20 reps. This means that for the duration of 10 reps, you under-loaded the exercise. To have achieved an adequate training effect, you needed to use much more intensity (weight).
You should really have to work for the last couple of reps in each set. If you are able to do the entire set with a straight face, you aren't working to a high enough intensity.
The lower the reps in a set, the closer you should come to hitting failure. For instance, for a set of 5 reps, you may use a weight that you can barely get the last couple of reps. The further down the rep scale you go, you should struggle more and more to get the last rep. If you go far enough down the scale, you would eventually come to your 1 rep max: the absolute heaviest weight you can manage for one single rep.
Knowing your 1 rep max (aka 1RM) can be helpful because once you know that, you can use percentages of your 1RM to make weight estimations for each set in your training program.
To sum up, make sure you are matching the intensity to the duration of your sets appropriately. If the workout calls for a lower amount of reps, make sure you are using heavier weight. And I will guess your interpretation of "heavy" isn't even heavy enough. True strength training (lower reps, using high weight) has an entirely different feel than moderate to high rep training. Many people never experience it because they spend too much time training with higher reps/low weight. Without ever using lower rep ranges, say sets of 5 or less reps, you will never learn how to continue to apply force as tempo slows and fatigue accumulates...and that is the type of training that builds legitimate strength. More on this in Part 2.
I'd love to hear your feedback. Comment below. I'd also love to elaborate on anything that I mentioned...maybe you'd like to learn what you can do if you if you're in the middle of a set and you find you are using either too light, or too heavy of weight?