Bench press vs. flys: which is better for the pecs? [Study review]

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Which is the better pec builder: the bench press or flys? A new study by Solstad et al. compared muscle activity of the pecs, front delts and triceps in the barbell bench press vs. dumbbell flys. Unlike many electromyography (EMG) studies, this study was properly conducted: both groups performed 6RMs and the 5th rep was taken to zoom in on different parts of each the movement. Most EMG studies are essentially useless, as they don’t equate the relative intensity of the different exercises, or don’t train remotely close enough to failure to see if the differences they find persist with maximal effort.

Can you guess the results?

The bench press beat flys in terms of average muscle activation for all target muscle groups, including the pecs: see the data below.

Flys did stimulate the biceps more. Interestingly, biceps activity was actually likely high enough to stimulate muscle growth with the flys. I don’t think this is universally true for flys. If you look at the subjects’ execution of the flys above, you can see they kept their arms bent, their wrists supinated and their shoulders externally rotated. This position stimulates a significant isometric contraction for the biceps.

Barbell bench press vs dumbbell flys EMG muscle activity

So case closed: flys are useless, presses are better for everything? No, that’s not how we should interpret EMG data. The difference in muscle activity came mostly from the top third of the movement, where the bench press still effectively trained the pecs but flys did not: see the data below. This makes perfect biomechanical sense of course: with dumbbells the external moment arm, which is the horizontal length of your literal arm in this case, is maximal in the bottom position, so you get a great stretch, but there is virtually no tension on the pecs in the top position, as there’s no external moment arm for horizontal shoulder flexion/adduction anymore then. Basically, bench presses train the pecs over a greater effective range of motion (ROM). Most research finds training with greater ROM stimulates greater muscle growth. That’s likely due to 2 reasons, but only 1 is relevant here.

First, using full ROM can stimulate more stretch-mediated hypertrophy. However, flys actually achieve greater a greater pec stretch than bench presses. Most people don’t achieve a full pec stretch with the barbell bench press, as the barbell can’t go down further than your chest. This is one of the reasons I generally favor dumbbells over a barbell. Flys should also beat the bench press in this regard.

Second, since different muscle fibers have a different length-tension relationship, at different muscle lengths different muscle fibers experience more or less tension. In other words, at each part of the ROM you stimulate slightly different muscle fibers. Neglecting a significant portion of the ROM means some muscle fibers may remain understimulated. In this regard the bench press decisively beats dumbbell flys, as this study shows.

Many people would probably argue flys also lose out on constant tension and thereby metabolic stress. I don’t think that’s relevant, as most research has debunked metabolic stress as an independent driver of muscle growth.

So should we do dumbbell flys? I basically never use them with my clients. Instead, you get the best of all worlds, as well as lower injury risk, with a properly set up cable fly: I call these Bayesian flys and you can read exactly how and why you should do them in this article.

Pecs EMG activity barbell bench press vs flys

Another interesting finding of this study was the pattern of triceps activity. Many powerlifters argue that the triceps is mostly active in the top position. That’s why block presses are commonly prescribed to build the triceps. I think this is misguided for 2 reasons.

First, as I’ve previously showed, it’s simply not true that the triceps is that much more active in the top of the movement: see the data below. Triceps activity is relatively constant during a bench press. There’s no biomechanical rationale as to why it would be much higher at the top.

(If you really wanted to overload the top, you should probably use bands or chains, as no matter how much you cut the ROM with a block press, without accommodating resistance you’ll always have to decelerate the weight at the very top.)

Triceps EMG activity barbell bench press

Second, bench presses of any kind are not great triceps builders in the first place. I previously posted a review of how most studies find bench presses don’t build the triceps nearly as well as the pecs. While many people thought this was circumstantial evidence, as it didn’t compare the growth rate vs. another exercise, recent research has validated it: bench presses don’t effectively stimulate the long head of the triceps, whereas triceps extensions do. The long head is a biarticulate muscle: it aids not just in elbow extension but also shoulder extension. During a bench press, maximally contracting the long head would cause a significant shoulder extension moment. Since the shoulder flexion moment is likely more limiting than the elbow extension moment during a bench press, this means full contraction of the long head of the triceps is often not desirable.

All in all, this study may seem like a big win for the bench press, and bench presses are a fine compound exercise, but they’re likely not perfect for either the pecs (no maximal stretch-mediated signaling), delts (only ~50% ROM) or the triceps (long head remains understimulated). You should add more targeted exercises to optimally stimulate each muscle.

Reference

Tom Erik Solstad, Vidar Andersen, Matthew Shaw, Erlend Mogstad Hoel, Andreas Vonheim, Atle Hole Saeterbakken. (2020) A Comparison of Muscle Activation between Barbell Bench Press and Dumbbell Flyes in Resistance-Trained Males. Journal of Sports Science and Medicine (19), 645 – 651.

Bench press vs. flys: which is better for the pecs? [Study review]

Reference

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Menno Henselmans

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