I took part of my title from a quote by Mark Rippetoe. In it, he states that “Strong people are harder to kill than weak people and more useful in general.” I love that quote. It makes me feel like Rip', as he is known in strength circles, is the kind of person who doesn't pay many compliments, but if he ever said "nice job" after seeing you do a lift, it would feel like you won gold in the Olympics. You could take that to the Wheaties board of directors and be on the box the next week.
It's with that quote in mind that we're going to get into a little something I like to call Stop Living a Lie. It's going to sound like I am berating you, but I'm not--I truly want what's best for you. The problem is that "what's best for you" might not exactly align with "what makes you happy"; unfortunately, I am not in the Happy Business so I can't tell you things that are going to achieve that. I'm in the Full Contact Business and so are you if self-improvement is what you are after. That means there is bound to be a little hurt along the way, but, hey, change ain't easy.
So if these next few paragraphs sting a little bit, feel free to fill one of these out after, but let's move on with making you a better athlete in the interim.
Here's the deal: you need to get in a weight room a couple of times each week and start building some strength. This isn't just about improving your running--it's about improving your life. Strength is one of those great things that makes everything easier, and nothing builds strength like weight training. One body, one barbell, some plates, and your favorite angriest music to get you in the zone. Lifting weights will give you so much back for the time you invest in it, but you have to get in there. Keep it short, keep it brutal, keep it honest, and reap the rewards.
What you don't need is anything with the word "circuit" in the title or description. That isn't going to build strength in the truest sense of the attribute unless you are (a) extremely weak to begin with, in which case anything will work at first, or (b) you are confusing your ability to move a light weight a bunch of times with being able to move a heavy weight a few times. Either way, we need to fix your thinking and get you on track; we aren't making fun of you or anyone else doing circuits, but we can't let you think you're building strength when you're really just improving your ability to move little things around a bunch until your GymBoss beeps and sends you to the next station. That isn't strength training and you need to stop calling it that.
First of all, beginners to lifting weights see gains quickly, but--sorry--they taper off in short order. Initial gains are usually coordination-based and have more to do with neurological improvements than actual muscle. That said, you have to learn to crawl before you walk; this is a fruitful time, so enjoy it. It's also why, when you follow that nifty YouTube circuit plan for runners, you feel like you're getting stronger and every day sees new gains. But your stronger and my stronger are not the same thing. Again, this isn't about not being inclusive--it's about being honest in our appraisal of strength and, more importantly, not perpetuating a definition that isn't accurate. Let's keep it real.
The problem is that strength is a very specific attribute that requires a precise stimulus to engage. You cannot call it strength training if you do not have a working definition of strength and what training for it entails. This is how you find yourself in a circuit training class moving weights around that may or may not meet the threshold for building strength by academic standards of practice, but are chosen arbitrarily based on what is either (a) available in that setting or (b) convenient for the trainer. Find a new gym and fire that person if that's what is happening.
Some of you out there might totally disagree with me at this point. I guess you could call any type of lifting strength training, but, in a very real sense, you would be incorrect or, at least, inaccurate. It is possible to build strength at lower percentages of your one repetition maximum (1RM), but 85% or higher is usually what is necessary to make the structural changes most would consider true strength gains and not simply neural adaptions or noob gains.
Nothing is written in stone, but, for the most part, this is. The above chart is sort of the gold standard for sets of reps in most elite strength programs and it comes from a gentleman by the name of Alexander Sergeyevitch Prilepin. Suffice to say, he was smarter than your personal trainer and if they don't know who he is? Yup, fire 'em--or at least give them a stern lecture about "shoulders of giants" and all that, because Prilepin is one of the modern fathers of successful weightlifting.
There are many variations on the sets and reps recommended in the chart (Jim Wendler's 5/3/1, Juggernaut Training Systems, Starting Strength), but the truth is that anything that veers too far out of those ranges is probably not worth following. Little periods over and under, sure; that's how periodization works. However, if you look at a program and it doesn't jive with at least some of these guidelines, you may want to look elsewhere for your strength plan. It's the age-old "principles versus methods" idea as it relates to strength building.
If you find yourself always siding with the latter, you're probably not going to be as successful as you would have been respecting the former; your trainer's feelings notwithstanding, you will be so much better off without their guessing and haphazard misapplication of training principles if that's what they are giving you. (There's a handy form up above if you want to pass it along to your PT.) With this in your back pocket, you'll be able to get a sense if that's the case and ask some intelligent questions about the whys and hows of program design.
The vast majority of trainees would benefit tremendously by not dicking around in the gym, focusing on a handful of lifts that most experienced lifters would call The Big Three, and performing a few accessory lifts to shore up weaknesses/imbalances. Short of powerlifting legend Ed Coan's deadlift program, not a lot of circuit training goes on in those types of programs. This is a good thing.
And for your strength gains? It's a great thing.