Overview of Cardiovascular Benefits and Mechanical Demands of the Kettlebell Swing Exercise: Implications for Work Economy

Daniel E. Vencel1,2,3 University of

Dayton

300 College Park, Dayton, OH 45469

1. Department of Health and Sport Science

2. Berry Summer Thesis Institute

3. University Honors Program

Thesis Mentors: Anne R. Crecelius, Ph.D. and Matthew Beerse,

Department of Health and Sport Science

Abstract:

Ph.D.

The kettlebell is a small piece of exercise equipment which originated from Russia and was popularized in the United States in the 20th century. Arguably the kettlebell swing is the most popular exercise within kettlebell training programs. Proponents of the kettlebell swing argue the exercise has multiple benefits including improving strength and cardiovascular health. This review will focus on articles specifically investigating the kettlebell swing to confirm the validity of these claims. These articles include topics such as motion analysis of the kettlebell swing, cardiovascular benefits of the swing, and lastly a combination of work and oxygen consumption known as work economy. Work economy is how much oxygen is consumed by the body to perform a given amount of external work. While work and VO₂ have each been investigated independently, they have yet to be combined in a kettlebell swing study.

History of the Kettlebell

The kettlebell is a spherical weight with a handle on top. It originated in Eastern Europe with sources dating as far back as the 16th century, particularly having an emphasis in the former Soviet Union (Tikhonov et al., 2009). In Russia, it was very popular among strongmen who displayed their feats of strength using a kettlebell. A strongman was often referred to as a girevik or “kettlebell man (Tsatsouline, 2006).” According to Tsatsouline, kettlebell training was and still is a relevant form of training for the Russian military and police. He even goes on to say that almost every Russian military unit has a “courage corner” where kettlebells are commonly found (Tsatsouline, 2006). With these considerations in mind, it is clear to see the popularity of kettlebell training in Russia.

Kettlebell training expanded and eventually became its own sport in 1948, called kettlebell sport also known as Girevoy sport (Ross et al., 2017). In kettlebell sport, participants do a variety of exercises such as the snatch while completing as many reps as possible in a 10 minute timeframe (Ross et al., 2017). The snatch in kettlebell sport is similar to that seen in olympic lifting, however, a barbell is used for olympic weightlifting instead of a kettlebell. Even though Eastern European countries frequently exercised with the kettlebell in the past, it would take much longer for it to become popular in the West, particularly the United States.

Pavel Tsatsouline is one of several people credited with popularizing the kettlebell in the United States in the late 20th century between his several publications of the kettlebell and introduction of his Russian Kettlebell Certification program (Meigh et al., 2019). Exercises most commonly found in the hardstyle program include the swing, snatch, squat, turkish get-up, and press (Meigh et al., 2019). According to Tsatsouline, the average man should begin exercising with a 16 kilogram kettlebell, and the average woman with an 8 kilogram kettlebell, with clients progressing in weight as needed (Tsatsouline, 2006). Even though all of these exercises can be beneficial and each have their own specific uses, this literature review will focus specifically on the kettlebell swing. Proponents of the kettlebell swing such as Tsatsouline praise the exercise for improving strength, cardiovascular health, injury prevention, and body composition (Tsatsouline, 2006). This review will focus on articles specifically investigating the kettlebell swing to confirm the validity of these claims. These articles include topics such as motion analysis of the kettlebell swing, cardiovascular benefits of the swing, and lastly a combination of work and volume of oxygen consumption (VO₂) known as work economy.

Motion Analysis and Mechanical Demand of the Kettlebell Swing

In the context of physics, work, measured in joules (J), is simply a change in energy because it is assumed that as energy changes, work has to be performed to cause those changes. This is also known as the work-energy theorem. Work is performed all the time in our daily lives. Whether someone is walking to class or exercising in the gym, work is completed at differing amounts and intensities. In this case, work for the kettlebell swing is energy required to successfully lift the kettlebell and perform the movement. Even though there are few studies that have calculated total work of the kettlebell swing, several studies (Back, 2016; Bullock et al., 2017; Oikarinen, 2016; Ross et al., 2017) have investigated kinematics which is the study and analysis of motion for variables such as joint angles and power of the kettlebell swing. Power is simply the rate at which a given amount of work is performed. Power is an important variable to investigate as power and strength are two performance variables which are trained extensively in many athletes including powerlifters and weightlifters.

A few studies have investigated the strength and power produced during kettlebell swings. Maximal strength is defined as producing a force to overcome the highest possible mass during a lift. Power is the ability to generate a force as quickly as possible. One study compared a 6 week kettlebell swing protocol to the same length jump squat protocol (Lake and Lauder, 2012a). The researchers found similar increases in strength as measured by a one-rep max half-squat, however, power increases as measured by jump squat height were greater in the jump squat training protocol compared to the kettlebell swing protocol (Lake and Lauder, 2012a). Jump squat height is a popular measure of power since it allows a subject to quickly contract muscles in an explosive movement since the subject’s body weight is not a relatively heavy load. Given the same body mass, a higher generated amount of power would increase the upward velocity of the jump leading to a higher jump squat height. Results from (Lake and Lauder, 2012a) likely favor the jump squat protocol due to training specificity since one training group was exercising using jump squats for several weeks which is the exact metric used to assess power. This makes sense why jump squat height would improve with that protocol and why it may give an unfair advantage over the kettlebell swing. In a future study, it would be interesting to see a power assessment that is not specifically used during the intervention. For example, a jump squat protocol could be compared to a kettlebell swing protocol, but a different movement such as a sprint could be utilized to measure increases in power.

There are two main reasons which drive strength increases when lifting weights: neural adaptations and increased muscle size (Reggiani and Schiaffino, 2020). Increased muscle size would occur in the targeted muscle group, but neural adaptations can be less specific as the central nervous system can improve motor unit recruitment and lessen antagonist muscle activation leading to whole body strength increases (Reggiani and Schiaffino, 2020). A study from (Manocchia et al., 2010) supports the idea that kettlebell swings improve neural adaptations as the researchers found significant increases in traditional lifts which have very different movement patterns from the swing. Strength significantly improved for the bench press, while almost reaching statistical significance for the back extension and clean and jerk during a kettlebell swing training protocol (Manocchia et al., 2010). However, those researchers found no statistically significant increase in vertical jump height which contradicts the results found by (Lake and Lauder, 2012a). Perhaps either the swing is insufficient for building power or other metrics for assessing power could be implemented. Furthermore, another study (Otto et al., 2012) found the kettlebell swing can improve back squat one rep max, however, not to the extent of a traditional weightlifting protocol. Since the load for the swing was lower than what was used in the weightlifting group, this could explain the differences in strength improvements.

However, the swing has been shown to have very different forces on the spine when compared to the deadlift. Deadlifts are very high in compressive forces which are forces that press on the spine in opposite directions, while kettlebell swings are high in shear forces which are opposing forces occurring when the vertebrae of the spinal column rub together (McGill and Marshall, 2012). The high shear forces can be explained by an arc-like trajectory with directions of forces constantly changing. This would have to be researched further but this may mean kettlebell swings could supplement a deadlift program so the spine is not overtrained by only one type of force.

Correct Form of the KB Swing

The kettlebell swing is characterized as a hip-hinge exercise in which the hamstrings are stretched at the bottom of the movement, and the pelvis, hamstrings, and gluteal muscles contract explosively to move the kettlebell in an arc-shaped motion. According to (Jay et al., 2011), the swing has an explosive concentric motion which is followed by a coasting phase in which the kettlebell reaches about shoulder height until momentum slows (occurring from picture B and C), and then lastly an eccentric muscular phase in which the kettlebell begins to return to its lowest point (occurring from picture A to B). One study (Back, 2016) further explains that experts perform the swing by first extending the hip joint explosively in the anterior direction, followed by the pelvis in a similar direction, and lastly at the shoulder which carries the momentum generated by the hips and pelvis, with the reverse order occurring in the eccentric direction. Since the hips are strongly involved throughout the movement, this takes possible strain away from the lower back with the lower back mostly acting as a stabilizer against shear forces as previously mentioned.

In Figure 1 presented by (Lake and Lauder, 2012b), the correct form of the swing is shown with only slight knee flexion, however, those who are unfamiliar with the exercise may perform the exercise with less hip flexion and compensate by having greater knee flexion (Back, 2016). Back also found that coordinating explosiveness of hamstring and gluteal muscles was found in experts, while novices tended to “lift” the kettlebell using their shoulders which contradicts the correct form previously described. In addition, another less common form of the kettlebell swing exists. It is known as the “American” style or overhead kettlebell swing and has some popularity in crossfit (Oikarinen, 2016). Overall, this style seems to be less popular and since most of the literature focuses on the shoulder height swing, this is the only form that will be discussed.

Methods Used to Calculate Work

As previously described, work is the transformation of one form of energy to another. The three types of energy most relevant to the kettlebell swing are gravitational potential energy, translational kinetic energy, and rotational kinetic energy. Firstly, gravitational potential energy changes as the body performs work against gravity to raise the kettlebell and center of mass of the body further from the ground. Secondly, translational kinetic energy changes as the kettlebell velocity increases or decreases. This can easily be seen during the exercise as the kettlebell follows a parabolic motion of a ballistic nature (Lake and Lauder, 2012b). For example, the kettlebell has a velocity of zero at the top and bottom positions of the swing, and as the kettlebell moves between those positions, either moving forward and upward or backward and downward, velocity constantly changes. Thirdly, rotational kinetic energy has a small contribution to total work performed as joint segments rotate about their axes during the swing. The amount of work performed during various exercises has been studied for over a century yielding methods such as point mass, segmental analysis and inverse dynamics.

The oldest and least accurate method of calculating work is the point-mass method. For the pointmass method, the body is collapsed into a single center of gravity, and the change in energy of that “point” is analyzed to determine the amount of work performed (Robertson et al, 2014). As shown in equations 1 and 2 above, point mass analysis calculates mechanical energy using the sum of change in potential energy (Eq. 1) and change in translational kinetic energy (Eq. 2) (Robertson et al, 2014). For Equation 1, m is the mass of the body, g is the acceleration due to gravity, and y is the height above the ground. To calculate change in translational kinetic energy for Equation 2, m is the mass of the body, vx is velocity in the x-plane, and vy is velocity in the y-plane.

As stated above, this is the simplest and most crude form of motion analysis and has been heavily criticized for not being accurate as it neglects change in energy of the body segments (Winter, 1978). Winter further explained how the point-mass method could potentially have an error of 70% since only change in energy of the trunk is captured. Thankfully, more accurate methods exist and are much more commonly used.

Segmental Analysis: Segmental Analysis is a second type of motion analysis used to calculate work which has been found to be more accurate than point mass analysis (Williams and Cavanagh, 1983; Winter, 1978). Segmental analysis adds on to point mass by calculating the amount of work performed for the trunk and every major body segment. In addition, segmental analysis includes rotational energy as seen in Equation 3, so more work is captured for a more accurate estimation. Rotational kinetic energy is solved by multiplying by the object’s resistance to angular acceleration or the moment of inertia (I), by the square of the angular velocity () multiplied by onehalf. Even though this method is more accurate than using point mass and can be sufficient, it is still not the most accurate method.

Inverse Dynamics: According to (Robertson et al., 2014), inverse dynamics is the gold-standard for analyzing motion capture data. The main addition to this type of analysis is that inverse dynamics allows for researchers to compute joint moments, or the resistance of a joint to rotate about its axis. Aside from being defined as a change in energy, work can also be described as force times displacement. So to find work at each joint, the joint moment is multiplied by the angular displacement. This is done for all of the joints to calculate total work. This is a more direct way to calculate work than segmental analysis as inverse dynamics attempts to directly estimate the forces and thus work done at the joint, while segmental analysis indirectly estimates work by looking at motion analysis which is a byproduct of the forces produced at the joints. Inverse dynamics involves using several dozen markers on the body, which are captured using many cameras, and ground reaction forces are collected using a force plate. At least one study (Bullock et al., 2017) has used inverse dynamics for the kettlebell swing in which it compared peak joint angles between the Russian kettlebell swing and the Indian Club swing which uses two clubs and a different movement pattern. This study found no significant differences between peak joint angles. Even though this approach used inverse dynamics, they only used it to analyze joint angles and not to calculate absolute total work performed during the kettlebell swing. This is an area that could possibly be studied in the future.

Kettlebell Swing Cardiovascular Overview

Kettlebell swings have also been studied on their ability to improve cardiovascular health and conditioning. A popular exercise prescription for kettlebell swings involves interval training of roughly a 1:1 work-rest ratio of alternating 30 seconds of swinging and rest for a total length of 10-12 minutes (Budnar et al., 2014; Lake and Lauder, 2012a; Raymond et al., 2021). According to Tsatsouline's recommendations, several studies involve female subjects swinging an 8-kg kettlebell and male subjects swinging a 16-kg kettlebell (Hulsey et al., 2012), with some studies lowering the weight to 4-kg and 8-kg respectively (Duncan et al., 2015). Several previous researchers have used this exercise protocol and compared it to the American College of Sports Medicine (ACSM) guidelines for cardiorespiratory exercise.

According to the ACSM, the optimal amount of cardiovascular exercise for most adults to improve health and longevity is at least 75 minutes of vigorous intensity cardiovascular exercise or at least 150 minutes of moderate intensity (American College of Sports Medicine, 2021). The authors also outline a few common metrics used to measure exercise intensity such as a percentage of maximum VO₂ or percentage of maximum heart rate (%HRmax). %HRmax is often recorded using a heart rate monitor, and a relatively higher heart rate indicates a greater stimulus for aerobic/ anaerobic performance (ACSM, 2021).

Another way to quantify intensity of an exercise for cardiovascular health and performance is %VO₂max, and similar to %HRmax, it shows the relative intensity of the exercise by looking at how much oxygen is being consumed compared to the individual’s maximum value. This is commonly measured using indirect calorimetry via a metabolic cart which is attached to a subject through a face mask. Air is drawn in as the subject breathes, and the expired air is captured by the gas analyzer which calculates changes of volume and flow rate of oxygen and carbon dioxide from the subject’s resting state to calculate VO₂. The extent to which oxygen is used to fuel metabolism during exercise is what defines aerobic metabolism, compared to anaerobic metabolism which occurs using low or no amounts of oxygen. These metrics as commonly used throughout the literature will be used as a reference to assess the capability of the kettlebell swing to reach a sufficient intensity which has previously been shown to improve aerobic health in other modalities.

Kettlebell Swing Cardiometabolic Studies

The majority of studies have concluded that kettlebell swings improve cardiovascular health (Farrar et al., 2010; Fortner et al., 2014; and Hulsey et al., 2012). Given the various metrics used to assess cardiovascular intensity and differing methods used for kettlebell swings, it is difficult to generalize the kettlebell swing as an exercise. For example, (Hulsey et al., 2012) argued kettlebell swings may improve aerobic capacity since the subjects’ heart rate exceeded 85% HRmax, and (Farrar et al., 2010) found the same to be true since subjects exceeded 65% of VO₂max. In addition, Hulsey et al. used a more traditional protocol of 35 seconds of swinging alternated with 25 seconds of rest for 10 minutes, while Farrar et al. used a very different set of instructing subjects to complete as many reps as possible in 12 minutes. Interestingly, Farrar et al. claimed to have studied the “man-maker: drill outlined by Tsatsouline, however, (Tsatsouline, 2006) mentioned the “manmaker” including intervals of swings followed by jogging, not 12 minutes of only KB swings. (Fortner et al., 2014) compared a Tabata (Tabata et al,. 1996) interval set, which is a four minute set with 20 second work intervals separated by 10 seconds of rest, to a more traditional weightlifting protocol and found both protocols exceeded the minimal aerobic threshold.

However, one study produced conflicting results compared to the others mentioned and found that kettlebell swings are not sufficient in producing gains in aerobic capacity (Jay et al., 2011). Unlike the previous studies mentioned (Farrar et al., 2010; Fortner et al., 2014; and Hulsey et al., 2012), this was an 8 week interventional study which compared estimations of VO₂max at the beginning and end of the study. Unfortunately, relative aerobic intensity was not measured during the actual exercise. Jay et al. also relied on using a submaximal ergometry test measured at the beginning and end of the study to estimate VO₂max which may not be sensitive enough to detect changes (Hulsey et al., 2012). Another limitation as described by Jay et al. could be the fact that there was not enough time to induce enough of an aerobic stimulus. However, most kettlebell swing studies have matched or exceeded the aerobic intensity needed to improve VO₂max according to other exercise modalities. A longitudinal study that records metabolic data during the kettlebell swing exercise may be needed to compare the exact intensity of the workout against the established values outlined by the ACSM.

Work Economy

Work economy is calculated by taking the amount of work performed and dividing it by the volume of oxygen consumed (Wilmore and Costill, 2004; Winter 1978). If an athlete is trying to improve aerobic capacity as much as possible, estimations of work economy can be useful to allow the athlete to choose a training style that maximizes the amount of oxygen consumed while minimizing the amount of external work performed. Also, work economy can show insight if a subject is consistently performing or showing signs of fatigue and form breakdown which may not be optimal to continue exercise as those two factors can increase risk of injury.

Conclusion

Given the increasing popularity of the kettlebell swing, particularly in the United States, kettlebells are becoming more common in gyms across the nation. Even though many useful studies have been published about the swing including topics of strength, motion analysis, and cardiovascular health, there is still a large gap in the literature. As of the author’s knowledge, no study has been published attempting to identify the relationship between the two main ideas mentioned throughout this article: work (Joules) and oxygen consumption (VO₂). These two variables have been studied independently, however, combining the two to find the ideal set from a work economy standpoint can either prove or disprove the effectiveness of the intervals outlined by Tsatsouline and those commonly studied in the literature.

Acknowledgments

I am very grateful for having been a member of the 2022 Berry Summer Thesis Institute cohort. I am so thankful for funding from the Berry Family and Berry Foundation which were so helpful in purchasing lab equipment for my study. Also, I would like to thank all of the faculty in the Honors Program who were dedicated to making this summer as inclusive and educational as possible during the program. Lastly, I am so thankful for my mentors Dr. Crecelius and Dr. Beerse who were both indispensable in helping create and continue with my current project.

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