IMECE BIOMECHANICAL LOADING OF THE AMERICAN KETTLEBELL SWING

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1 Proceedings of the ASME 2015 International Mechanical Engineering Congress & Exposition IMECE2015 November 13-19, 2015, Houston, Texas, USA DRAFT IMECE BIOMECHANICAL LOADING OF THE AMERICAN KETTLEBELL SWING Jefferey Mitchell, EIT Mechanical Designer Savannah, GA, USA Wayne M. Johnson, Ph. D. Engineering Studies Program Armstrong State University Savannah, GA, USA Bryan Riemann, Ph. D., ATC, FNATA Sports Medicine Program Armstrong State University Savannah, GA, USA Cameron W. Coates, Ph. D., P.E. Engineering Studies Program Armstrong State University Savannah, GA, USA ABSTRACT The American kettlebell swing is a variation of the Russian kettlebell swing where the kettlebell is swept in an arc from between the legs to an overhead position with straightened arms. Previous studies involving the kettlebell swing have examined the aerobic and cardiovascular impact of the swing, the variation of mechanical impulse and power generation with kettlebell weight, and compared its efficacy to other types of exercises. However, there have been limited studies examining the dynamic biomechanical loads of the swing on the arm and shoulder. The aim of this study was to establish the mechanical demands of the American kettlebell swing exercise on the arms and shoulders to determine the regions of highest force output and the variation of the forces throughout the swing, all based on percentage of the swing completed. In order to obtain kinematic data, two female subjects with prior kettlebell exercise experience performed one set of fifteen American swings with 8, 12, and 16 kg kettlebells. Position and orientation data was recorded during trials for the kettlebell, joints, and centers of mass of arm segments. Velocity and acceleration data was found using finitedifference approximations. An inverse dynamics method applied to (2-D) planar motion using Newton-Euler equations was used to determine the forces and moments at various joints along the entire arm including the wrist, elbow, and shoulder joints. Data was time normalized as percent of swing, where 0% and 100% indicated the beginning and end of the swing respectively, and approximately 50% denoted the transition between upswing and downswing halves. Results revealed that the arm was under tension during 0% to 35% and 67% to 100% of the swing, indicating the upper torso works to provide the normal force to support the curved motion of the kettlebell. Inversely, during 36% to 66% of the swing the arm muscles worked in order to support the weight of the kettlebell. While the lower extremity mechanical demands associated with kettlebell swings have been studied, the current results help clarify the upper extremity mechanical demands associated with kettlebell swing exercise. The results of this analysis will better help practitioners to understand the prerequisite upper extremity function needed to perform the full American style swing. The American kettlebell swing carries risks its Russian equivalent does not have, typically breaking form to make the shoulder extension involved with raising the kettlebell above the subjects head. These results suggest that the extra range of motion in the American kettlebell swing prompts very different mechanical demands which, in turn, targets different muscle groups from the lower half of the American swing or the full Russian kettlebell swing. Finally, because increasing mechanical stimuli is an important component to exercise progression, this analysis fills the void of understanding the effects of changing kettlebell loads on the upper extremity demands. Future research will consider the symmetry of the upper extremity mechanical patterns revealed by this analysis. INTRODUCTION Kettlebells have seen increased use in American gyms and athletic training in the last couple of decades. Although kettlebells have been in widespread use throughout Russia 1 Copyright 2015 by ASME

2 for centuries and a lesser extent through Europe, there are relatively few papers quantifying the mechanical demands associated with kettlebell exercises. Beyond a few recent studies examining strength and cardiovascular changes following a kettlebell training program, there are many unsubstantiated claims about their effectiveness versus other exercise regiments and their effectiveness on improving strength and power. Strength in this study is the ability of muscles to apply force, often associated with heavy lifting and one repetition maximum. Power considers the ability of muscles to apply force quickly and is an integral part of many functional movements such as jump height or throwing a ball. [4] The kettlebell itself is a round weight with a handle protruding from the top similar in appearance to a tea kettle, thus the name. Most weights such as dumbbells and barbells have their handle located through the center of mass. However, the kettlebell's handle is offset from the center of mass which readily lends itself to a number of unique exercises such as the snatch, clean and jerk, bottoms-up carry, and kettlebell swing. The focus of this study is on the two-handed American kettlebell swing. The American swing starts with both hands gripping the kettlebell handle between both legs with slight knee flexion. The posterior chain musculature is then rapidly extended to drive the kettlebell in an upward arc. The arc ends with the arms and kettlebell extended above the head before returning to the starting position. This is in contrast to the Russian kettlebell swing which stops at chest level. [5] Existing studies have mixed results on the effectiveness of kettlebell training. Jay et al. found kettlebell training to help improve postural stability, but only a statistically insignificant result on jump height verses the control group and pre-training levels. [2] Otto et al. in contrast found kettlebells increase jump height, but strength gain was greater with focused weight training rather than with kettlebells. [7] Beardsley and Contreras concluded positive results for power improvement, but uncertainty with strength gains. They concluded that the mechanics and joint movement are not well understood for kettlebell exercises. [1] McGrill and Marshall addressed the mechanics primarily of the lower back and leg loading where they found loading was highest during the initial stages of kettlebell exercises. [6] McGrill and Marshall also found a shear force component on the lower spine which is in contrast with traditional exercises, and credit this shear force as a possible explanation for improved back health. They also offer this fact as a possible reason for reports of lower back pain due to kettlebell exercises. [6] Jonen and Netterville respond by stressing the need for evidence to support proper instruction for the complex movements involved in kettlebell exercises in order to ensure safety and reduce risk of injury. [3] To improve the understanding and safety of the kettlebell swing, we have employed inverse-dynamics similar to the methods of Lake and Lauder to determine the forces experienced by the joints in the arms and shoulders for based on kinematic data [5]. Dynamics involves examining the relationships between the forces and moments applied to objects and the subsequent motion; kinematic data being the data associated with an object s position, velocity and acceleration with no regard for the object s mass. Inverse-dynamics is the reverse of this process and works using the kinematic data and mass/inertial information to determine the resultant forces and moments that created motion. The work herein provides an analytic framework for which more exhaustive studies can be completed to examine and characterize various aspects of the kettlebell swing including swing styles (Russian vs. American), and gender differences. EXPERIMENTAL METHODS Two female test subjects with minimum 6 months kettlebell swing experience performed 15 repetitions of two-handed overhead (American style) swings. Subject 1 was 167cm tall and weighs 75 kg; subject 2 was 161cm and 53 kg. Both subjects were early 20 s in age. Each variation of the kettlebell swing was performed with three different kettlebell masses; 8kg, 12kg, and 16kg (one set each) in a random order. Subjects were given at least one minute of rest in between sets. Swings were performed on two AMTI force plates while 12 Vicon infrared cameras (Vicon, Oxford, UK), captured upper extremity motion of each subject at 100Hz sampling rate. Each subject was outfitted with four marker sets, placed on the trunk and upper extremity. Specific locations for the marker sets were; thorax, approximately one to two inches below the C7 vertebra, left upper arm three inches above the olecranon process, the mid-forearm three inches above the head of the radius and the dorsal portion of the hand covering all five metatarsals. The proximal and distal segment endpoints were digitized and combined with anthropometric data to compute the location each segments center of mass. The 3- D position and orientation of the marker sets were streamed through The Motion Monitor (Innovative Sports Training, Chicago, IL) and recorded for offline analysis; a built-in Butterworth filter was applied to remove discontinuities and smooth recorded position data. Five swings with clean (few interruptions) data were identified in each set. All subjects signed an informed consent form and experimentation has IRB approval. DYNAMIC MODELING A Matlab program was written to take recorded data from the Motion Monitor software and apply inverse dynamics to determine the forces and moments acting on the kettlebell and arm joints up to the shoulder socket. Several assumptions were made to facilitate the modeling of the system. First, the kettlebell and the arm sections: hand, forearm, and upper arm, were assumed to be rigid. Second, the kettlebell swing was simplified to a two dimensional problem in the sagittal plane. Examination of 2 Copyright 2015 by ASME

3 the position data for the kettlebell indicated a m difference between the maximum and minimum values of the z-axis/lateral movement during 13 complete 8kg kettlebell swings. This is relative to 1.000m of travel in the x-axis/anterior and m of travel in the y-axis/vertical. Third, it was assumed that forces were symmetrical between both arms. In reality, the results in this study indicate the average of both arms on the kettlebell's motion. Asymmetry between the forces would be attributed to asymmetry within the body structure and muscle application if taken into account. Fourth, it was assumed that frictional forces and other external forces were negligible. In order to perform inverse dynamic calculations, kinematic data needed to be paired with mass and inertial data. The kinematic data provided from recording software consisted of position, acceleration, rotation, and angular acceleration of the bodies' center of mass and position data of the joints at specific times. The combined data is then passed through Newton-Euler equations generated from the free body diagrams and relationships between bodies. Figure 1 contains the free body diagram of the kettlebell and Figure 2 contains the free body diagram of a generic arm segment. Note that in modeling the arm anatomy, the elbow, wrist, and hand grip are hinge joints while the shoulder is a ball and socket joint. The rationale for modeling the first three joints as hinges is due to capacity for the muscles in the arm segment above a joint to provide torques across that joint which must be taken into account. For example, the triceps and biceps in the upper arm provide a net torque on the elbow causing it to bend or straighten. Figure 2. Generic arm segment free body diagram. The generic Newton-Euler equations generated from the free body diagram in fig. 2 for the n th arm segment is as follows: ΣF x = ma x = F (x,n) F (x,n 1) (1) ΣF y = ma y = F (y,n) F (y,n 1) + mg (2) ΣM z = Iα z = M (z,n) M (z,n 1) (3) +(r (a,n) F (n) ) + (r (b,n) F (n 1) ) Where equation (1) shows the forces on the x-axis, (2) shows the forces on the y-axis, and (3) shows the torques about the z-axis. A similar set of equations for the kettlebell are shown in equations (4)-(6): ΣF x = ma x = F (x,n) (4) ΣF y = ma y = F (y,n) + mg (5) ΣM z = Iα z = M (z,n) + (r (a,n) F (n) ) (6) Figure 1. Kettlebell free body diagram Most of the data was directly available from the experiment, but data for the kettlebell was incomplete. Although recording software can approximate position data of joints and the center of mass (COM) of appendages on a human body, it cannot correctly determine the COM for non-anatomical objects (e.g., kettlebell). All kettlebell kinematic data was based the centroid of a triad of data markers rather than based around the actual center of mass as a result of this issue. In order to correct this, the kettlebell was digitally scanned and modeled in CAD software to determine the location of the centroid relative to the center of mass. From here, a position vector was created and combined with kettlebell rotational data (i.e., angular velocity and acceleration) to determine the position and acceleration of the kettlebell s center of mass in the data as depicted in Equations (7) and (8), respectively. Note that in Equations (7) and (8), cdm is the centroid of the data markers, and kb is the kettlebell. Further, the location of the grip on the kettlebell was unknown. Again, a vector was created between the center of the handle's cross member 3 Copyright 2015 by ASME

4 and the center of mass and the rotation of the kettlebell was applied to determine the location of the hand's grip. Com is the center of mass of the kettlebell and cdm is the centroid of the data markers in equations (7) and (8). r com = r cdm + r cdm/cdm (7) a com = a cdm + α kb r cdm/cdm (8) ω (kb) r cdm/cdm Once all of the forces and moments acting at each joint were determined, the forces were converted to tensive/shear (T-S) forces. These T-S forces were aligned to each segment of the arm as the segments rotated during the swing. This is a relative coordinate system where the positive tensive forces always point up the body segment (proximally). It represents the muscles and joints being pulled in tension by the weights further down the arm when positive, and the arms under compression or supporting the weights further down the arm when negative. The forces at the grip point on the kettlebell were aligned with the hand, the wrist aligned with the forearm, the elbow with the upper arm, and the shoulder with the distal thoracic (T12) vertebrate. The vertebrate was chosen since it best aligns the T-S axis with the spine and shows the nature of the loading the shoulders put on the spine. Note that since the spine is supporting the arms and kettlebell, the resulting tensive values are normally negative, indicating compression whereas the other joints are normally in tension. Positive shear forces represent force normal to the compressive axis, and are positive when the shear force propels the segments below the joint up and around the arc of the kettlebell swing. A positive shear on the shoulders indicates the muscles in the upper torso are counterintuitively pulling the shoulders backwards. Figures 3 through 5 represent the T-S coordinate system at three temporal instants during a swing and show how the T-S coordinate system for the shoulders differs verses the other T-S coordinate systems. Figures 3 through 5 apply to both the up and down swing that make a complete kettlebell swing. The numerical annotations in these figures represent the following: 1 Kettlebell, 2 Hand, 3 Forearm, 4 Upper Arm, 5 Thorax. Figure 4. Tensive/Shear force conventions during the middle of a representative kettlebell swing. Figure 5. Tensive/Shear force conventions during the top of a representative kettlebell swing. RESULTS When handling the results, skeleton plots of the kettlebell swing were used for a preliminary examination of the data for errors and movement throughout a swing. In the skeleton plots in Figures 6 and 7 the asterisk represents the kettlebell location at various instants in time, while the circles show the location of the grip on the kettlebell handle, wrist, elbow, and shoulder joints in addition to the T-12 vertebrate. The dashed line that connects between each joint and the kettlebell to show which markers correspond to the same time, while the solid line show the path of travel for the selected points on the X-Y plane throughout a swing. Figure 6 shows the various locations of the kettlebell and joints for a single upswing of an American kettlebell swing with a 12kg weight swung by subject 2. Notice the elbow travels almost vertically during the majority of the swing, while the shoulder and back travel up then forward on the second half of the upswing, in contrast to the smooth arc of the kettlebell and hand data points. During the second half of the swing, the torso swings forward in an effort to counterbalance the kettlebell being held out. For this particular subject, the torso and kettlebell balance out near the elbow, which gives it a nearly vertical path. Figure 3. Tensive/Shear force conventions during the bottom of a representative kettlebell swing. 4 Copyright 2015 by ASME

5 Figure 6. Select locations of a 12kg kettlebell up swing by subject 2. Figure 7 shows the corresponding down swing for the same kettlebell swing shown in Fig. 6. There is little difference in either the up or down half of the swing; this is typical of all swings in the data sets. Figure 7. Select locations of a 12kg kettlebell down swing by subject 2. In order to produce meaningful data from the forces and moments found with the Newton-Euler equations (1 to 6), the data was broken into complete swings by a simple algorithm. The data points were then assigned to discrete percentages of a swing based on their time in the swing where 0% represents the bottom of the swing, 50% the top, and 100% as the 0% of the following swing with ensemble averaging. This was done to allow for statistical calculations of the data across multiple swings despite minor time variations in each swing. Only swings without notable discontinuities or other recording errors were used for statistical calculations. Figure 8 (See Annex A) is the first plot generated with this method. The plot contains the mean Y-position for the swings of a single trial as a function of swing percent completed. The dashed lines represent the maximum and minimum values at each percentage in the data set. The standard deviation of the data set was computed, but would 5 Copyright 2015 by ASME

6 have little meaning with a minimum of five swings in each data set. Figures 9-12, 13-16,17-20 are the compression, shear forces, and moments respectively at each joint for all four trials. Figures show the resultant force while figs show the corresponding power at each joint for each trial. The power is based on the net force, linear displacement between two moments in time, and the constant sample period for each data point as seen in equation 9. Figures 8-20 can be found in Annex A. All sets are shown for comparison. P = F (x,n) 2 + F (y,n) x (n,n+1) + y (n,n+1) 100 Table 1 below shows the maximum force in Newtons on each kettlebell with respect to percent of swing completed. Table 2 is similar but shows the peak power in Watts for each kettlebell. Subject 1 2 Subject 1 2 DISCUSSION Figures 9-12 show that moving closer to the torso increases magnitude of tensive forces. This intuitively makes sense as the arm segments further up the arm not only have to provide the tensive force on the kettlebell, but also the additional tensive force to move the body segments below. Note that the back provides the reactionary forces against the shoulder/upper arm and that the positive tensive forces is aligned down the spine which explains why the upper arm is mostly in compression while the other segments are in tension. When the swing is 30% to 70% complete, indicating the kettlebell is near the peak of the swing, the weight of the kettlebell begins to create compression on the arms rather than tension. This region is also marked by low force created by the subject and low displacement per time interval as the kettlebell slows to a stop which, by equation (9) Weight (kg) Percent of Swing Peak Net Force Mean (N) 8 90% % % % Table 1. Peak Force Weight (kg) Percent of Swing Peak Power Mean (W) 8 88% % % % Table 2. Peak Power 9, means the 30%-70% region of the swing requires minimal power from the subject. A positive shear force in figs indicates the subject is exerting force to pull the kettlebell towards the top of the kettlebell swing. With one local maximum on the upper arm provided by the torso near 17% as the swing starts and another near 88% completion as the kettlebell s motion retards nears the bottom. The presence of high shear on the shoulder with comparatively low shear elsewhere indicates most of the force that drives the kettlebell around the arc originates elsewhere in the body, rather than in the arms. This supports the conclusion in existing biodynamic literature that states the main force and power comes from the posterior chain [5,6]. Examining the tensive forces in fig. 9 and shear forces in fig. 13 verses figs and reveals an anomaly with forces on the kettlebell for subject 1 s 8kg swing. By examining the positional information for this trial, it was revealed that the subject had locked their wrist in flexion. This flexion forced the tensive direction associated with the hand as seen in fig. 3-5 further out of line with the arm. This explains why the net forces in fig. 21 do not appear out of line. Although the net force on subject 1 s 8kg swing is nominal, the moments required to maintain that flexion are approximately 2 times as large as subject 1 s 12 kg swing despite being a lighter weight. The greater moments acting on the kettlebell indicate greater use of the extensor/flexor muscles in the forearm. Compared to both of subject 2 s swings which have minimal moments on the kettlebell, it seems that the muscles in the arms can be engaged by maintaining flexion at joints that counters the natural tension created by the kettlebell. The significance of the muscle engagement in the arms is, however, beyond the scope of this analysis. CONCLUSIONS The regions of highest force and power both occur at the beginning and end phase of the kettlebell swing, while the middle section of the swing provides little in the way of difficulty for the weightlifter. One possible avenue of future study is the examination of the Russian kettlebell swing. The Russian swing stops at approximately chest level verses over the head as in the American version. As it avoids the mostly idle region of the American swing, the Russian swing may provide constant tensive force rather than switching to compressive near the peak. Further study needs greater verity of weights, test subjects, and multiple kettlebell exercises, not just the two handed swings. Seeing trend data as weight increases for various individuals could help determine effectiveness of the kettlebell swing and potentially lead to recommendations for ideal kettlebell weight to maximize force or power based on user. The addition of data on baseline exercises which engage the posterior chain would allow for more complete conclusions regarding the effectiveness of the kettlebell swing. 6 Copyright 2015 by ASME

7 Refinement to the recording and analysis of the data is needed to determine forces and moments at the bottom of the swing to create a complete picture of the swing data. Finally, additional understanding and research needs to be completed to determine the significance of moments on the joints. It is currently believed they represent the net torque acting on a joint from the muscles directly above, but this is speculation REFERENCES [1] C. Beardsley and B. Contreras, The Role of Kettlebells in Strength and Conditioning: A Review of the Literature, Strength and Conditioning Journal, vol. 36, no. 3, pp , Jun [2] K. Jay, M. D. Jakobsen, E. Sundstrup, J. H. Skotte, M. B. Jørgensen, C. H. Andersen, M. T. Pedersen, and L. L. Andersen, Effects of Kettlebell Training on Postural Coordination and Jump Performance: A Randomized Controlled Trial, Journal of Strength and Conditioning Research, vol. 27, no. 5, pp , May [3] W. Jonen and J. T. Netterville, Kettlebell Safety: A Periodized Program Using the Clean and Jerk and the Snatch, Strength and Conditioning Journal, vol. 36, no. 2, pp. 1 10, Apr [4] J. P. Lake, B. S. Hetzler, and M. A. Lauder, Magnitude and Relative Distribution of Kettlebell Snatch Force-Time Characteristics:, Journal of Strength and Conditioning Research, vol. 28, no. 11, pp , Nov [5] J. P. Lake and M. A. Lauder, Mechanical demands of kettlebell swing exercise, J Strength Cond Res, vol. 26, no. 12, pp , Dec [6] S. M. McGill and L. W. Marshall, Kettlebell Swing, Snatch, and Bottoms-Up Carry: Back and Hip Muscle Activation, Motion, and Low Back Loads:, Journal of Strength and Conditioning Research, vol. 26, no. 1, pp , Jan [7] W. H. Otto, J. W. Coburn, L. E. Brown, and B. A. Spiering, Effects of Weightlifting vs. Kettlebell Training on Vertical Jump, Strength, and Body Composition:, Journal of Strength and Conditioning Research, vol. 26, no. 5, pp , May Copyright 2015 by ASME

8 ANNEX A FIGURES Figure 8. Average Y-Positions for 8kg swing by subject 1. 8 Copyright 2015 by ASME

9 Figure 9. Average tensional forces for 8kg swing by subject 1. 9 Copyright 2015 by ASME

10 Figure 10. Average tensional forces for 12kg swing by subject Copyright 2015 by ASME

11 Figure 11. Average tensional forces for 8kg swing by subject Copyright 2015 by ASME

12 Figure 12. Average tensional forces for 12kg swing by subject Copyright 2015 by ASME

13 Figure 13. Average shear forces for 8kg swing by subject Copyright 2015 by ASME

14 Figure 14. Average shear forces for 12kg swing by subject Copyright 2015 by ASME

15 Figure 15. Average shear forces for 8kg swing by subject Copyright 2015 by ASME

16 Figure 16. Average shear forces for 12kg swing by subject Copyright 2015 by ASME

17 Figure 17. Average moments for 8kg swing by subject Copyright 2015 by ASME

18 Figure 18. Average moments for 12kg swing by subject Copyright 2015 by ASME

19 Figure 19. Average moments for 8kg swing by subject Copyright 2015 by ASME

20 Figure 20. Average moments for 12kg swing by subject Copyright 2015 by ASME

21 Figure 21. Sum of forces for 8kg swing by subject Copyright 2015 by ASME

22 Figure 22. Average power for 12kg swing by subject Copyright 2015 by ASME

23 Figure 23. Average power for 8kg swing by subject Copyright 2015 by ASME

24 Figure 24. Average power for 12kg swing by subject Copyright 2015 by ASME

25 Figure 25. Average power for 8kg swing by subject Copyright 2015 by ASME

26 Figure 26. Average power for 12kg swing by subject Copyright 2015 by ASME

27 Figure 27. Average power for 8kg swing by subject Copyright 2015 by ASME

28 Figure 28. Average power for 12kg swing by subject Copyright 2015 by ASME