Can kettlebells help develop power for sprinting?

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Can kettlebells help develop power for sprinting?

Kettlebells are often seen as a novelty or even a fad by many in the strength and conditioning community. However, because the path taken by the kettlebell swing exercise is so completely different from the bar path of a squat or deadlift, they present a different challenge to the body. Kettlebells could therefore be a very useful tool for strength coaches, particularly in respect of improving power outputs in a horizontal direction, which could mean faster sprinting. In this article, Chris Beardsley (@SandCResearch) reviews a study that breaks it all down.

The study: Mechanical demands of kettlebell swing exercise, by Lake Lauder, in Journal of Strength and Conditioning Research, 2012

(Too much detail? Skip to the practical implications)


What is the background?

Kettlebells are often promoted in the general fitness community as being useful tools for developing both aerobic fitness and muscular strength simultaneously. Indeed, some researchers have reported significant aerobic demands during bouts of exercise with kettlebells but others have found that this does not translate to improvements in aerobic fitness when measured over a period of a number of weeks. In terms of muscular strength, some reviewers have proposed that kettlebells are unable to produce any significant gains. However, research has since shown that it is possible to improve strength, power and muscular endurance using common routines with kettlebells lasting for several weeks.

However, despite these chronic investigations into the effects of kettlebells on muscular strength and aerobic fitness, there is actually surprisingly little acute research investigating the kinetics (such as the forces, moments, impulses and power outputs) or kinematics (joint angles and bar path) of swinging a kettlebell. This is probably partly because of their novelty and partly because the angular path traveled by kettlebells makes for some challenging calculations in certain respects. But it is exactly the angular path that makes kettlebells so interesting from a strength and conditioning point of view.


What did the researchers do?

The researchers wanted to understand the mechanical demands of exercise with kettlebells, starting with the most fundamental exercise, the two-hand kettlebell swing. They also wanted to compare the mechanical demands of the swing with the jump squat. Specifically, when thinking about mechanical demands, the researchers were most interested in the ground reaction forces and the power outputs, both of which can be measured by standing on a force plate when performing the swing.

For the study, the researchers recruited 16 men, who then performed two sets of 10 repetitions of two-handed swings with single 16, 24, and 32kg kettlebells while standing on a force plate. Additionally, the researchers recorded the sagittal plane of motion movements using a digital camera positioned to the side. In addition to the swings, the subjects performed back squats with 40, 60 and 80% of 1RM and jump squats with 0, 20, 40 and 60% 1RM, having first established their back squat 1RM. This enabled the researchers to compare the results of the kettlebell swings with two exercises that are very commonly used in strength and conditioning programs for athletes.


What happened?

Comparison between kettlebell weights

The researchers observed that the mechanical demand of the kettlebell swings, as determined by ground reaction forces, power outputs and impulse, were all maximized when using the 32kg kettlebell. Of no surprise, both mean and peak velocity were maximal when using the 16kg kettlebell.

Comparison between kettlebell swings and squats: impulse

The researchers observed that the results of comparing the mechanical demands of the jump squat and the kettlebell swing were mixed. The largest values of net impulse were produced when using the 32kg kettlebell. The following chart shows the order in which the different exercises were ranked in respect of impulse:

Impulse kettlebells

The 32kg kettlebell is clearly superior to the other exercises in respect of impulse. This is probably because of the unusual path that the kettlebell follows through the swing motion, which is unlike the straight bar path of the jump squat and squat. In the swing, because of the angular motion, there is never a point in which the kettlebell is not accelerating away from the lifter and therefore exerting a force against them.

However, during the jump squat and squat, there is a deceleration phase in which no force is exerted. The longer period of time spent accelerating provides the opportunity to generate a greater impulse, as impulse is the product of force and time spent producing that force. This longer period of time spent accelerating may translate to greater anatomical adaptations, as greater periods of time in the acceleration phase are typically associated with improved training effects.  However, exactly what this greater impulse means in this context is not clear.

Comparison between kettlebell swings and squats: force

The researchers found that the highest peak and mean ground reaction force occurred during the back squat, followed by the jump squat. The following chart shows the peak ground reaction forces.

Peak GRF - kettlebells

The chart shows that heavy back squats produced the greatest peak ground reactions forces. It also shows that peak ground reaction forces were higher with most jump squat loads than during the kettlebell swing. This is likely because of the heavier loads used in the back squats and jump squats, along with the pure vertical path of the squat movements. It would, however, be interesting to compare the resultant forces of very heavy kettlebell swings with the same loadings of back squats and jump squats. Additionally, although the data was not provided, the researchers noted that the element of horizontal force was much greater in the kettlebell swing than in the squat or jump squat. This may have important ramifications for transfer to training and therefore for the use of kettlebells in strength and conditioning programs for athletes.

Comparison between kettlebell swings and squats: power

The researchers found that power output during the swings with the 32kg kettlebell was greater than the power output during back squats but similar to the power output during jump squats. The following chart shows the peak power outputs of the various exercises tested:

Peak Power - kettlebells

As is typically found, the load that maximizes power output in the jump squat is no load. It would be very interesting to see whether heavier kettlebell loads led to greater power outputs, as this would make them a very useful training tool in respect of training power.


What did the researchers conclude?

The researchers noted that the back squat with 80% of 1RM produced the highest measurement of peak force. They observed that since peak force is closely related to muscular strength, strength is best developed using the heavy back squat rather than with the jump squat or kettlebell swing.

The researchers suggested that since the back squat produced much greater ground reaction forces than the kettlebell swing with 32kg, kettlebell swings are likely not sufficient for developing muscular strength. However, they were not able to conclude this with any degree of certainty from this study, as it was an acute investigation and not a chronic one.

The researchers observed that the 32kg kettlebell produced similar mean and peak power to jump squats. They therefore propose that kettlebell swings could be suitable for a power-based program as an alternative to jump squats.


What were the limitations?

The researchers note that the ground reaction forces were stated in total and included both vertical and horizontal forces. Moreover, the kettlebell swings produced much greater horizontal forces than the squats and jump squats. Therefore, it may be the case that for sports-specific applications where the application of force-direction is important, there may be a significant difference between the use of jump squats and kettlebells for power.

Moreover, the researchers stopped at 32kg and did not use a heavier kettlebell. However, different results might have been obtained with heavier weights. Furthermore, experienced kettlebell trainees might be able to produce more power in the swing compared to the jump squat, which remains to be proven.


What were the key points?

Kettlebell swings with 32kg produced the greatest impulses. This is likely because of the different path taken by the swing than the straight bar path in the squat and jump squat. Exactly what this means in this context is not clear.

Ground reaction forces are greatest in the following order: heavy back squats > jump squats > kettlebell swings. Kettlebell swings may therefore not be optimal for developing strength.

Power outputs are similar in jump squats and kettlebell swings, particularly with heavier kettlebells such as the 32kg weight. Kettlebell swings may therefore be appropriate for substituting into a power-based program.

Kettlebell swings produced more horizontal forces than the jump squats. Therefore, where power is being trained with a view to transfer to horizontal movements, such as sprinting, kettlebell swings may be superior to jump squats.


What are the practical implications?

Vertical ground reaction forces with 32kg kettlebells are smaller than during jump squats and squats. Therefore, heavier kettlebells are needed if athletes wish to use them to develop strength.

Power outputs are similar in jump squats and 32kg kettlebell swings. Kettlebell swings with moderate loads can therefore be used by athletes as an alternative to jump squats to develop power.

Kettlebell swings with 32kg loads are probably better for developing power in a horizontal direction (i.e. for sprinting) than jump squats.

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