The final two reps of any set should be challenging, approaching technical failure.
And, without the aid of a trainer, that's an element many of us tend to fudge up. We either go too heavy and risk injury—or we go too light, shorting our results.
Research shows most of us fall into that second category. Which is why this simple trick from Mike Boyle, cofounder of Mike Boyle Strength & Conditioning, is incredibly helpful: “Select your weights for your exercise, then do one set of 20 reps without resting. If you can finish all 20 reps without pausing for more than a few seconds, you need to increase the load.”
Boyle has many of his clients, especially newer ones, try this test before performing new exercises, or when he thinks they’re ready to go up in weight. It works because “in a lot of cases, you’re strong enough to go past the 10- or 12-rep count you set for yourself, but you would never know it since you never it attempt,” he says. “Seeing just how far you can really go can completely change your mental dynamic and empower you for the rest of your workout.”
Boyle also abides by the strength-training standard known as the "Last Two Reps Rule": The final two reps of any set should be challenging, approaching technical failure (a.k.a. you can’t do another rep without cheating). But if you can get to 20 reps (or at least close to it) without fail, he recommends increasing your weight by 2.5 to 5 pounds for upper-body moves and 5 to 10 pounds for lower-body ones. (And then doing the "One Set of 20" trick to check yourself on that weight too.)
You really should be choosing weights that you can handle for only 12 reps, says Boyle; otherwise, you "aren't getting much of a strengthening benefit." So if time is of the essence—which, let's be honest, it always is—and you're looking for legit results (which, again, let's be honest, we always are), sticking to the "One Set of 20" rule can be a real game changer.