Recently, I was asked by a friend working in the sports rehabilitation industry to write my next post on the difference between trunk activation during kettlebell swings and traditional trunk exercises.
First I would like to establish what I mean by kettlebell swing; it’s not a squat and front raise. The swing itself is a hip hinge movement utilising the posterior chain (glutes, hamstrings etc). The arm is essentially used as a rope or chain to hold on to the kettlebell. It is a very similar movement to a RDL or good-morning, in the sense that it is a hip dominant hinging movement. The nature of the swing is for the kettlebell to take a curvilinear path, from between the legs up to roughly shoulder height. However, this does increase the shearing and compressive forces on the lumbar spine (McGill & Marshall, 2012) and to reduce the spinal shearing forces some people swing and don’t allow the arm to raise fully in-front of the body, (by reducing the moment arm and flexing at the elbow). There are many different techniques for swinging a kettlebell and each one has its place. You need to be aware of what you’re loading and how. This will help you to decide which swing technique is for you.
For the purpose of this post I am not going to talk about swing technique. There are videos available on youtube, but I may upload a video for this along with some others in the near future.
I am going to explain briefly what I mean by conventional trunk training. I won’t be going into great detail on each exercise but will outline the basic movements and major muscles recruited. Baechle and Earle (2008) show some conventional abdominal exercises such as the bent-knee sit-up and the abdominal-crunch, where the aim of each exercise is to utilise the rectus abdominis to bring the anterior thoracic region closer to the pelvis, (requiring flexion of the spine). There are also some conventional exercises like the plank, where the aim is to contract the “core” and stabilise the body while in a neutral spine position and not rotate, (requiring isometric contraction of the musculature around the trunk). Some new approaches (functional training) have been shown to have more benefits than standardized trunk training and are also less tedious. Michael Boyle (2004) has a great section of torso targeting exercises that’s starts off with the very simplistic exercises and builds up to some more functional rotational/resistance stuff. These are not only fun but become quite demanding.
Without going off on a tangent, I personally believe that there is a time and place for all trunk training whether it’s a conventional method or a more functional style; if you can strengthen your athlete/client in a particular method, then why not? If your athlete/client can perform a skill as required without causing injury then why not allow it? There is no right or wrong answer when it comes to the question of “core” training, just the fact that your athlete/client may be weaker in certain postures so will need to strengthen/correct the problem. And with today’s society and lifestyle there is more emphasis on isometric work to try and strength the trunk in a neutral position, because everyone is always slumped over while sitting etc. There is also a huge emphasis on posterior chain exercises to “Pull” the body back up into a neutral and correct posture. This is why the kettlebell swing seems like the ideal exercise to use. It is the believed “isometric” trunk contractions in a neutral posture with heavy emphasis on posterior chain eccentric & concentric contractions that make the kettlebell swing a desirable exercise.
Notice that I put believed in speech marks! This is because people normally associate kettlebell swings with bracing your core and not allowing your spine to move. McGill and Marshall (2012) dispelled this belief in their study, stating that “of all the participants, lumbar spine motion (specifically L1 to the sacrum) ranged from 26o in flexion at the beginning of the swing to 6o of extension at the top of the swing. There was 2o of lateral bend and only 4o of spine twist at the beginning of the swing.” This proves that there is movement in the lumbar region during a swing, which is possibly why some people get irritated lower back musculature. This has been put down to incorrect movement patterns. Even with the movement established in the lumbar region the musculature of the trunk contracted to ~50%, Maximal voluntary contraction (MVC) in the back muscles, ~20% in the abdominals and ~30% in the oblique’s (McGill & Marshall, 2012). This is a significant enough contraction in all areas to stabilise the spine and create some potential strengthening around the trunk. The thing that is great about the kettlebell swing is that it does not isolate, or try to, one particular section of the trunk. There is a pattern of rapid contraction/relaxation in all of the major musculature of the trunk. This is why so many people deem the kettlebell swing a “core” exercise, but McGill, (2010) also hit on the fact that the trunk is a power transmitter and contrary to popular belief the trunk, more often than not, braces to stop motion. A review article on squats (Clarke, Lambert & Hunter, 2012) shows a range of research taken on squatting and how, when loads are increased to >50%, there is a significant recruitment in trunk activation. This may be down to trying to stabilize the body to stop anterior/posterior movement as the weight is driven upwards. This also proves that the trunk acts in a fashion that limits/stop movement.
Now we are aware of the fact that the kettlebell swing does activate the muscles of the trunk, especially the back, we can have a look at some conventional exercises to compare results. Leporace et al. (2010) conducted research on 8 core exercises typically used by exercise professionals, these where; front bridge, right side bridge, left side bridge, bird dog (left & right arm), single leg back bridge (left & right) and double leg back bridge. Their findings showed that a larger recruitment of external oblique’s and rectus abdominal, as a synergist, occurred during unilateral exercise. With the greatest activation of external oblique’s during side bridging, up to ~50%. However overall the highest recruitment off all musculature during their study was for front bridging, where 50-65% MVC of external oblique’s and 30-40% MVC of rectus abdominals occurred, this is down to the nature of the exercise and again using the trunk to resist movement. They also showed that during unilateral back bridging the lumbar erector spinae and multifidus had the highest recruitment, between 30-40% and 20-30% respectively. Other traditional methods such crunches and some slightly more functional methods of trunk training where studied by Escamilla et al. (2006), even though this is a slightly older study there is some pretty good findings. The more functional exercises used, such as ab roll outs and hanging leg raises, promoted a larger increase of muscle activation then traditional crunches etc, especially the external oblique’s and rectus femoris. The main finding within this research was how a bent knee sit up recruited hip flexors more than a crunch with feet raised off the floor.
From my recent reading it is pretty clear to see that there is a limited amount of research done on trunk activation during kettlebell swings, but what there is shows ~50% MVC of erector spinae, ~20% in the abdominals and ~30% in the oblique’s, with rapid contraction/relaxation occurring within these muscles, and others, which can lead to a major increase in impulse. This will in-turn lead to a greater motor recruitment and ability to produce power, so as a whole-body power training tool the swing is a great place to start for improvements in motor recruitment. It does have some effects on core stability, but more traditional exercises also show major activation of trunk musculature. What I have taken out of this research project given to me by my friend, is that trunk activation occurs in different magnitudes across different exercises and before prescribing any form of trunk exercise to any one you need an understanding of their injury history and a decent screen of their current lumbo-pelvic control as well. There is a place for prescribing some more conventional trunk training exercises if the athlete/client needs them, such as more traditional trunk bracing exercises like the plank/bridge for people with injury/weakness which occurred during flexion. Major parts of the trunk that have been neglected by many people are the quadratus lumborum, psoas and latissimus dorsi, as these are major muscles that connect the trunk to the limbs and usually problems in the lower back or pelvis are related to quadratus lumborum or psoas issues.
Would I say that the kettlebell swing activates the core more than traditional exercises? Not necessarily. It doesn’t try to isolate muscles like most traditional exercises, but doesn’t give as much activation as some of them. It does however increase the potential for whole-body power output, by rapid contraction/relaxation of muscles and increasing impulse. This could help to improve muscular endurance in the musculature of the trunk and be a reason why so many people seem to see good benefits and improvements in posture if kettlebell swings are used as part of a prescribed training program.
If there are any questions please comment, I would also like to know what you think/believe about kettlebells and whether you have had good or bad experiences with them.
Baechle, T. R., & Earle, R. W. (2008). Essentials of strength training and conditioning. Champaign, IL: Human Kinetics.
Boyle, M. (2004). Functional training for sports: superior conditioning for today’s athletes. Champaign, IL: Human Kinetics.
Brumitt, J., Gilpin, H. E., Brunette, M., & Meira, E. P. (2010). Incorporating kettelbells into a lower extremity sports rehabilitation program. North American journal of sports physical therapy, 5, 257-265.
Clarke, D. R., Lambert, M. I., & Hunter, A. M. (2012). Muscle activation in the loaded free barbell squat: a brief review. Journal of strength and conditioning research, 25, 1169-1178.
Escamilla, R. F., Babb, E., Dewitt, R., Jew, P., Kelleher, P., Burnham, T., Busch, J., D’Anna, K., Mowbray, M., & Imamura, R. T. (2006) Electromyographic analysis of traditional and non-traditional abdominal exercises: implications for rehabilitation and training. Physical therapy, 86, 656-671.
Leporace, G., Praxedes, J., Metsavaht, L., Pinto, S., Changas, D., Pereira, G., & Batista, L. A. (2010) Muscular synergism during core stability exercises. International symposium on biomechanics in sports: conference proceedings archive.
McGill, S. M. (2010). Core training: evidence translating to better performance and injury prevention. Strength and conditioning journal, 32, 33-46.
McGill, S. M., & Marshall, L. W. (2012). Kettlebell swing, snatch, and bottoms-up carry: back and hip muscle activation, motion, and low back loads. Journal of Strength and Conditioning Research, 26, 16–27.