Methods Study design Twelve amateur kettlebell sport lifters performed 6 min of the kettlebell snatch exercise with one hand change, as is commonly performed in training by GS competitors. GRF was recorded with two AMTI force plates and kettlebell trajectory was simultaneously recorded with a nine-camera VICON motion analysis system. The GRF from the force plates allowed us to determine the external mechanical demands applied to the lifter and kettlebell system centre of mass, whilst the reverse kinematics calculated the force applied to the kettlebell. Force was determined using the kettlebell’s known mass (kg) and the acceleration (m s−2) determined via reverse kinematics. The aim was to identify the external demands placed upon each leg and the changes in kinetics during a prolonged kettlebell snatch set over 6 min. The dependent variables were resultant kettlebell force (N), resultant absolute and relative GRF (N) for: resultant, anterior–posterior, medio-lateral and vertical bilateral, GRF impulse (N s) and resultant velocity of the kettlebell (m s−1). These were measured at the following time points: time of peak GRF, point of maximum kettlebell acceleration, point of maximum kettlebell velocity, end of back swing, lowest kettlebell point, midpoint and highest kettlebell point. Subjects Twelve males with a minimum of three years kettlebell training experience (age 34.9 ± 6.6 years, height 182 ± 8.0 cm and mass 87.7 ± 11.6 kg, handgrip strength non-dominant 54.5 ± 8.0 kg and dominant 59.6 ± 5.5 kg) gave informed consent to participate in this study. They were free from injury and their training regularly included 6 min kettlebell snatch sets. Prior to taking part in the study, the participants performed 6.0 ± 2.1 training sessions per week, of which 3.3 ± 1.9 were with kettlebells. All had previously competed in kettlebell sport and kettlebell sport was the primary sport for nine of the 12 participants. A 24 kg kettlebell was selected, as this is the weight used by ‘amateur’ lifters within a kettlebell sport competition. This is in contrast to 32 kg weight for ‘professional’ lifters and 16 kg for ‘novice’ lifters. The Australian Catholic University’s ethics review panel granted approval for this study to take place (ethics number 2012 21V). All participants gave written consent to take part in this research. Procedures During a single testing session, athletes performed one 6 min kettlebell snatch set with a hand change taking place at the 3 min mark. A 6 min set was chosen as opposed to the GS standard 10 min set, as it was attainable for all subjects and is a common training set duration for non-elite kettlebell sport athletes. Handgrip strength was tested with a grip dynamometer with a standardised procedure 10 min pre-set and immediately post-test (ACSM, 2013). They were provided with chalk and sand paper (as this is standard competition practice) and asked to prepare the kettlebell as they would before training or competition. A range of professional-grade kettlebells of varying masses (Iron Edge, Australia) were available for the lifters to perform their typical warm ups. Following the athletes warm up, each 6 min set was performed with a professional-grade 24 kg kettlebell, as is the standard for kettlebell sport within Australia. Three markers were used, one (26.6 mm × 25 mm) was placed on the front plate of the kettlebell, and two markers (14 mm × 12.5 mm in diameter) were placed on the kettlebell at the base of each side of the handle. The markers were placed in these positions to help avoid contact with the lifter during the set. Nine VICON infrared cameras (six MX 13+ and three T20-S) sampling at 250 Hz, were placed around two adjacent OR6 AMTI force plates sampling at 1,000 Hz. The point of origin was set in the middle of the platform, to calibrate the cameras’ positions. The athlete was instructed to stand still with one foot on each plate and the kettlebell approximately 20 cm in front of him before the start of the 6 min set in order to process a static model calibration. A self-paced set was then performed as if they were being judged in a competition. To initiate the set, the kettlebell was pulled back between the legs. VICON Nexus software was used to manually label markers, and a frame-by-frame review of each trial was performed to minimise error. Average marker position was computed at rest from initial position. The initial position of the markers was used to compute vectors from centroid to the centre of gravity. Kettlebell motion was computed using singular value decomposition of the marker transformations into a translation, a rotation and an error value (Duarte, 2014). Root mean square error was calculated and time steps with high error values were dropped from analysis. The centre of gravity locations were computed from the translation and rotation of the kettlebell geometry. A third order B-spline was used to interpolate and filter the three-dimensional trajectories using the python function (‘scipy.interpolate.splprep’). The spline functions (‘knots’) were then used to compute the velocity and acceleration. Time steps of the kettlebells trajectory that contained the kettlebell maximum velocity, maximum acceleration (peak resultant kettlebell force) and the following points: end of the back swing, lowest point, midpoints and highest point (overhead lockout position) were identified. At these time steps the resultant kettlebell force, resultant bilateral GRF, and resultant velocity were recorded. Time steps moving from the overhead lockout position to the end of the back swing were allocated a relative negative time in seconds, with the end of the back swing as zero. The time steps from the end of the back swing moving to the overhead lockout were given a positive relative time. Over the entire set at the point that peak bilateral absolute resultant force or peak resultant force for the ipsilateral and contralateral leg was reached, the three-dimensional force was reported. In addition to the entire set, the three-dimensional bilateral forces were reported for the first and last 14 repetitions. Fourteen repetitions were chosen because it was the closest whole number to the mean repetitions per minute performed by the subjects over the 6 min. The forces were presented in both absolute units and relative to each subject’s body mass. As the majority of the work occurred between the end of the back swing and the midpoint of the upwards and downwards phases of its trajectory, absolute and relative impulse for each leg was calculated over this period. Statistical analyses Data were placed into the Statistical Package for the Social Sciences (SPSS; IBM, New York, United States), Version 22. The data were screened for normality using frequency tables, box-plots, histograms, z-scores and Shapiro–Wilk tests prior to hypotheses testing. One univariate outlier was detected and removed from three of the data sets, relative unilateral vertical GRF, relative and absolute upwards phase medio-lateral GRF. In order to satisfy normality, the medio-lateral GRF for the absolute upwards phase was transformed using the base 10-logarithm function. Following data screening, the final sample numbered 11–12 participants. A 2×2 two-way ANOVA was used to evaluate the difference within peak resultant kettlebell force, absolute and relative GRF for: resultant, anterior–posterior, medio-lateral and vertical bilateral vectors for both the first and last 14 repetitions and the upwards and downwards phases. Additionally, absolute and relative unilateral GRF vectors were compared with a 2×2 two-way ANOVA between the ipsilateral and contralateral legs as well as the upwards and downwards phases. Temporal measures of kinetics were compared within different points of the kettlebell trajectory with two-tailed paired t-tests and a Bonferroni adjustment. An intra-repetition analysis compared the kinetics at six points of the kettlebell trajectory (highest point, midpoints, lowest points and end of the back swing), additionally peak bilateral GRF, maximum acceleration and peak resultant velocity were compared to their peak value (this was done to determine the different demands throughout a single repetition). The magnitude of the effect or effect size was assessed by Cohen’s D (ESD) for t-tests and Cohen’s F (ESF) for two-way ANOVA. Trials from both right and left hands were assessed. If the lifter performed an uneven number of repetitions with each hand, the side with the greatest number had repetitions randomly removed in order to allow for an even amount of pairs. Removed repetitions were evenly allocated between each minute. Within each minute, randomly generated numbers corresponding to each were used to determine removed repetitions. The magnitude of the paired t-test effect was considered trivial ESD < 0.20, small ESD 0.20–0.59, moderate ESD 0.60–1.19, large ESD 1.20–1.99, very large ESD 2.0–3.99 and extremely large ESD ≥ 4.0 (Hopkins, 2010). Statistical significance for the paired t-tests required p < 0.001. The magnitude of difference for the two-way ANOVA was reported as trivial ESF < 0.10, small ESF 0.10–0.24, medium ESF 0.25–0.39 and large ESF ≥ 0.40 (Hopkins, 2003). The two-way ANOVA required p < 0.05 for statistical significance.

Results A total number of 972 repetitions were analysed for the 12 amateur kettlebell sport lifters, each performing an average of 13.9 ± 3.3 repetitions per minute. Grip strength of the hand that performed the last 3 min of the set had a reduction (p = 0.001, ESD = 0.77) of 9.8 ± 4.4 kg compared to pre-test results. Tables 1 and 2 show descriptive statistics for the three-dimensional GRF and kettlebell force during the first and last 14 repetitions for the absolute and relative values, respectively. The absolute peak resultant kettlebell force was significantly larger for the first repetition period compared to the last (i.e. first 14 vs last 14) when a full repetition was analysed (i.e. upwards and downwards phases combined) (F (1.11) = 7.42, p = 0.02, ESF = 0.45). First 14 repetitions Last 14 repetitions Downwards Upwards Downwards Upwards GRF (N) 1,766 (240) 1,775 (277) 1,782 (249) 1,797 (285) GRF x (N) 47 (43) 70 (33) 59 (51) 63 (42) GRF y (N) 308 (74) 299 (80) 320 (88) 315 (92) GRF z (N) 1,736 (235) 1,746 (271) 1,748 (246) 1,766 (278) Resultant peak kettlebell force (N) 809 (74) 895 (76) 826 (85) 879 (101) DOI: 10.7717/peerj.3111/table-1 First 14 repetitions Last 14 repetitions Downwards Upwards Downwards Upwards GRF (BW) 2.06 (0.24) 2.08 (0.31) 2.08 (0.24) 2.10 (0.31) GRF x (BW) 0.06 (0.05) 0.08 (0.04) 0.07 (0.06) 0.07 (0.05) GRF y (BW) 0.36 (0.08) 0.35 (0.10) 0.37 (0.10) 0.37 (0.11) GRF z (BW) 2.03 (0.24) 2.04 (0.30) 2.04 (0.25) 2.07 (0.30) DOI: 10.7717/peerj.3111/table-2 Tables 3 and 4 show the descriptive statistics for the absolute and relative GRF of the ipsilateral and contralateral leg. At the point of peak resultant GRF for either the ipsilateral and contralateral side, a large significant increase was found within the ipsilateral leg in the anterior–posterior vector (F (1.11) = 885.15, p < 0.0001, ESF = 7.00). In contrast, a large significant increase was found within the contralateral leg of the medio-lateral force vector over a full repetition for both the absolute GRF (F (1.11) = 5.31, p = 0.042, ESF = 0.67) and relative GRF (F (1.10) = 9.31, p = 0.01, ESF = 0.54). No significant differences were found for the absolute and relative impulse of the upwards or downwards phase. Figure 2 demonstrates a typical three-dimensional GRF of the ipsilateral and contralateral side. Ipsilateral downwards Contralateral downwards Difference Ipsilateral upwards Contralateral upwards Difference GRF (N) 897 (133) 939 (175) 42 (4.6%) 936 (110) 949 (110) 13 (1.38%) GRF x (N) 34 (16) 59 (56) 25 (53.7%) 46 (25) 33 (33) 13 (32.9%) GRF y (N) 165 (42) 154 (38) 11 (6.9%) 164 (39) 146 (42) 18 (11.6%) GRF z (N) 885 (126) 939 (166) 54 (5.9%) 905 (93) 942 (106) 37 (4.0%) Resultant impulse (N·s) 380 (29) 365 (64) 15 (4.0%) 382 (52) 378 (63) 4 (1.0%) DOI: 10.7717/peerj.3111/table-3 Ipsilateral downwards Contralateral downwards Difference Ipsilateral upwards Contralateral upwards Difference GRF (BW) 1.07 (0.14) 1.11 (0.15) 0.04 (3.7%) 1.13 (0.14) 1.11 (0.13) 0.02 (1.8%) GRF x (BW) 0.04 (0.02) 0.08 (0.04) 0.04 (66.7%) 0.06 (0.04) 0.04 (0.04) 0.02 (40.0%) GRF y (BW) 0.20 (0.05) 0.18 (0.04) 0.02 (10.5%) 0.20 (0.06) 0.16 (0.03) 0.04 (22.2%) GRF z (BW) 1.04 (0.13) 1.07 (0.13) 0.03 (2.8%) 1.08 (0.19) 1.08 (0.12) 0 (0%) Resultant impulse (BW·s) 0.42 (0.50) 0.44 (0.05) 0.02 (4.7%) 0.45 (0.05) 0.43 (0.05) 0.02 (4.6%) DOI: 10.7717/peerj.3111/table-4 Figure 2: Typical three-dimensional GRF of the ipsilateral and contralateral legs for an 87 kg athlete. (A) midpoint (down), (B) lowest point (down), (C) end of back swing, (D) lowest point (up), (E) midpoint (up), x, medio-lateral; y, anterior–posterior; z, vertical. Tables 5 and 6 provide data on how the kinematics and kinetics of the kettlebell snatch changed throughout the range of motion. Specifically, these tables list the relative times, resultant velocity and temporal changes in both kettlebell force and GRF with a comparison to their respective peak values during the downwards and upwards phases, respectively. Within the downwards phase there was no significant difference between peak bilateral GRF and bilateral GRF at the point of maximum acceleration, peak resultant velocity and resultant velocity at the midpoint. All other points had significant differences (see Tables 5 and 6). Relative time (s) Resultant kettlebell force (N) Resultant velocity (m/s) Resultant bilateral GRF (N) Highest point overhead −1.72 (0.49) 222 (15)1,5 0.28 (0.22)1,5 1,054 (93)1,4 Midpoint −0.60 (0.04) 284 (53)1,5 3.62 (0.21)1,2 866 (153)1,5 Peak resultant velocity −0.53 (0.05) 466 (69)1,5 3.81 (0.21) 1,139 (165)1,4 Maximum acceleration −0.40 (0.04) 814 (75) 3.23 (0.27)1,4 1,660 (299) Peak resultant GRF −0.34 (0.11) 775 (73) 3.08 (0.29) 1746.68 (217) Lowest point −0.31 (0.04) 694 (79)1,3 2.69 (0.34)1,5 1,595 (276)1,2 End of the back swing 0.00 (0.00) 127 (43)1,5 0.21 (0.08)1,5 940 (169)1,5 DOI: 10.7717/peerj.3111/table-5 n = 972 Relative time (s) Resultant kettlebell force (N) Resultant velocity (m/s) Resultant bilateral GRF (N) End of the back swing 0.00 (0.00) 127 (43)1,6 0.21 (0.08)1,6 940 (169)1,6 Lowest point 0.32 (0.05) 788 (112)1,3 2.90 (0.37)1,6 1,701 (320)1,2 Peak resultant GRF 0.33 (0.05) 798 (81)1,3 2.89 (0.52)1,5 1,768 (242) Maximum acceleration 0.39 (0.04) 885 (86) 3.51 (0.29)1,5 1,634 (289)1,2 Peak resultant velocity 0.51 (0.05) 596 (62)1,5 4.16 (0.23) 1,095 (164)1,5 Midpoint 0.60 (0.04) 314 (38)1,6 3.82 (0.20)1,4 838 (122)1,6 DOI: 10.7717/peerj.3111/table-6

Conclusion In summary, the GRF and force applied to the kettlebell changes during different stages of the kettlebell snatch. Additionally, the kettlebell snatch places different external demands upon the ipsilateral and contralateral legs within the AP and ML force vectors. Thus, despite the kettlebell snatch being performed with two legs, each leg may be loaded differently, thereby offering a different stimulus to each leg. There are rapid changes within the kinetics during different phases of the lift. In the upwards and downwards phases there were extremely large significant intra-repetition differences within GRF, kettlebell velocity and force applied to the kettlebell. Applied force to the kettlebell during the first and last 14 repetitions at the point of peak resultant kettlebell force is altered over the course of a prolonged set, possibly due to muscular fatigue, which is further supported by a marked reduction in hand grip strength. The data from this investigation suggest that the kettlebell snatch may provide a unique training stimulus, compared to other exercises (e.g. barbell snatch), as it has a downwards phase and places different demands upon the ipsilateral and contralateral legs. In addition, within their respective sports the barbell and kettlebell snatches sit on different ends of the strength–endurance continuum.