One of 
the questions I frequently hear asked is whether or not I “like” compound exercises. That’s a very broad question; it’s like asking if I like beverages. It depends on the exercise. Some I like, and some I don’t. But let me be clear - this particular discussion is specifically for the purpose of building the physique. It’s not for those who use resistance training as preparation for a particular sport, and it’s not for those whose goal it is to lift the maximum amount of weight. It’s for those who want to maximally develop the physique with the least amount of wasted effort, and the least amount of unnecessary risk of injury.
For those unfamiliar with the term “compound exercise”, it is a reference to a type of resistance exercise which involves multiple joints and muscles working simultaneously or alternately - as compared with an “isolation exercise” which typically involves only one joint and one muscle working during a set. There are quite a number of compound exercises, but let’s first look at a couple of examples to get a sense of what we mean by a compound exercise.
A squat is a compound exercise, because it works the quads, glutes, a portion of the hamstrings, a portion of the adductors, and the lower back. The primary joints involved are the knees and hips.
Another
example is the “pullover and press”. This exercise is typically not done much these days (...it was more popular in past decades), but it also falls into the category of compound exercises. As the name implies, (while lying flat on a bench) one first does a pullover (with a pair of dumbbells), and then presses the weight over one’s chest - and then repeats. It differs from the squat in that it involves two separate movements, done alternately, as compared with a squat which has its movements occurring simultaneously. The pullover and press works the lats, the pecs, the front deltoids and the triceps. The primary joints involved are the shoulders and the elbows.
A Closer Look
I believe that a compound exercise is only as good as its weakest component. In other words, each aspect of the compound movement must be “good” (productive and safe), or else I would vote it down. As an example, a squat involves two primary movements (knee extension and hip extension), and both of those movements are productive and safe. There is also some isometric stimulation to the spinal erectors during a squat (“static” muscle contraction, without range of motion). Isometric tension is not “ideal” for muscular development, but it’s not especially risky, and therefore acceptable.
In further analyzing the squatting movement, we can see that the lower leg “lever” is less perpendicular to the direction of resistance, than is the upper leg “lever” (femur). This suggests that the quads get a lesser percentage of the load, than the glutes (...in my estimation, it’s about 40% quads and 60% glutes). As it turns out, this is probably ideal, since the quads have less capacity for power than do the glutes. It might actually be an ideal ratio. So the squat involves two safe joint movements, as well as a productive amount of resistance for each of the muscles involved - and this is “good”.
Not so with the “pullover and press”. For starters, the pressing part of the movement has much more capacity for strength than does the pullover part of the movement. The problem this creates is that one will either get short-changed on the pressing part, because he (or she) is being held back by the limitations of the pullover; or one will end up using a weight that is challenging enough for the press, but potentially hazardous for the pullover part of the movement. These two movements are essentially miss-matched, in terms of the resistance. But perhaps more worrisome is the fact that the pullover is not an entirely safe movement for the shoulder joint. So while the press is entirely “natural”, the pullover isn’t. That makes this particular compound movement - “bad” - in my view.
Analyzing a Few More Exercises
The Arnold Press - This exercise combines a standard, overhead dumbbell press - which typically begins with the dumbbells alongside your shoulders - with what would appear to be the beginning of a front press (with the dumbbells held in front of one’s chest). I’m not crazy about overhead presses to begin with, but this move is especially strange.
After having 
done a regular, overhead dumbbell press (usually from the seated position), the person then brings the dumbbells around to the front - as if he were going to press them up from there, but doesn’t. He then simply brings them back to the position where the overhead dumbbell press normally begins. This really makes very little sense.
The act of moving them toward the front of the body is caused by the pectoral muscles, but without any opposing resistance - so there’s no real benefit to the pecs. The act of holding the dumbbells in front of one’s chest for a moment involves more of the front deltoids, but only isometrically. Since there is no upward press from there, there is no contraction, so very little additional benefit to the front deltoids.
It’s really a wasted movement, since there is essentially no additional benefit than what is provided by the simple act of pressing a pair of dumbbells overhead. This one also gets a “no” vote from me.
Upright Rows - If you were to observe someone performing this exercise, take notice of the fact that the upper arm bones (levers) are doing something very similar than they would be doing if the person were performing a standard dumbbell side raise. Sure - the arms are slightly more in front of the body, and the elbows are bent as the arms come up. But the deltoid is pulling on that upper arm bone (humerus) in a very similar way than it would be during a side raise. Therefore, a similar benefit is being affected to the deltoid during an upright row - not a better benefit - just a similar benefit. That part is good.
However, the fact that the elbows are bent as the arms come up, combined with the fact that the forearms are essentially parallel to the ground (i.e. therefore perpendicular to gravity) means that they are now “active” levers. But because of their angle, they are “activating” the muscles that control the inward / outward rotation of the upper arms. In this case, it’s the external rotators (because they are preventing the upper arms from collapsing downward). Working the external shoulder rotator is probably not the goal of a person doing an upright row. And worse, the weight typically used could easily strain the external rotator because it is meant to work the lateral deltoid, which is a much stronger muscle.
Often times, people doing this exercise intend to also work their traps. It does indeed do that, because it’s difficult to bring the bar up to one’s chin without incidentally pulling the shoulder carriage up. This action is performed by the traps. But it’s an incidental part of the movement - not the primary movement. Therefore, the benefit to the traps is not be as great as a regular dumbbell shrug would be, or any other exercise that performs that action as the primary part of the movement.
Lastly, the position of the wrists causes strain - especially if one is using a straight bar. So, while the benefit to the lateral deltoids is relatively good, the benefit to the traps is compromised, and there is rotator cuff strain as well as wrist strain. Personally, I’d rather do heavy side dumbbell raises, and then heavy dumbbell shrugs - more benefit and less strain (i.e. risk of injury).
Bent Over Barbell Rows - We don’t see the exercise being done in gyms very often these days, and that’s generally because we now have better options. People who exercise at home are more likely to do it, especially since it’s still “taught” by many of the fitness magazines. The pitch is usually this: “How to Work the Whole Body with Five Basic Exercises” - or something like that.
The problem with this exercise is that while the lats being worked, the lower back, glutes and hamstrings have to maintain the body in that bent-over position. The lower back, glutes and hamstrings - therefore - are working isometrically (rather than isotonically - that is “static” versus “dynamic” - without movement versus with movement), and that is much less beneficial. But the bigger problem arises from the fact that in order to use a weight that is sufficiently challenging for the lats, it is likely too much for the lower back.
This is especially true if one rounds the back while bent over, as the intervertebral discs distort, and could herniate.
A supported
rowing movement would be much better for working the upper back - either a rowing machine or a One Arm Dumbbell Row. And while some trainers pitch this exercise as a good “functional” exercise, I would prefer to work the muscles separately, because I can work my lats as heavy as I want without risking injury to the lower back, and I can work the other muscles isotonically, thereby providing them with more benefit.
Yes - this exercise “saves time”, in the sense that a person can do one exercise that works four body parts, as compared to doing four different exercises. But it’s not that simple. If each of the four body parts were to receive as much benefit as they would during four separate exercises, without any risk, it would be a “no brainer”. But that is not the case. Personally, I’m not willing to risk injury to save time. I’m also not willing to receive a compromised benefit, in order to save time. But that’s just me.
The Wrap Up
As you can see, each compound exercise has its own individual risks and benefits. Some are worth doing, and some are not. Some are actually risky.
But 
here’s the ultimate question: is there some kind of “synergistic” benefit that a particular body part gains by participating in a compound exercise, above and beyond the benefit that that same body part would get from an isolated exercise? For example, does the quadriceps benefit MORE from participating in a Squat, than it would from performing a Leg Extension? Of course, the Squat provides benefits for several other muscles, in addition to the quadriceps - and that’s good. But the REAL question is does the quadriceps itself benefit more?
Certainly, there are many who believe it does. And - if so - the question is “why?”. How COULD the quadriceps benefit more from a maximum effort Squat, than it would from a maximum effort Leg Extension? Logically speaking, it couldn’t. Maximum effort is maximum effort.
No studies have ever been done to prove or disprove this - either way. Empirically, people always do both, so no one ever knows for sure how much each exercise contributed to their quadriceps development. And - generally speaking - few people are willing to conduct that experiment for themselves. Plus, it would be difficult to measure only the quadriceps development, without measuring the circumference of the entire upper leg - which includes the hamstrings and adductors. People who “swear” that Squats build the quads more than Leg Extensions cannot possibly be speaking with any real authority. It’s conjecture. We’ve all been told “Squats are the best for building the quads”, and we buy into it. We then invest more energy into squats, and it becomes a self-fulfilling prophecy. But it may not be an accurate, or honest, assessment.
The broader question is do ANY compound exercises build a particular muscle more than an isolated exercise would? Or do they only “save time”, because they work a group of muscles during a given set? Again - logically - it does not seem possible to work a muscle beyond “maximum effort”. How could the quads work “harder” while other muscles are assisting, than they if they were working alone? How would the quads even know when they are working as part of a group, versus working alone?
I do squats, but I don’t do them with the belief my quads are gaining MORE benefit than they do from Leg Extensions. To me, Squats is simply a good quad and glute exercise, which happens to also recruit the collaboration of some other lower body muscles. I don’t consider it a Holy Grail.
My rule of thumb - again - is that a compound exercise is worth doing, as long as the muscles that are working are benefiting as much as they would be benefiting on their own (during an isolate exercise), and there is no unnecessary strain to any particular joint, tendon or muscle. Certainly, some compound exercises are “fun” or “gratifying”, but that alone is not reason to do it. I want to gain maximum benefit with minimal risk of injury or wasted effort, and I constantly seek the best way to do that. I assume that others who want maximum muscle gain, minimum wasted time and wasted energy, and minimum risk of injury, would have a similar thought process.
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