A couple of days ago I mentioned the principle of specificity, which says that physiological adaptations to exercise are specific to the system or part of the body being worked. Today I might as well toss out another principle of exercise science for your consideration, and that is the principle of overload.
This principle states that in order for the system or part of the body being exercised to adapt to training and make progress, a greater-than-normal stress must be placed on it. In other words, your workout should tax you a bit.
If you want to improve your cardiovascular endurance, a moderately paced half-hour walk around the local track every day won't cut it. Or, if you want to build leg strength, squats with an Olympic bar minus weight plates are not going to do it.
I mention this because over the years I've heard a number of friends bemoan the fact that they were not getting faster or stronger, or whatever it was they hoped to achieve, despite working out regularly.
But upon further questioning, it usually turned out that they were still doing the same old exercises at the same intensity as when they first began working out. Their muscles, heart and lungs were probably crying out: "Help! We're bored! We need a challenge!"
Now "overload" does not mean "overdo." When adding weight to a lift, or distance to a run, you want to challenge yourself, but within safe limits. I won't speak about running, because I think I've spent enough time this week documenting the fact that I'm not a runner.
And although I'm not a personal trainer — at least not yet — I have spent 20 years engaging in regular weight-training, so I'll limit my comments to that subject.
With weight-training, the best way to judge whether a weight load is appropriate for any given exercise is by the number of reps you can do until fatigue (fatigue meaning you could probably do only one or two more reps before your form would be compromised). In general, 8 to 12 reps is considered a safe range for experienced exercisers (if you're just starting out, 12 to 15 is a better bet).
So if you're easily banging out 15 reps while whistling and mentally composing your grocery list, and feel like you could still do a whole bunch more, that's probably a good sign you can go heavier.
Why spend all that time at the gym without getting results?
Sadly, a lot of people who end up asking themselves that question give up without understanding why success is eluding them. Just remember: at the gym, "heavier" is a good word.