First it depends on your body type and genetics, one size fits all doesn’t work for muscular development. It sounds like you can add mass, but most of the time it’s just muscular tone. Don’t worry about the gain in muscle as much as the loss of flexibility that sometimes occurs with increased strength training. There are many schools of thought with high reps/ low weight being advocated for endurance athletes. But this type of work out is very time/intensity specific for any benefit to be received. Go with the basics to start and progress in the body areas that will give you the most benefits for swimming, biking, and running. AND the best benefits received from lifting are for injury prevention. Keep your current body part specific routines, lift a weight that allows 10-12 reps with muscle exhaustion near the last rep. Do 3 sets. Don’t increase the weight from set 1 to 3 (those who want to increase muscular development would be encouraged to exhaust completely during the last two reps and increase weight between sets). Three times a week are plenty. Watch out for too much shoulder work and quad work, and make sure to increase your calf stretching before running.

I've always gone by the pythy "You can’t accidentally get too big, it’s like assuming that learning how to drive means you’ll immediately turn into a Nascar driver." but maybe you're the exception that proves the rule.

How much did your aerobic volume decrease? I'm loathe to advocate caloric restriction while strength training and endurance training simultaneously but maybe keeping an eye on your intake would help too.

I gain muscle by thinking about lifting…but I came from a lifting & sprint/power background and just did my first 70.3.

No weight training in the course of 8+ months of half prep.

Thought I’d trim down the bulk but instead…gained 10lbs!

Yes longer and leaner muscles but still heavier and thicker overall. So, I feel you. If it helps, I plan to incorporate lifting again as strength is important with the approach of 15-20 reps and lighter weights. YMMV and good luck!

I don't mean to seem flippant but if you weigh yourself every morning, you will catch the weight gain a lot faster and then you can cut back on your caloric intake. You have to regulate what you put in your mouth to avoid weight gain. I suspect you'll still get stronger but just not gain mass.

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"Anyone can be who they want to be IF they have the HUNGER and the DRIVE."

p90x3 works great for me. x and x2 i bulk up quite easily, but x3 is my sweet spot for strength without bulking.

Here is my confusion: I've read that 3 sets of 10 reps is exactly how you build muscle bulk, which is the exact opposite of what I am trying to achieve. I want wiry triathlete muscles...

I'm certainly not "big" - 5'9" and 165 pounds right now - I'm just "bigger" than before I started lifting (by about 10 pounds). I agree that the gain could be from over-eating, it's just that my measurements have all increased (e.g. chest size and back width) so it doesn't feel like it's just chub. My t-shirts are tight across my shoulders, chest and upper back, my trousers fit the same as always (i.e. my waist hasn't changed, and I would expect that to be where the chub would go). Aerobic volume in September was about 8-10 hours per week; right now I'm at about 6-8 hours per week, so about a 20% decrease I guess.

I probably weigh myself three times a week, always in the morning, so this is something I have watched creeping up. The reason I am asking about my lift protocol is because I can see (and feel) that I have added quite a bit of bulk across my chest, shoulders, and upper back, while my waist has stayed the same (and my stomach looks the same as it did in the summer). I'm sure some of the 10 pounds is chub, hidden around my body, but I also feel as though some (most?) of it is muscle bulk that I was trying to avoid.

First of all, I'd argue you're better off with heavier 3x5 as a triathlete. N=1 but that has given me my best bang for buck in terms of correcting imbalances and improving mobility. Secondly, building bulk doesn't happen by accident, as someone said above. You only get bulky if you eat lots of protein and have a fairly significant caloric surplus. Fuel the work but eat normally and you'll have nothing to worry about. As with most things, big muscles come from the kitchen, not the gym alone.

When you do your 3 x 5 how long are you resting between sets (for example during front squats)? What do you do between sets? Pure rest or some sort of stretching/core work?

It depends on what I'm doing. Front squats, 2-3 min, more if needed. You're trying to get strong, so do quality work, which requires recovery. Sitting if possible, nothing else. IMO, biggest mistake of most triathletes is not resting enough when necessary. Happens all the time in swimming. Anyway, a session would look something like this:

10 min warm up, stationary bike, dynamic stretches, etc.

Single leg exercise, usually Bulgarian split squat or b-stance split squat.

Upper body move: overhead press, bent over row, bench

Heavy double leg exercise, squat or deadlift

Second upper body move

Core

I can get through all that in an hour and it's an extremely productive workout. 2x a week during base season, 1x a week or no lifting in-season. Doing only two leg movements each session was key for letting me get quality work in the gym without significantly impacting other workouts. That being said, if you're trying to do work in the gym, limit the interval work on the bike and run especially.

Thanks for this, I'm cutting and pasting your post into my training schedule.

Do you see any value in sessions involving high reps/low weight, in terms of building endurance?

unless you increase calorie intake or decrease activity, you should not worry about weight gain. That being said, the only probable weight gain will be water, which muscle mass consists 79% of it.

Sets and reps is a simplistic approach, but it’s one that’s easy to understand. You build muscle with progressive overload. If you don’t overload the muscles they won’t grow. If you go to just under overload you can strengthen the muscle in smaller increments, so whatever will suit you. If you work a muscle group and you’re sore the next day that’s doms or delayed onset muscle soreness. You broke down muscle fiber and it’s rebuilding. This isn’t a bad thing but in your case it may be more than you want. Just workout, and if you are too sore, back off on the weight and/or weight.

Okay, that makes sense, thanks for clarifying. I have had DOMS after most sessions, so perhaps I am just lifting too much too soon.

Happy to help! First season I'd focus on form and lower weight high rep. As you progress, in order to continue making gains you'll need to periodize between lower weight, higher reps and higher weight, lower reps. For those you're not really building endurance though, that's more hypertrophy for building muscle, although again, that doesn't mean you'll get bulky, you can build lean muscle as well.

What is your goal? "Getting stronger" is not a functional goal, unless your goal is to lift more weight. Is your goal to go faster at your IM race next year? Are you maxing out your swim/bike/run time?

Nobody puts on ten pounds of muscle in a month, sorry. Most of that is fat (or water or both). Do you really think that lifting for about five or six hours would cause you to gain ten pounds of muscle?

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"Go yell at an M&M"

Obviously I know it’s not ten pounds of muscle - if you read the thread you would see references to the chub I accept is there as well. The fact remains that I have bulked up in the shoulders, pecs and upper back, and I’m pretty sure I haven’t suddenly got fat shoulders. My goal is to strength train without bulking up (the clue is in the thread title). If you have some constructive advice to add then feel free to chime in. (Note that telling me to swim/bike/run more is not helpful here as I have asked about strength training protocols that allow for strength gains without inducing muscle hypertrophy, I haven’t asked whether you think strength training is a good idea.)

 Eat low protein?

I have no advice for you, since you insist on saying that strength training is your goal. Sometimes the best advice is not in answering a question, but in asking different questions. I guess you just want to be stronger for stronger's sake. My mistake.

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"Go yell at an M&M"

I’ll try one last time: my goal is to strength train through the winter and become stronger without putting on too much muscle mass. The reason I want to get stronger is that being stronger allows me to perform better in all the different parts of my life (triathlon is one of my hobbies, but I have other hobbies, as well as a career and a very large family with young kids). The reason I don’t want to add too much muscle mass is because I enjoy triathlon and want to be able to do it well; if I can be just as strong at 155 pounds as I am at 165 pounds, then I would rather be 155. The point of this thread is to find out what lifting protocols (in terms of how much weight, how many reps, how many sets, how much rest, which exercises to do etc.) will allow me to get strong at 155, rather than getting strong at 165 (which I already know how to do).

I've maintained the same amount of lean body mass (muscle) for the last five or six years...I can tell you that I haven't maintained the same amount of fat hahaha. Getting to muscle atrophy is pretty hard when you're using your muscles through some type of training while still maintaining decent calories.

I'm also a hard gainer and hard loser...hard to gain muscled and hard to lose fat.

But as long as you're not starting from being a body builder with 50lbs of excess muscle and 2500 extra KCals to maintain that size per day, most people don't atrophy too much.

Now as far as this guy gaining bulk, if he just focuses on strength and limits his reps, he'll gain some lean tissue from being under trained, but not too much at all. Unless he decides to go on a 18 month hypertrophy journey there will be very limited change to his mass. What he could see is a change in body composition.

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Washed up footy player turned Triathlete.

Still need a metric to chase. For example 2x bodyweight deadlift . Chasing a goal requires test phases with time deadline

Just lift the same way powerlifters or bodybuilders do but don’t eat a caloric surplus. It’s what I do and it works great. It’s a lot harder to look like a bodybuilder than endurance athletes think.