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The latest issue of the Strength and Conditioning Journal includes a debate on the use of kettlebells in strength and conditioning programs.1 A key focus of the debate is the effectiveness of kettlebell training for improving strength and power. Safety was also discussed, but to a limited extent. Having invested the time and money to complete the infamous Russian Kettlebell Challenge (RKC) certification (now the StrongFirst SFG certification), I am no stranger to kettlebell training and have developed a few opinions of my own. As such, I would like to share a few thoughts on the question posed by this debate, particularly in regard to a potentially significant safety concern.
Is the Kettlebell Really That Special?
Some people regard kettlebell training as a revolutionary breakthrough in fitness training. I agree to some extent, but is the breakthrough because of the kettlebell itself or how it is used? People who use kettlebells for shoulder raises and bicep curls make a great case for them being nothing more than expensive dumbbells. And the fact that most kettlebell exercises can be effectively performed with dumbbells or other common pieces of fitness equipment further supports this argument. However, there are indeed some unique aspects of the kettlebell that can add value to a training program.
When properly pressed and locked out overhead, the off-centered position of the kettlebell promotes optimal shoulder positioning, which can be a great asset when training shoulder strength and stability. Another unique benefit of the kettlebell is the forward displacement of its center of mass during the squat. Even compared to the barbell front squat, this makes it possible to maintain a more vertical torso while squatting to full depth. For anyone who appreciates quality in their movement, this is a great feeling, not to mention that it reduces the amount of shear load on the lumbar spine. Finally, swings and snatches, which are two of the more prominent kettlebell exercises, flow much more naturally with a kettlebell than a dumbbell.
A Failure to Plan …
As common as it is to hear the phrase that failing to plan is planning to fail, there are still a lot of people failing at meeting their fitness goals. In my opinion, much of this is due to a lack of focus. To be more specific, people are going to the gym with vague directives such as elevating their heart rate, working up a sweat, and burning calories, and they do all of these things without any specific sense of purpose. This is mindless exercise, and it is about as logical as going for a drive without having a destination in mind.
The point here is that you cannot know the value of kettlebell training unless you know what you want to achieve from it. As you will soon see, kettlebell training is effective for improving strength, power, and aerobic capacity. The degree of effectiveness for each of these areas, however, could have significantly different meaning for different people. A powerlifter, for example, would not care much about aerobic capacity, but a marathon runner certainly would. In contrast, most other athletes would have a more evenly balanced need for all three qualities. Furthermore, although everyone should care about each of these training goals, the average person who simply wants to lose weight has no need to develop them to the extent that an athlete would want to.
In my opinion, a few of the key benefits of kettlebell training are convenience and efficiency. For example, just ten minutes of swings done two or three times per week can improve strength, power, and endurance all at the same time. Furthermore, this type of training can be done just about anywhere, including at home, and with minimal need for equipment. However, would such a routine provide enough of a strength stimulus for a power athlete? Would it provide enough of a metabolic stimulus for a long distance runner? Might some aspects of this routine even be counterproductive? And what about safety?
Evidence of Effectiveness
As noted in the debate article,1 relatively little research has been done on kettlebell training. To my knowledge, fewer than 20 articles have been published in the peer-reviewed literature, and only a portion of them are trials related to performance or safety. As such, the reliability of any conclusions that can be drawn from the available evidence is limited.
One of the authors of the debate article states that kettlebell training is effective for improving strength and power.1 In one of the studies cited, two similarly designed training programs based exclusively on either kettlebell or barbell exercises were compared for their effects on strength and power.2 Vertical jump, 1 repetition maximum (1RM) barbell back squat , and 1RM power clean were used to measure progress. Although both groups significantly improved all three measures, which clearly shows that kettlebell training is indeed effective for improving strength and power, barbell training led to a significantly greater improvement on the back squat. Two other studies have also found kettlebell training to increase strength and power,3,4 but unfortunately, neither study included a comparison to a traditional strength training program. As suggested by the other author in the debate,1 kettlebell training is unlikely to promote strength gains to the same extent as traditional barbell training. While this might not matter much for the average person seeking weight loss, it could make a tremendous difference for an athlete.
Even Pavel Tsatsouline, the man who is primarily responsible for making kettlebells as popular as they are in the United States, has bluntly stated that kettlebell training is not as effective as barbell training for improving strength. In fact, he has written that “if your sport is what Russians call a ‘speed-strength’ sport, you need the bar. … You need the bar because kettlebells and your body are just not heavy enough.”5
In regard to conditioning, one study has shown that the average oxygen uptake during a maximal 12 minute set of swings is approximately 65% of max, which the authors regarded as an aerobic intensity that is higher than that of circuit weight training and is high enough to improve aerobic capacity.6 Kenneth Jay has written a book called Viking Warrior Conditioning, which is completely dediciated to using kettlebell training to increase aerobic capacity. Based on Jay’s protocol, an unpublished study has shown that a 20 minute kettlebell workout consisting of 15 seconds of snatches followed by 15 seconds of rest produced an average oxygen uptake of 78%,7 further indicating that kettlebell training is indeed useful for improving aerobic capacity. In addition, an unpublished master’s thesis has shown that this same workout, repeated three times per week for four weeks, significantly increased aerobic capacity by approximately 6%.8
When compared to continuous running on a treadmill for the same duration and at the same heart rate and perceived level of exertion, a 10 minute interval workout consisting of 35 seconds of kettlebell swings followed by 25 seconds of rest was found to have a significantly lower metabolic demand.9 The treadmill run resulted in significantly greater oxygen uptake and total calorie expenditure. Based on this, for a given level of perceived effort, running is not only more effective for challenging the aerobic system, but also for increasing energy expenditure. In fact, an average of 512 kcal of energy was expended during the treadmill run compared to 375 kcal for the kettlebell workout. Furthermore, compared to the 6% improvement in aerobic capacity observed in the study above, multiple studies have shown traditional sprint interval training to increase aerobic capacity by as much as 13%,10 which is more than twice the amount of improvement.
Risk of Low Back Injury?
As mentioned in the debate, shear and compressive forces generated within the lumbar spine are a common concern related to kettlebell training.1 One of the studies cited in the debate to support the safety of kettlebell training showed a program partly based on swings to actually reduce low back pain.11 However, the average weight used for swings in this study was only 12 kg. Unfortunately, no other studies have assessed the occurrence of back pain in relation to kettlebell training.
Dr. Stuart McGill, a spine researcher who is well known in the fitness industry, conducted the only published biomechanical analysis of compressive and shear loads on the lumbar spine during a kettlebell swing.12 Based on this study, which was done with a 16 kg kettlebell, one of the authors of the debate stated that the compressive loads generated during the swing are within established limits of safety.1 However, there was no mention of the shear loading observed in this study or the intriguing remarks McGill and his coauthor Leigh Marshall made in reference to it.
Although the compressive load generated by the kettlebell swing was determined to be within established safety limits, McGill and Marshall noted that the ratio of shear to compression was much greater than what typically occurs during other exercises that load the spine, such as the squat and deadlift.12 Furthermore, the shear force generated during the swing occurred in the opposite direction, which was regarded as a possible explanation for the anecdotal reports of kettlebell swings causing low back pain in individuals who can deadlift very heavy weights without any discomfort. McGill and Marshall also noted that since this type of shear is rare, there is no established guideline for safety.
Another concerning factor that McGill and Marshall alluded to but unfortunately did not elaborate on is that after the peak compressive load occurs during the initiation of the concentric portion of the swing, the spine is actually under tension, which reduces compression of the facet joints and, in turn, reduces tolerance for shear.12 Because of the ballistic nature of the swing, the weight of the kettlebell decreases and eventually becomes weightless for a moment at the end of the concentric phase. As a result, compressive load decreases as well, which is presumably why McGill and Marshall referred to the spine as being under tension and the compression at the facet joints being dissipated.
Based on the orientation of intervertebral discs and ligaments, the spine has a lower tolerance for shear than compression, and this lower tolerance is further reduced during repetitive activity.13 Among automotive assembly workers, who experience a considerable amount of repetitive spinal load, average peak shear between the fourth and fifth lumbar vertebrae in individuals with back pain was calculated to be 465 N.14 In comparison, the average peak shear between the same two vertebrae during the 16 kg kettlebell swing was 461 N.12
Based on the pictures provided in their paper, the style of kettlebell swing evaluated by McGill and Marshall involved a considerable amount of anterior knee displacement, effectively making the swing more similar to a squat and, in turn, allowing the torso to remain more vertical at the bottom of the movement. In contrast, the swing commonly taught by kettlebell instructors and recommended in Pavel Tsatsouline’s book Enter the Kettlebell emphasizes minimal knee displacement, resulting in a more horizontal torso. Basic biomechanics makes it pretty clear that the latter style will result in greater shear load. This is supported by the observation of sumo deadlifts, which allow for a more upright torso, causing less shear load at the lumbar spine than conventional deadlifts.15 This, in combination with the common use of high volume and heavier kettlebells, suggests that swings are likely to produce enough repetitive shear load on the lumber spine to exceed levels known to be associated with back pain. Granted, there are many variables involved here, but in my opinion, it is pretty clear that more evidence is needed before a high volume of kettlebell swings can be confidently described as safe.
Looking at the safety of the kettlebell swing from the perspective of the barbell deadlift, which is a fundamentally similar exercise, provides some additional insight. Based on data from a national powerlifting competition, the deadlift produced an average of 6.8 N of shear load per kilogram of weight lifted by men and about 7.6 N/kg for women.15 In contrast, the kettlebell swing produced a much greater 28.8 N of shear load per kilogram of weight lifted.12 In all cases, shear load was assessed between the fourth and fifth lumbar vertebrae. Based on these numbers, swings with a 24 kg kettlebell, which is commonly regarded as the standard kettlebell size for men, will produce the same amount of lumbar shear as a 225 lb deadlift. As great of an exercise as the barbell deadlift is, most people appreciate the strain it puts on the low back. Doing hundreds of repetitions of swings with a 24 kg kettlebell in a single workout is common practice, and such workouts are commonly repeated multiple times per week. However, most qualified fitness professionals would probably regard the same volume of 225 lb deadlifts as an irresponsible injury risk. Furthermore, although this argument is based on shear load, with the high volume and the heavier kettlebell, perhaps compression could be a concern as well. Granted, the above comparisons are based on some extremely crude math and some major assumptions, but I think it clearly illustrates the point that we may want to figure out what kind of impact kettlebell swings have on the spine before we casually declare them as safe.
A Potential Alternative
Although the kettlebell snatch has a steeper learning curve than the swing and can present additional concerns related to overhead movement, it may be a safer alternative for the low back. The data provided by McGill and Marshall suggests that the snatch produces less shear than the swing.12 The difference was not statistically significant at the initiation of the two lifts, but it was at the completion. Although, the completion of the snatch is much different than the completion of the swing.
The snatch clearly involves a greater range of movement than the swing, which suggests that it is also more metabolically demanding. This is anecdotally well supported by the infamous RKC snatch test. For relatively well trained men, completing 100 swings in five minutes with a 24 kg kettlebell is easy. In contrast, it often takes months of intense training for the same men to complete 100 snatches in five minutes with the same kettlebell. Although there are no studies directly comparing the metabolic demands of the snatch and the swing, the difference is supported by two individual studies. As described earlier, a 20 minute interval style snatch workout with a 16 kg kettlebell has been shown to result in an average oxygen uptake of 78% of max7 while oxygen uptake during a maximal 12 minute set of swings with the same size kettlebell only reached 65% of max.6 Unfortunately, the differences in the two protocols make it difficult to compare the results.
Based on the limited evidence, it is conceivable that the aerobic training effect provided by swings can be duplicated by lighter weight snatches, which implies the benefit of reduced load on the lumbar spine. The program outlined in Viking Warrior Conditioning is a good example of this. For a man who would typically require a 24 kg kettlebell to produce a notable training stimulus from swings, it is recommended that a 16 kg kettlebell be used for the snatch workouts. However, the exceptionally high volume required by this program may offset the benefit of reduced weight.
As with any form of training, the value of kettlebell training is dependent on the goals it is being used to achieve. For someone looking to maximize strength or aerobic capacity, the preliminary evidence currently available indicates that kettlebell training is not the best choice. In contrast, given the modest effectiveness of kettlebell training for improving both of these qualities, as well as power, it is an appropriate choice for people who are willing to sacrifice performance gains for increased training convenience and efficiency. However, the question relating to safety is one that was largely ignored in the debate1 and has yet to be adequately answered. Until more conclusive data is available, I have personally strayed away from using kettlebell swings for metabolic conditioning, but I certainly see a lot of value in including specific components of kettlebell training into a more traditional strength and conditioning program.
If anyone has additional insight relating to the effectiveness or safety of kettlebell training, please share!