Unraveling The Kettlebell Swing

The kettlebell swing is the fundamental kettlebell movement. It’s the first thing a practitioner learns with a kettlebell in their hands, and for good reason. After all, it’s the movement so many others are built around.

However, it’s not really understood in a lot of ways.

That’s understandable in a lot of ways. It’s a movement that doesn’t quite work to the way many of us learned to train. There’s no simple movement pattern clearly designed to train a couple of muscles at a time. There’s no grinding out a rep to get a max or anything else.

It’s an odd approach to a lot of our minds, but it’s time to start trying to sort this mess out. Bear with me, though, because this will take a while.

Let’s start with where the misconception, with regard to body parts trained, in a swing.

Earlier this week, I did a Google search to see what people were saying. I like to flip over to the image search because you sometimes find some fascinating images illustrating the muscles worked in a given exercise.

That’s when I saw this particular bit that I’ve screenshot and cropped for your viewing pleasure.

One exercise with such widely divergent views on what’s trained?

On the left is an illustration that is conservative in which muscles being worked. On the right is one that’s basically saying a heavy enough swing will do a whole lot of your training for you right there.

To be fair, the one on the left points to primary muscles worked, and they do mention on their website that numerous other muscles are worked in the swing, but claim the glutes and hamstrings are the primary muscles trained.

Yet the image on the right gives you a completely different list of muscles being worked.

As there are several different kinds of swings, the two most dominant being the traditional Russian or Hardstyle Swing that the Crossfit-focused American Swing, I thought maybe that was the issue.

Well, sure enough, the one on the left is a Hardstyle Swing while the one on the right is an American Swing.

However, it’s also worth noting that the internet can be a very unreliable place. A lot of bad information is passed along via the internet by well-meaning people. (See also: Bro Science)

While trying to find some answers, I came across the American Council on Exercise (ACE) and their page on the kettlebell swing. Here are the muscles they say are being worked:

The prime movers of the kettlebell swing are the posterior-chain muscles responsible for creating closed-chain hip extension; specifically, the hamstrings (semitendonosus, semimembranosus and biceps femoris), gluteus maximus and adductor magnus. Additionally, the flexor muscles of the forearm (brachioradialis and flexor digitorum) are responsible for establishing grip strength, while the latissimus dorsi and triceps are responsible for shoulder extension. 

The synergists of the kettlebell swing are the muscles responsible for closed-chain knee extension; specifically, the quadriceps (rectus femoris, vastus lateralis, vastus intermedius and vastus medialis), gastrocnemius and hamstrings (in a closed-chain position, both the calf and hamstring muscles can assist with knee extension). Additionally, the deep abdominal stabilizers (thoracolumbar fascia, the transversus abdominis, the lumbar multifidii and posterior fibers of the internal oblique) help maintain intersegmental stability of the spine, while the anterior deltoids and biceps brachii are responsible for shoulder flexion.

Bear in mind that synergist muscles are the muscles that help, but aren’t the primary muscles used in the exercise.

This, of course, varies from what both images indicate so far as primary and secondary muscles involved.

Now you can start to see why there’s some confusion.

Of course, as mentioned previously, it’s past time we actually looked at the science of kettlebells and see what studies have to say about the topic. While ACE does include a list of studies they used to compile their post on the swing–an excellent sign of evidence-based thinking, but not a slam dunk–I also had to wonder whether primary and secondary muscles even matter.

When I train, I want to work the muscles, but does it matter?

The difference between primary and secondary movers when it comes to training is generally which is the muscle that is primarily moving the load, but secondary movers are still needed to accomplish the work. They’re training too, after all.

So with that, I turned to the science and looked at what was being activated.

So far, I’ve found one particular paper that I think sheds some light. Written by Dr. Stuart McGill and Leigh W. Marshall, it used electromyography to test to see whether certain muscles were activated during a one-handed kettlebell swing as well as the same swing with something called a “kime” (more on that later) and a swing to a snatch position.

To do that, they attached EMG electrodes to the following body parts:

• right and left rectus abdominis (RRA and LRA)
• right and left external obliques (REO and LEO)
• right and left internal oblique (RIO and LIO)
• right and left latissimus dorsi (RLD and LLD)
• right and left lumbar erector spinae (RLES and LLES)
• right gluteus medius (RGMED)
• right gluteus maximus (RGMAX)
• right rectus femoris (RRF)
• right biceps femoris (RBF) 

Keep those acronyms in mind, because here is what they found:

In addition to the modeling described above, the normalized EMG amplitude at the start, middle, and end points of the kettlebell swing and kettlebell swing with kime trials, and at the start and finish of the snatch, was reported for the following muscles: RRA, LRA, REO, LEO, RIO, LIO, RGMED, RGMAX, RBF, RRF, RUES, LUES, RLES, LLES, RLD, and LLD. Peak amplitudes were also tabulated together with when they occurred within the swing cycle as a percentage of the swing. Average activation for the same muscles was reported for the carrying tasks as the participant’s left foot was in contact with the force plate.

Translation from someone who isn’t a scientist but has read a lot of papers: They looked at precisely which muscles were actually used and how significant the effect was.

The answer? Well, McGill and Marshall tell us.

Swing exercise had a significant effect on only 3 muscles: REO (F = 4.27, p < 0.05), RRF (F = 4.16, p < 0.05), and LIO (F = 5.45, p < 0.05); however, Bonferroni t-test post hoc analyses revealed that there were no significant differences in the REO activation between the 3 kettlebell swing exercises. Post hoc t-tests with Bonferroni corrections showed that peak RRF and LIO activation was significantly greater during the swing with Kime compared with the swing without Kime (p < 0.017).

Now, understand the “kime” is basically a pulsed tensing of the abdominal muscles as the kettlebell reaches the apex of its arc.

That said, it looks like the obliques (both inner and outer) and the rectus femoris of the quadriceps are what’s actually doing the lion’s share of the work. It also appears that all of those muscles are activated, but not to a significant degree.

Here’s what McGill and Marshall have to say on the practical applications of their research:

The message for coaches is that the kettlebell offers several unique training opportunities, for example (a) the opportunity to train rapid muscle contraction-relaxation cycles emphasizing posterior chain power development about the hip. However, the large shear to compression load ratio on the lumbar spine created during swing exercises suggests that this training approach may be contraindicated for some individuals with spine shear load intolerance and (b) enhanced activation of the core musculature during the bottoms-up carry.

The short version is that if you have a bad back, you may want to skip the kettlebell.

However, we also have to temper science with experience. For example, I occasionally get some lower back pain from the kettlebell as well. It’s not a bad thing, though. It’s a reminder to keep my glutes tight. Once I do that, the pain goes away.

Is that universal, though? Probably not. McGill spends a great deal of his work focused on back pain, so I’m inclined to believe that if it was something as simple as that, he’d have seen it.

Still, it’s worth a try.

Now, some may have questions about the odd way only muscles only fired on sides, but that’s not surprising. This was, after all, a one-handed swing back in 2012. In 2016, Anderson et al compared the one-handed swing and the two-handed swing and found a similar dispersion of muscle activation in the abdominal region.

For the upper erector spinae, the activation of the contralateral side during 1-armed swing was 24% greater than that of the ipsilateral side during 1-armed swing (p < 0.001) and 11% greater during 2-armed swing (p = 0.026). Furthermore, the activation in 2-armed swing was 12–16% greater than for the ipsilateral side in 1-armed swing (p< 0.001). For rectus abdominis, however, 42% lower activation of the contralateral side was observed during 1-armed swing compared with ipsilateral sides during 2-armed swing (p = 0.038) and 48% compared with the ipsilateral side during 1-armed swing (p = 0.044).

In other words, when you swing one-handed, you get a create more muscular activation in some muscles on the side you’re swinging on and less in others while also causing more muscular activation on the other side with some muscles and less on others.

While the two-handed versus one-handed swings discussion is beyond what I’m ready to get into today, I only bring this up because it’s relevant to understand that muscles do weird things when you swing one-handed. (That discussion is coming, though.)

Now, back to the discussion of muscles activated in the swing.

For one, we can say definitively that the inner and out obliques are impacted, as are at least part of the quadriceps. Those showed significant activation.

However, another study found significant activation of the biceps femoris and medial hamstrings. While it didn’t find activation in the rest of the hamstring muscles, it also wasn’t looking for it either. I think if they’d have looked, they’d have seen it as well.

A fourth study looked at comparing the swing to the kettlebell snatch and kettlebell clean, wanting to see if muscles were activated in similar ways. That study, however, looked at activation of the vastus lateralis muscle, another part of the quadriceps. It found that the swing did a good job of activating it.

So that means we can also safely say that it works at least parts of the quadriceps, the inner and outer obliques, and the hamstrings. Further, I suspect that further study would find more quadriceps activation taking place.

However, here I also need to consider experience for a moment. For example, my first day back to working kettlebells led to the better part of a week with sore lats. That tells me something I was doing was training those muscles, and while I was doing a one-arm bent row with the kettlebell, the weight was light enough it shouldn’t have been an issue.

I’m inclined to think that the activation of the latissimus dorsi is more significant than reported by McGill and Marshall. However, that’s anecdote, not science.

After all, I may also have been more detrained than I believed.

Instead, for my purposes, I’m going to cite this one as using at least the majority of the muscles in the thigh, the rectus abdominis (the “abs”) and the obliques are recruited to do the majority of the work. While there may be work for other muscles such as the anterior deltoids, I’m not seeing the studies to back that up.

If you’re looking to incorporate the kettlebell swing into your routine, don’t consider this a replacement for any of your various shoulder exercises nor for any other muscles outside of these mentioned.

By now, you can start to see where some of this confusion about this one exercise comes from. It took four studies to figure out it works the thighs and core. How many more studies would it take to find anything else?

Let’s also consider that the kettlebell swing is probably the definitive kettlebell exercise. It’s the basis for kettlebell training as a whole, after all. Many other exercises simply can’t be performed until the practitioner understands the mechanics of performing a swing properly.

So, it’s kind of important.

Despite that, we’re still out in the wilderness in a lot of ways when it comes to scientific studies looking at kettlebell training. While there were at least four studies that looked at the swing and the muscles activated, we still don’t have a complete picture of the most important and popular movement.

And that’s not really even delving into the different types of swings.

With that in mind, you can start to see the problems someone wanting to develop evidence-based kettlebell training principles would run into, especially with regard to many other exercises. That’s not to say there’s no information out there, necessarily. I’ve found some interesting pieces so far that I’ll bring to you in due time, mostly after I’ve had time to digest them completely.

Frankly, I have multiple posts in the works already, all on this one training implement. It makes me wonder what all else I’ll see in the future on this topic.

In addition, expect more on this particular topic–the kettlebell swing–as I also find it.