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Is it a good idea to have some overlap between engaged muscle groups in different exercises within a single day?

If you do, for example, hip thrusts and then some squat variation, your performance in the second exercise would be limited primarily by your tired posterior chain, not "fresh" quads. It may be good news for the former which would get more sets, but bad news for the latter which would be kind of undertrained.

If that is true, it would be a better idea to first hit only your hamstrings and glutes, and then only your quads (with some exercise that involves knee extension but not hip extension). Or vice versa.

Is this logic faulty?

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Is it a good idea to have some overlap between engaged muscle groups in different exercises within a single day?

It's hard to avoid, really, even if you're doing full-body workouts, and by definition, any regime that isn't full-body will focus on one or a few specific muscle groups. There's push/pull/legs splits, upper/lower body splits, antagonist splits, "bro splits", etc, which all focus each workout on one or a few muscle groups, and they're all used by lots of people with good results.

There are theoretical physiological advantages to full-body workouts with low overlap between exercises on individual workouts and instead hitting each muscle group several times a week, compared to workouts with overlapping exercises (for example, muscle protein synthesis in a muscle is elevated for 24-48h after stimulating it). On the other hand, you don't want a full-body regime without at least one rest day between each workout (because you need time to recover), so if you're working out on consecutive days, some kind of split is probably better. Also, many splits actually have some degree of overlap between days; for example, you'll hit both triceps and shoulders when you're benching on chest day.

Focusing on a specific muscle group during one workout also means that you get "free" warmup for the rest of the workout from your first exercise, which might even improve your performance (and definitely makes the workout more time efficient).

For all practical intents and purposes, which regime is "better" really depends on how it fits into your schedule. From what we know, total volume is more important than frequency, so whatever lets you lift the most during the time you are willing to spend in the gym is the Optimal Exercise Plan™ for you.

If you do, say, hip thrusts and then some squat variation, your performance in the second exercise would be limited primarily by your tired posterior chain, not "fresh" quads. It may be good news for the former which would get more sets, but bad news for the latter which would be kind of undertrained. If that is true, it would be a better idea to first hit only your hamstrings and glutes, and then only your quads (with some exercise that involves knee extension but not hip extension). Or vice versa. Is this logic faulty?

Well, you could of course choose to only do isolation exercises to ensure minimal overlap (like leg curls for hamstrings, hip thrusts for glutes, and leg extension for quads) and therefore optimal performance in each exercise, but compound exercises bring a lot more to the table than just strength and muscle gains for the prime movers, such as coordination, balance, activation of a slew of minor supporting muscles, etc, so leaving out seems a poor choice.

You could also try and spread exercises out over the week to get minimal overlap during workouts, but that could make it hard to reach the desired volume in the time available while also allowing for recovery between workouts. Looking at myself, I'm pretty sure I'd do worse if I did hip thrusts the day after a heavy squatting session rather than directly after.

Instead, I'd consider the "conventional wisdom" which says the following about organizing workouts (regardless of if you're doing full-body or a split):

  • Do the exercises you prioritize first. If you want to get better at squatting, squat before you do any other exercises having any significant overlap.
  • Do compound exercises before isolation exercises. Compound exercises are more complex and you want good form when you do them, for isolation exercises you can just lower the weights a bit and still get a good burn and/or pump. Squats are a compound exercise, hip thrusts are an isolation exercise, so start with squats and then move on to hip thrusts.

Having said this, some lifters (particularly bodybuilders) like to reverse the order and do isolation exercises first, which is known as pre-exhaustion, but science says that exercise order doesn't matter much for hypertrophy, while you tend to get stronger in the exercise you do first:

However, there was a difference between the MJ-to-SJ and SJ-to-MJ orders for strength gains in the MJ exercises, favouring starting the exercise session with MJ exercises (ES = 0.32; p = 0.034), and the strength gains in the SJ exercises, favouring starting the exercise session with SJ exercises (ES = -0.58; p = 0.032).

So you build the same amount of muscle regardless of order, but you'll increase more in strength in whatever exercise you start with.

Anyway, it seems that "conventional wisdom" is well-backed by evidence:

  • If you're an athlete, you start off with the lifts that are important to your sport (which for most sports, incidentally, are compound exercises).
  • For general health and fitness, I'd argue that compound exercises have more carryover to everyday activities, so start with them before doing isolation exercises. (Or you could even skip isolation exercises, if your schedule is tight.)
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You should generally be more concerned about managing overall fatigue, rather than local muscle fatigue.

Fitness adaptations occur in response to the accumulation of fatigue through exercise, but not all exercises are equally fatiguing. Movements that use lots of muscles, called compound movements, are the most fatiguing. Naturally, using more muscle mass means using more metabolic resources to get the job done. So exercise like the barbell back squat and deadlifts from the floor are highly fatiguing exercises. They consume lots of resources and impose higher recovery demands than any other movements because the prime movers are huge muscles (quadriceps, hip extensors), and they both use the entire musculature of the back for support. Lots of muscles, lots of fatigue.

In contrast, movements that use less muscles or smaller muscles do not impose the same amount of fatigue or recovery demands. Think about it, what's more likely to leave you on the floor of the gym? Five sets of five on back squat with 85% of your 1RM or five sets of five on leg extensions with 85% of your 1RM? You could probably do the leg extensions on 60 seconds rest without really elevating your heart rate, but the back squats are going to be tough, even with plenty of rest.

So what I do with my training is I spread my big compound movements out across the week, and fill in the rest with lower fatigue exercises. This keeps my overall fatigue about the same workout to workout, but different muscle groups get fatigued more or less in different workouts.

Lets break it down for a three day per week program just as an example of these principles.

Monday

First day of the week, I'll hit the exercise that fatigues me most after I've recovered over the weekend: barbell back squats. For me, this looks like 4 to 6 sets of 4 to 6 reps with a fairly challenging load, somewhere in the 75-80% 1RM range. By the end of this, I'm pretty beat up, but I still need to do some other work.

Next I will do a lower fatigue pressing movement, something like a overhead press or feet up bench. OHP uses small muscles (mostly delts) so the fatigue cost is minimal. Feet up bench forces you to limit the load because you cannot use leg drive, and does a good job of isolating the pecs, again, smaller muscles means less fatigue.

Then I will do some kind of pulling movement, like cable rows or dumbell rows, to hit my back without imposing any significant fatigue. The back has lots of muscles, but by doing a machine or dumbell row, you limit the demands put on all the supporting musculature. Something like a barbell row is not what I'm looking for here - it places significant demands on the trunk and legs to support the position for rowing a barbell.

Wednesday

The big squats are done, and I'm still a bit tired from them two days later, so today I'm doing my main press, the flat barbell bench press. I'm going heavy, using full leg drive, so it really is a full body exercise. It isn't as fatiguing as back squats, but it still takes a good bit out of me.

Since the bench wasn't as fatiguing as Monday's squats, I've got more room to work, so I might do some medium fatigue leg movement, some sort of squat variation. We can lower the fatigue cost of the squat by modifying the movement in a way that limits the load. For example, I might do a Bulgarian Split Squat or a Barbell Lunge. These are single leg movements, so they greatly limit the amount of weight you put on the bar, which means less overall fatigue, even though they can easily tap out your quads and glutes pretty quickly.

Finally, I might throw in some extra shoulder work, this time even lower fatigue than the overhead press, something like light dumbbell lateral raises.

Friday

Deadlift day. Like Monday, I'm going heavy, and I'm not going to have much left in the tank when I'm done. Once I've finished deadlifting, it's going to be isolation movements. I'll hit the quads with leg extensions, hit the chest with incline dumbbell press, might hit the back with reverse flies, etc.


Anyway, the idea here is that you manage fatigue by spreading out higher fatigue movements, arranging your exercise selection so that the total fatigue each day adds up to be about the same, and hitting all your muscle groups two or three times per week, maybe more on the smaller muscles.

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  • beat me to it. I was going for a similar answer but with an upper/lower split :)
    – Luciano
    Commented Oct 11, 2022 at 13:36
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    @Luciano Yeah, the general principle should translate to any sort of program idea. Except Texas Method. Texas Method is quite literally designed to not do what I describe here. (Which is one of the many reasons it is terrible).
    – Thomas Markov
    Commented Oct 11, 2022 at 13:40

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