Athletes have access to a wide array of training methods and equipment. One such training implement is the kettlebell, which has re-emerged in the United States in recent years as a popular option for athletic conditioning. Kettlebells have been a consistently popular training tool in their native Russia, for many years. The Russian word for kettlebell, girya, first appeared in a Russian dictionary in 1704 (9). However, a sport discus search for the keyword “kettlebell” finds no English references before the year 2002.
Currently, kettlebell training has become a staple of popular strength and conditioning programs such as Crossfit and Pavel Tsatsouline's Russian Kettlebell workshops. Training with kettlebells is touted as a viable way to increase muscular strength, muscular endurance, cardiorespiratory fitness, and reduce body fat (9). However, no empirical evidence (at least in English) exists to support these claims. In response to this paucity of information, we examined the cardiorespiratory demand of one popularly recommended kettlebell routine.
Experimental Approach to the Problem
Despite the recent emergence of kettlebell training, there is little documentation of the physiological effects of training routines using this device. The purpose of this descriptive study was to document the heart rate (HR) response and oxygen cost of performing a kettlebell exercise routine that is intended to improve cardiorespiratory fitness. The kettlebell routine we chose to examine is termed “the US Department of Energy Man-Maker” and is described in a popular text on kettlebell training (9).
Subjects performed 2-handed swings, in accordance with the routine's published description, for 12 minutes in duration. Heart rate and o2 were continuously recorded during the exercise.
Ten college-aged men (age = 20.8 ± 1.1 years, height = 179 ± 3 cm, body mass = 77.3 ± 7.7 kg, o2max = 52.78 ± 6.22 ml·kg−1·min−1) were recruited as subjects. Although all subjects were recreationally active, only one had previous experience exercising with kettlebells. Subjects reported to the laboratory for testing on 2 occasions. During the first testing session, subjects completed a health history form, and upon determination that they were of “low risk” for cardiopulmonary or metabolic disease according to ACSM guidelines, they completed a treadmill test for maximal oxygen consumption. On the second testing session, subjects completed the kettlebell exercise routine. All test procedures were approved by the University's Institutional Review Board, and all subjects provided their informed consent before participation in the experiment.
Test Session 1-o2max Testing
After the completion of a medical history form, and the determination that the subjects were eligible for maximal exercise testing, subjects' anthropometric data were obtained using a stadiometer and a calibrated digital scale. Subjects were then outfitted with a chest strap HR monitor (Polar USA, Lake Success, NY) and connected to a metabolic cart (Truemax 2400; ParvoMedics, Salt Lake City, UT). Subjects then completed a graded exercise test for the determination of o2max using the Bruce protocol (4) on a motorized treadmill (Quinton model Q45; Bothell, WA). All subjects were deemed successful in achieving o2max, as they obtained at least 2 of the following criteria: an HR within ± 12 b·min−1 of age-predicted maximum, a respiratory exchange ratio (RER) of ≥1.10, and a rating of perceived exertion of ≥17 on the 16-point Borg scale (3). After completion of the o2max test and a brief rest, subjects were familiarized with the technique for the kettlebell swing exercise and allowed to perform a few practice swings.
Test Session 2-Kettlebell Exercise
Two to 7 days after the o2max test, subjects returned to the laboratory. On arrival, they completed a 2-minute warm-up on a cycle ergometer (Monark, Varberg, Sweden) at a self-determined workload. Subjects then donned an HR monitor and were connected to the metabolic cart to allow continuous monitoring of HR and expired gases.
Subjects completed a 12-minute exercise bout, known as the “US Department of Energy Man-Maker” (9). The bout consisted of performing 2-handed swings, using a 16-kg kettlebell (Perform Better, Cranston, RI) for 12-minute duration. A 16-kg kettlebell was used in this study because that is a recommended weight for beginning men (9). Subjects were told to work at their own pace, resting as needed, while aiming to complete as many swings as possible in the 12-minute time frame. Heart rate was monitored continuously and recorded every minute of the bout.
All data are presented as mean ± SD. Average HR and o2 during the kettlebell routine were calculated, and the percent of HRmax and o2max achieved during the bout were determined. Percent HRmax and %o2max were compared using a paired samples t-test. A linear regression analysis was used to describe the relationship between %HR and %o2. Statistical significance was set at the p ≤ 0.05 level.
Subjects completed an average of 265 ± 68 swings during the 12 minutes, for an average work rate of 22 ± 6 swings·per minute. Relative o2 averaged 34.31 ± 5.67 ml·kg−1·min−1 (65.3 ± 9.8% of o2max) during the exercise bout, for an accumulated oxygen consumption of 26.5 ± 4.78 L·min−1 in 12 minutes. The average RER during the bout was 1.00 ± 0.05. The HR was 165 ± 13 b·min−1 (86.8 ± 6.0% of HRmax) during the exercise bout. Figure 1 illustrates the average o2 and HR data for the 12-minute exercise bout.
The average %HRmax during kettlebell exercise was significantly higher (p < 0.001) than the average %o2max. The equation describing the regression line to predict %o2max from %HRmax was %o2max = 0.714%HRmax − 4.57, with a significant correlation of 0.58 and an SEE of 6.6%. Figure 2 illustrates the relationship between %o2max and %HRmax.
Performing kettlebell swings at a self-determined pace for 12 minutes, attempting to complete as many swings as possible during that time, maintained subjects' HR and o2 at an average of 87 and 65% of their respective maxima. Based on these observations, the “man-maker” kettlebell drill provided a metabolic challenge of sufficient intensity to increase o2max. The American College of Sports Medicine recommends an optimal intensity of 60-85% o2max to improve cardiorespiratory fitness (1). The average HR and o2 achieved during this exercise bout would classify it as “hard” exercise according to ACSM standards (1). Furthermore, the high RER values (average of 1.0), relative to the moderate exercise o2 (∼65% of maximal), are indicative of a substantial contribution by nonoxidative metabolism to the total energy requirement of the bout. These gas exchange data observed during kettlebell exercise were of a similar pattern (high RER, moderate o2) to those previously reported during circuit weight training (2,5,8).
The kettlebell exercise in this experiment produced an increased HR that was substantially higher than the increase in o2. While the pattern of the relationship between %HRmax and %o2max was similar to those previously reported during circuit weight training (2,5,8), the manmaker drill required a greater oxygen consumption and HR values than previously reported (approximately 30-47% o2max 62-76 %HRmax) for circuit weight training (2,5,8). The slope of the linear regression predicting %o2max from %HRmax during the kettlebell exercise was substantially lower (0.714) than the slope previously reported (1.369) during treadmill running (7) but higher than the slope reported (0.582) during circuit weight training (6). Thus, the “man-maker” kettlebell drill appears to impart a greater challenge to cardiorespiratory system than has been shown with traditional circuit weight training.
Kettlebells provide a useful conditioning tool for coaches. Performing 2-handed kettlebell swings in the “man-maker” drill can improve the cardiorespiratory fitness of athletes. However, coaches should be aware that HRs achieved during continuous kettlebell exercise are significantly higher than the corresponding exercise o2 demand. Furthermore, the relationship between the 2 variables is greater than that noted for circuit weight training but less than that for treadmill exercise.
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