Kettlebell Research Update
Studies show cardio and strength benefits, but suggest caution about spinal loads.
Kettlebells have seen growing popularity as a total-body training tool to improve cardiovascular health and musculoskeletal fitness. Yet for all the enthusiasm among personal trainers, experimental research on the effects of KB training was scant until last year, when studies began showing up in peer-reviewed journals. This column updates IFJ readers with recent research on KB training.
Study 1. Metabolic Demand of Kettlebell Training
Hulsey, C.R., et al. 2012. Comparison of kettlebell swings and treadmill running at equivalent rating of perceived exertion values. Journal of Strength and Conditioning Research, 26 (5), 1203–07.
Experimental question. The two-handed KB swing (see Figure 1) is generally viewed as a foundational training approach for KB training. Researchers wanted to determine whether performing continuous KB swings would create an energy cost capable of improving cardiorespiratory fitness. To find out, they compared the metabolic demand of KB swings to treadmill running at equivalent ratings of perceived exertion (that is, subjective assessments of exercise intensity).
Subjects. Thirteen subjects (11 male, 2 female, average age = 21.4, average height = 69 inches, average weight = 170 pounds, average BMI = 23.5) volunteered for the study. All were deemed moderately trained and had no KB experience. Prior to testing, a certified KB instructor familiarized them with the two-handed KB swing exercise.
Study procedures. In the first experimental trial, subjects performed 10 minutes of two-handed KB swings, alternating 35-second swing bouts with 25 seconds of rest. The men used a 16–kilogram KB, and the women an 8 kg KB. Subjects were encouraged to maintain a steady swing count averaging 22–25 per minute. A certified KB instructor gave continuous feedback, helping subjects maintain correct swing execution and posture.
After 48 hours of rest, study participants returned to the testing lab and completed a 10-minute treadmill run at a subjective intensity (RPE of 15.3–15.5 on a 6–20 scale) equivalent to what they experienced during the KB swing trial. An automated metabolic gas analysis system commonly used in exercise studies measured oxygen consumption and kilocalorie expenditure. The final 7 minutes of each work bout were used for data analysis.
Results and discussion. Table 1 compares key cardiovascular and metabolic values obtained in this investigation. At an equivalent subjective RPE, subjects had no significant difference in heart rate. Oxygen consumption and calorie expenditure were significantly higher for treadmill running than they were for KB swings. However, it should be noted that subjects completed both 10-minute exercise bouts at above 85% of their age-predicted maximum heart rate. Thus, KB swings, albeit for just 10 minutes in this investigation, did elicit a physiological response necessary for improving cardiorespiratory fitness.
The study did not determine an optimal KB workout duration or a weekly frequency necessary to obtain a meaningful improvement in cardiorespiratory fitness; further research is needed to answer those questions. However, the kilocalorie expenditure (375 kilocalories) obtained from this study suggests that KB training is a viable exercise option for inclusion in a weight loss program.
Study 2. Kettlebell Training’s Effect on Strength, Vertical Jump and Body Composition
Otto, W.H., III, et al. 2012. Effects of weightlifting vs. kettlebell training on vertical jump, strength, and body composition. Journal of Strength and Conditioning Research, 26 (5), 1199–1202.
Experimental question. Previous research has demonstrated that weightlifting can improve the vertical jump of athletes and recreational enthusiasts. In particular, the snatch, clean and jerk and other explosive weightlifting exercises seem to resemble the explosive movement patterns of a vertical jump. KB training could be a viable exercise option for developing similar performance characteristics, but there has been no research in this area until now. This study compared the effects of weightlifting to KB training on strength, power and body composition over a 6-week period.
Subjects. Thirty healthy males (average age = 23, average height = 69 inches, average weight = 176 pounds, average BMI = 26.5) who had 1 year of resistance training experience (but no KB training) volunteered for this study. Subjects were randomly assigned to either the weightlifting group or the KB training group. All participants attended two to three exercise sessions in the week before the start of the study to learn how to execute their exercises correctly.
Experimental variables (described below) were tested before and after the intervention. Subjects trained twice a week with at least 72 hours between training sessions and were encouraged to maintain their normal dietary lifestyle during the 6-week period.
Study testing procedures. Vertical jump was assessed with a scientific testing vertical-jump apparatus (EPIC Jump Station), using the best of three trials for analysis. One-repetition maximum was employed to test maximum back squat and power clean ability, following established testing guidelines. A three-site skinfold test for chest, abdomen and thigh was used to assess body composition.
Kettlebell and resistance training procedures. Participants used a 16 kg KB and completed KB swings, accelerated KB swings and goblet squats (see Figure 2), with the volume (number of sets) progressively increasing during the course of the 6 weeks. Emphasis was placed on proper form and technique in conjunction with an appropriate speed of movement.
The weightlifting group performed high pulls, power cleans and back squats in a linear-periodization model that attempted to replicate the number of sets and reps of the KB swings during the study. Although the researchers attempted to match the volume (reps sets) of exercises performed by both training groups, the weightlifting group used heavier loads, which represented a limitation of the study’s findings.
Results and discussion. The resistance training group improved vertical ability jump about 2% after 12 training sessions in 6 weeks, while the KB group results showed no change. The KB group increased power clean performance by 4.2% (a significant improvement), while the weightlifting group showed a 9% increase. With the back squat, both groups showed significant improvements, with the weightlifting group showing greater gains (14% increase) than the KB group (4%). This may have been expected since the weightlifting group lifted heavier loads. There were no changes in percent body fat for either group.
The findings of this study clearly show the potential power and strength benefits of KB training as an alternative to traditional resistance-training methods.
Question: Is Kettlebell Training Safe for the Spine?
McGill & Marshall (2012) completed the first and so far only biomechanical study attempting to quantify spine loading during various KB exercises. Electromyography, ground reaction forces and 3D kinematic data were recorded as seven male subjects performed the exercises using a 16 kg KB.
The KB swing (regardless of style of swing or snatch) appears to create a hip-hinge squat movement pattern together with patterns of rapid muscle activation–relaxation cycles that elicit considerable magnitudes of load on the spine. That is why this unique exercise may be very appropriate for exercise programs emphasizing posterior power development about the hips. In contrast, the KB swing also appears to result in unique compression and shear load ratios in the lumbar spine, which may explain why it causes discomfort in the lower backs of people who otherwise tolerate very heavy loads.
The message for personal trainers from this research is that KB training offers several distinct training opportunities. However, the large shear- to compression-load ratio on the lumbar spine created during KB swing exercises suggests that this training approach may be contraindicated for some individuals. Personal trainers interested in introducing KB training to clients should follow careful progressive overload training procedures to ensure safe and effective training outcomes.
McGill, S.M., & Marshall, L.W. 2012. Kettlebell swing, snatch, and bottoms-up carry: Back and hip muscle activation, motion, and low back loads. Journal of Strength and Conditioning Research, 26 (1), 16–27.