Kettlebells are frequently used by both athletes and recreational lifters. So there are many anecdotes about their use.
One anecdotal feature of using kettlebells is that athletes often report increased hip extension strength or improved function, despite performing a high volume of other hip extension exercises. Another anecdotal feature is that some individuals experience lower back pain when performing swings despite not experiencing pain in other hip extension exercises.
The following study set out to find out the reasons behind these two common reports.
The study: Kettlebell swing, snatch and bottoms-up carry: Back and hip muscle activation, motion, and low back loads, by McGill and Marshall, Journal of Strength and Conditioning Research, 2012
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What is the background?
Kettlebells are now being increasingly used by many weight lifters and fitness enthusiasts both as their sole training tool and in conjunction with barbells and calisthenics. However, the anecdotal evidence from weight lifters appears to be mixed.
Some weight lifters credit certain kettlebell movements, such as the swing, as being instrumental in assisting rehabilitation from lower back injury or in increasing hip extension strength despite using other movements for the same purpose. Others indicate that the swing in particular is one of the few movements that aggravate their lower back, while they can perform barbell lifts without pain.
In addition, some kettlebell practitioners perform the swing with a technique attributed to the revolutionary martial artist, Bruce Lee. This technique, called “kime” is a brief muscular pulsing to train muscle contraction and relaxation. It is done at the top point in the swing movement.
Research has identified that mixed martial artists do in fact use rapid muscle contraction and relaxation during striking to enhance the effect of their blows. However, no previous studies have been done that seek to quantify the mechanics and back loading during typical kettlebell exercises.
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What did the researchers do?
The researchers set out to quantify spinal loading and muscle activity of various trunk, leg and back muscles during the following kettlebell exercises: kettlebell swings, kettlebell swings with kime, kettlebell snatches, kettlebell bottoms-up carry, and kettlebell racked carry.
The researchers wanted to find out whether there is a unique feature of the kettlebell swing that makes it therapeutic or beneficial for some but discomforting for others. They also wanted to find out what effects kime has. The researchers used seven subjects for the swings and snatches and five subjects for the carries.
The researchers also obtained permission to record the characteristics of a swing performed by Pavel Tsatsouline. A 16kg kettlebell was used for the study. 3D body segment kinematics was assessed using reflective markers placed on anatomical markers and a nine-camera motion capture system. Ground reaction forces were measured using two force plates, one under each foot.
EMG data were recorded using surface electrodes. The researchers normalized the EMG data to a maximum voluntary isometric contraction (MVIC). The MVIC position used for the gluteus maximus was the higher of either the Bierring-Sorensen position or hip extension with the knee in 90 degrees of flexion, while lying prone on a table. The normalization position for the gluteus medius was resisted side lying hip abduction at 45 degrees of abduction combined with slight external rotation.
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What happened?
EMG activity: swings
The researchers looked at where the activity of the various muscles were highest in the swing. They found that the muscles that were most active during the swing were the gluteus maximus and medius and the latissimus dorsi and erector spinae, as the chart below shows:
The researchers also noted that the gluteal activation peak occurred late in the swing cycle and was closely associated with the final point of hip extension. This is useful for two reasons. Firstly, the peak in gluteal activity during the swing therefore occurs at the degree of hip flexion where gluteal activity is most capable of being maximal (i.e. full hip extension). This indicates that the kettlebell swing may activate the gluteals to a greater degree than others exercises that activate the gluteals at their peak in a position of hip flexion.
Secondly, this peak in gluteal activity in the kettlebell swing is in contrast to the peak in gluteal activity during axial exercises such as the squat and deadlift, where gluteal activity is believed to be greatest in high degrees of hip flexion because hip extension torque is greatest at that point. This indicates that the kettlebell swing may be a useful complementary exercise to the squat and deadlift, as it activates the gluteals at different degrees of hip flexion.
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EMG activity: swings with kime
The researchers reported that the addition of kime to the swing mostly affected the abdominal muscles, with the largest increases in activation occurring in the external oblique muscles (101% increase in the right external obliques and 140% increase in the left). The following chart shows the differences between the various muscles:
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EMG activity: snatch
The researchers found that the snatch increased the activation of three muscles significantly: the right external obliques, the right rectus femoris and the left internal obliques. This was likely because of the need to propel the kettlebell higher. The following chart shows the differences between the various muscles:
However, reviewing the pictures of the subjects performing the swings and snatches shows that the form used was not a classic hip hinge but more of a squat style. This may have influenced the results here and led to the greater activity in the rectus femoris. Hip hinge style snatches might have instead produced greater biceps femoris activity. However, it is not possible to know this without further research.
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EMG activity: Kettlebell carries
The researchers noted that the muscle activity was very low in all walking exercises. In fact, average gluteus maximus activity never reached above 1% of MVC during any of the walking bouts, and average gluteus medius activity never reached above 3% of MVC, indicating that much greater loads than 16kg are necessary to achieve a training effect in these muscles. However, they did also note that the EMG activity of all muscles except the left external obliques was higher during the bottoms-up carry when compared with the other walking tests.
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Spinal motion: swings
It should be noted that the lumbar spine did flex and extend 32 degrees, 26 degrees of flexion at the bottom of the movement and 6 degrees of extension at the top of the movement, during the execution of the kettlebell swing. Whether or not more advanced kettlebell trainees achieve similar or different levels of spinal motion during the swing requires future investigation.
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Spinal loads: swings, swings with kime and snatches
Most importantly, the researchers observed that the ratio of shear loads to compressive loads was much greater in the swing, swing with kime and snatch than in typical barbell exercises, which may explain why individuals sometimes find kettlebell exercises uncomfortable.
The researchers also reported that both shear and compressive loads were the highest at the beginning of the swing. They found that both shear and compressive loads were similar in the swings and the swings with kime with the exception of the top of the swing, where the shear and compressive loads during the swings with kime remained high while they reduced significantly during the normal swings. The following charts show the shear and compression loads for each type of swing and snatch:
The above chart shows how the shear loads decrease from the start to the middle of the movement in all three exercises (N.B. there was no middle part of the movement measured for the snatch). However, the chart also shows how the shear loads remain high in the swing with kime in comparison with the standard swing. The researchers note that less shear loading is considered to be more optimal. Therefore, swings without kime may be a better option for those who have lower back pain or a history of previous injury.
The chart also shows that the compression loads in the swing and the snatch decrease as the movement progresses. However, the compression loads remain high in the swing with kime. It is likely that this increased compressive load has been generated by the increased abdominal muscle activation in order to increase spinal stiffness and reduce the adverse effects of the higher shear forces, although it is not possible to establish this for certain from this study.
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Spinal loads: Kettlebell carries
The researchers reported that the joint compression and shear loads were also significantly greater in the bottoms-up position compared with that in the racked position and normal walking.
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Case study
The researchers recorded the EMG activity of Pavel Tsatsouline while he performed a swing with a 32kg kettlebell with his right hand and with two hands. During this experiment, Pavel displayed activation of 150% of MVC in his left erector spine and over 100% in his left gluteal muscles.
This case study is in contrast to the results of the EMG activity data recorded during the kettlebell swing for less advanced subjects, where the gluteal muscles were found to be the most active of all the muscles. The gluteals are clearly an important muscle in the kettlebell swing. However, Pavel was also able to stiffen his erectors to a greater degree in order to provide core stability during the swing action.
It would be interesting to record the hip extensor and core EMG activity of a group of untrained subjects, using a variety of loads, before and after a kettlebell training intervention focused on learning to perform a hip-hinge style swing to see where they achieved greater activity through improved form and practice at using the hips during the movement.
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What were the limitations?
The study was limited in that only light kettlebells were used in all of the movements and the subjects were mostly inexperienced. Moreover, the form shown on the picture included in the study did not look like ideal hip hinge swing form. This may have led to greater hip flexor, quadriceps and erector spinae activity and lower abdominal, gluteal and hamstring activity.
Finally, it is noted that no data for spinal loading was included for the case study, which might have been more useful for those coaches working with athletes who are more capable of correct hip hinge swing form and gluteal activation.
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What did the researchers conclude?
The researchers conclude that the kettlebell swing seems to create a hip-hinge movement together with patterns of rapid muscle activation and relaxation of large magnitude.
However, the researchers also note that the kettlebell swing appears to result in unique compression and shear load ratios in the lumbar spine in that the shear loading is very high compared to the compressive loads. Shear stability and tolerance to posterior shear loading are therefore required in order to gain the benefits of this movement pain-free.
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What are the practical implications?
For athletes and recreational lifters
The ratio of shear to compressive spinal loads is much greater in the kettlebell swing than in barbell exercises, which may mean that individuals with intolerance to shear loading may find them uncomfortable, despite being comfortable during axially-loaded barbell exercises.
The fact that some lifters find movements like the reverse hyper and kettlebell swing helpful is likely due to the slight extension reached in the lumbar spine. This slight extension is acceptable so long as form is kept in check.
Shear loads remain high in the swing with kime in comparison with the standard swing. Swings without kime may be a better option for those who have lower back pain or a history of previous injury.
Gluteal activity during the swing peaks near full hip extension, which makes the kettlebell swing a useful exercise to complement squats and deadlifts, where hip extension torque (and probably therefore also gluteal activity) is maximal in hip flexion.
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