Exercising can sometimes seem a little confusing. There are so many options, so many philosophies, and so many contradictions out there that it can sometimes lead to to not want to exercise at all, for fear of doing it wrong. The one thing that always seems straight forward when you're doing an exercise program, and typically the first thing people think about when their trainer tells them to do a certain movement, is how many reps are you supposed to do.
Typically, you'll look at a program designed to transform your body, and you'll see something like 4 sets, for 8 repetitions. Or, it might say 4 sets for 16 repetitions. Most people will feel like they at least understand that fundamental part of the exercise programming. Unfortunately, this is untrue. If your goal is to transform your body in some way, whether your goal is fat loss, putting on lean muscle, or bulking, your objective is to force your muscles to endure a new stimulus, or a novel stimulus. That happens by pushing your muscle to the limits of its current capabilities, so that it grows and adjusts to the new demands that you're placing on it.
With that premise in mind, aiming to complete 4 sets for 8 repetitions is not actually the goal. You want to FAIL while striving to achieve 8 repetitions. This can easily be the most difficult part of body transformation and resistance training to understand. There is no benefit in lifting a weight that you can comfortably lift for 8 repetitions, and then putting the weight down. If your programming is for 8 repetitions, you must select a weight that makes you FAIL at or near 8 repetitions. You should be on the 6th repetition, and your muscles should say "we don't have 2 more reps!" But, more often than not, when people see their rep ranges and goals, they grab a dumbbell or a weight that they know they can comfortably lift, and then lift it. Your workout program is not an achievement checklist-- look at it as a failure checklist. You gain significantly more muscle and reach your goals sooner by failing at or near that 8th rep. There are no gold stars for finishing 8 reps when your muscles could have done significantly more work!
If this were a race, imagine yourself nearing full exhaustion as the finish line is within sight. Achievement would look like you running through that finish line tape, chest high, still going at your race pace. That is not what we want when we're transforming your body through resistance training. Rather, you want to see that finish line in sight, and then stumble just 4 steps shy of the finish line! You catch your breath, gather yourself, stand up and grit yourself through those final four steps as you cross the finish line in last place. THAT is what you're looking for as you complete a set.
Are you familiar with that metaphor for life, the one stating that we learn more from our failures than from our successes? That is exactly what your muscles do, and building that lean muscle is will accelerates your metabolism and allows you to burn more fat, helping you achieve your body composition goals! You are far more "successful" from a body transformation standpoint if you only get 9 out of those 12 reps on the first attempt, and then catch your breath for 10 seconds to get those final 3 reps. If you see yourself approaching that programmed final rep and know that you're about to cross the finish line with your chest nice and high, be sure to select a heavier weight for your next set. What that tells us is that you're stronger than you thought!
Reach your body composition goals-- fail consistently! Once you learn how to fail, you will consistently succeed.