Kettlebell (KB) exercise is enjoying a resurgence as an alternative methodology for training both the muscular system and cardiovascular system. The various forms of KB exercises typically emphasize more of a total body approach to the movements (4,10). Among the exercises performed with KB, the 2-handed swing is commonly regarded as the foundational training maneuver using this mode of exercise (4,10). Despite the popularity of this form of exercise, limited research appears to have been done regarding the metabolic demand of various KB swing exercise routines. Some studies support the exercise as a viable alternative cardiovascular training procedure (7–9,17), whereas others have noted that it does not provide sufficient aerobic stimulus to be useful (3,13). Methodological problems may explain the lack of sufficient cardiovascular stimulus noted in several studies because the subjects used both light and heavy KBs and performed relatively short-duration exercise periods (3,13). In some of the studies showing higher metabolic intensity, various exercises were used in cyclic fashion making it difficult to isolate the metabolic cost of particular exercises (7,9,17).
Given the popularity of the KB swing (4,10) and its use as a basic exercise in this modality, it might be beneficial to strength and conditioning specialists to assess the energy cost and cardiorespiratory demand of this exercise relative to a more conventional form of aerobic exercise. If the KB swing provides sufficient aerobic demand to suggest it can produce a training effect, it might be considered an acceptable alternative to more traditional modes of exercise and serve as a viable means of crosstraining. Therefore, the purpose of this study was to compare the metabolic demand of a typical KB swing routine with a treadmill (TM) run at an equivalent rating of perceived exertion (RPE).
Experimental Approach to the Problem
To assess the metabolic demand of KB swings and compare them with TM running, moderately trained subjects volunteered to perform a 10-minute swing routine using a 16-kg KB (men) or 8-kg KB (women). The subjects were familiarized with the swing procedures under the direction of a certified KB instructor before measurement (10). The routine consisted of 35-second swing intervals followed by 25-second passive rest intervals. After a minimum of 48 hours of rest, the subjects completed a 10-minute TM run at equivalent RPEs as measured during the swing workout. Heart rate (HR) and RPE were assessed at minutes 5, 7, 9, and 10 during each exercise session. Metabolic data were monitored each minute during each exercise using an automated open-circuit unit. The first 3 minutes of each routine were discarded, and the remaining 7 minutes used for analysis of the aerobic component of the exercise.
Thirteen subjects (11 male, 2 female) volunteered to participate. The subjects were moderately trained but had no experience exercising with KBs. Before testing, each subject was familiarized with the swing procedures (4,5,10), and technical corrections were given by a certified KB instructor. The participants were informed of the risks and benefits of the testing program and signed an informed consent document before testing. All the testing protocols were approved by the university's Institutional Review Board for Studies Involving Human Subjects. Physical characteristics of the subjects by group are shown in Table 1.
During the first test session, each subject completed a 10-minute KB swing routine consisting of 35-second swing intervals followed by 25-second rest intervals. These intervals were selected based on past performance criteria (9). Men used a 16-kg KB, and women used an 8-kg KB (10). The subjects were not assigned specific swing counts but were encouraged to maintain a steady rhythm throughout the 10 minutes; swing counts were monitored during each interval using a hand-held counter to ensure a constant rhythm. Continuous feedback was given by the certified KB instructor to maintain correct form.
After a minimum of 48 hours of rest, the subjects completed a 10-minute TM run (Quinton, model Q45, Bothell, WA, USA) at equivalent RPEs as measured during the swing workout. Heart rate was monitored using a chest-strap telemetry system (Polar USA, Lake Success, NY, USA). The HR and RPE were assessed at minute 4 to 32% (+/− 10%) at minute 10 (Figure 2). Treadmill speed was adjusted to maintain an equivalent RPE to that noted during the KB swings. Metabolic data were monitored each minute during each exercise using an automated cart (Truemax 2400, Parvomedics, Salt Lake City, UT, USA). The first 3 minutes were discarded, and the remaining 7 minutes were used for analysis.
A 2 × 7 (routine × time) mixed factorial analysis of variance was used to identify significant differences in the dependent variables. When significance at p ≤ 0.05 was noted, a Bonferroni post hoc test was used to isolate the differences. Statistical power ranged from 0.47 to 0.99 for all analyses. Pearson correlations were calculated to determine the relationships between selected variables. Significance was established at the 0.05 level.
The RPE and HR increased by 2–3 and 7–9%, respectively, across the 4 measurements for each exercise, producing a significantly increasing pattern for each exercise but no significant difference between them (Figure 1). The HR and RPE were not significantly correlated at any time point for either KB swings (r = −0.01 to −0.51) or TM running (r = −0.08 to 0.48) nor when all time points were collapsed (TM: r = 0.28; KB: r = 0.09). The HR averaged 85–93% of age-predicted HRmax, whereas the RPE averaged 76–77% of maximum.
Oxygen consumption, METS, pulmonary ventilation, and calorie expenditure were significantly higher for TM running (25–39%) than for KB swings (Table 2). Oxygen consumption was significantly higher for TM running at each time point, producing a significant interaction effect ranging from 19% (±12%) at minute 4–32% (±10%) at minute 10 (Figure 2). The RER also had a significant interaction, becoming significantly higher during the TM run for each successive minute after minute 4; the KB RER remained relatively constant throughout the exercise session (Figure 3).
This study indicated that when KB and TM exercises were matched for RPE, the subjects were likely to have higher oxygen consumption, work at a higher MET level, and burn more kilocalories per minute during TM running than during KB swings. However, according to American College of Sports Medicine (ACSM) standards (1,2), this KB routine could provide sufficient exercise stress to produce gains in aerobic capacity since the %HRmax exceeded 85% in most of the subjects. Therefore, on days when a subject wanted an alternative to TM running or stationary cycling, KB swings might be substituted to maintain cardiovascular training levels (7,8,17).
Ancillary benefits to KB swing training may be related to its potential to enhance core muscular strength. Jay et al. (11) evaluated the effect of an 8-week KB training program on industrial workers. Training progressed from dead lifts with KB to double-and single-arm swings. Work-to-rest ratio decreased from the initial weeks of 30 seconds of work followed by 60 seconds of rest to 30 seconds of work and 30 seconds of rest by week 5. Isometric back extensor muscle strength increased significantly, whereas shoulder elevation strength and trunk flexion strength did not. This seemed reasonable because both the dead lifts and swings specifically activated trunk extensor muscles more than flexors (15). An ancillary benefit reported by the authors was a reduction in reported back and neck pain. Brumitt et al. (6) noted that the KB exercises are especially useful in a rehabilitation process using momentum actions because the handle allows swinging in a curvilinear motion, which may be more difficult to perform with traditional dumbbells. Given the variety of exercises that can be employed with KBs (6), it seems reasonable to assume that various specific strength or muscle endurance improvements might be possible. The degree to which these 2 performance areas might improve with KB training remains to be investigated.
A controversial area that has been noted with regard to KB training centers on the effect on aerobic capacity or conditioning. Jay et al. (11) found no improvement in aerobic fitness (i.e., V[Combining Dot Above]O2max) after KB training. Two factors might help explain the lack of aerobic improvement. The program they employed consisted of 5–10 minutes of warm-up followed by 10–15 minutes of interval KB training. This duration may not be sufficient to invoke a significant change in aerobic capacity. Furthermore, Jay et al. (11) measured aerobic fitness using a submaximal cycle ergometry test, which may not have been sensitive enough to detect changes in V[Combining Dot Above]O2max. Lanier et al. (13) found that a 5-set, 10-repetition sequence of 2-arm swings, snatches, and clean-and-press exercises provided an aerobic training stimulus of only 32% of V[Combining Dot Above]O2max, which is below the recommended aerobic training level (1). Bishop et al. (3) felt that the low aerobic stress level of swings and snatches might be because of the momentum imparted to the KB that reduced the work demand on muscle. Yet, other studies have noted that various KB routines offer sufficient training stimulus to produce improvements in aerobic capacity (7–9,17). Part of this controversy could stem from the varied training programs with regard to load, lifting rate, and duration. Fung and Shore (9) suggested that a KB weight ≤13 kg should be used to maintain the aerobic nature of the exercise by holding the RER under 1.00. Most of the subjects in this study maintained RERs <1.00, with only 2 subjects produced RERs >1.00. Obviously, further work in this area may be required before a definitive statement can be made with regard to the aerobic training benefit of KB work.
In a recent study determining the metabolic cost of a widely used KB swing routine, the subjects performed as many swings as possible in 12 minutes in a self-paced cadence (8). The subjects averaged 22 swings per minute, which was the approximate self-selected rate noted in this study (22–25 swings per minute). Their average oxygen consumption (34.3 ± 5.7 ml·kg−1·min−1) was not significantly different from that noted in this study (34.1 ± 4.7 ml·kg−1·min−1). Although the average HR in the previous study (165 ± 13 b·min−1) was significantly lower than in this study (177 ± 11 b·min−1), the %HRmax was similar between the previous (86.8 ± 6.0 b·min−1) (8) and current studies (89.1 ± 5.3%). The results of these 2 studies would suggest that the exercise intensity is sufficient to produce aerobic capacity gains. The major question remaining might be to determine the duration of the exercise that would produce meaningful gains in V[Combining Dot Above]O2max when using a KB swing routine.
An addition factor supporting the current 10-minute KB swing routine for meeting the ACSM recommendations for moderate physical activity (1,2) was the caloric expenditure noted during the steady-state portion of the activity. The current caloric expenditure was 1.7 times greater than a modified ACSM single-set resistance training routine (16) and required 60% less time to achieve (10 vs. 24 minutes). It could be argued that the 2 protocols were expressly different in that the modified ACSM resistance protocol had subjects perform a 15 RM with each of the 8 exercises and rest 2 minutes between sets (16). This approach may be designed more for muscle strength and endurance improvement than for caloric expenditure. Perhaps a more suitable comparison might be with a continuous 28-minute functional resistance protocol using resistance levels of 11.2 kg for men and 6.5 kg for women (12). That protocol produced a total caloric expenditure of 289 kcal at an average of 10.2 kcal·min−1, which was 18–23% less than the values produced in this study despite the longer duration. The subjects' RPE on the OMNI scale for resistance exercise (5.9 ± 1.5) was rated as “somewhat hard.” In this study, the RPE on the Borg scale was 15.4 (±1.2), which could be rated as “hard.” This may point to the difficulty of assessing and comparing the “stress” imposed by various resistance training modes, especially when relying on subjective rating.
Regarding the increased metabolic costs of the TM run compared with the KB swing routine, it should be noted that the intensity of the TM run was not consistent with normal training methods. For the participants to match the increasing RPEs noted during the KB routine, they continued to self-select an increasing speed of the TM near the end of the 10-minute run. Many individuals do not train in this manner, choosing instead to maintain a steady pace for the majority of a run. If this strategy had been adopted here, the KB swing routine would have likely produced more comparable values for many of the metabolic variables, as compared with the TM run.
Finally, it is obvious with the KB swing, as with any weight training exercise, care should be taken to execute the maneuver in proper form. Foremost in the procedure should be the proper position of the back during the entire movement. McGill and Marshall (15) recently reported muscle activation of around 50% of maximum voluntary contraction in the back muscles during the swing. These authors indicated that the inertia created by the swing could place some individuals at greater risk of sheer forces on the lower back; however, they noted moderate lumbar spine compression loads and deemed them below levels for clinical significance or concern. They did emphasize that spine stability and alignment were essential in performing the swing to make it a beneficial rather than detrimental exercise. This would require a “neutral” back position throughout the exercise and resist allowing the upper back to curve as the individual becomes fatigued. For novice lifters, a progressive technique might serve to facilitate the learning of proper spinal alignment and prevent damage to muscle and bone structures of the lower back (10,14,15).
The KB swing exercise completed in this study can be considered a basic skill, which can be used by beginners as well as experts. The intensity of this 10-minute routine was sufficient to meet current criteria for aerobic capacity development (1) and could offer an alternative training method to more conventional techniques. Other factors to be explored with regard to the aerobic training potential of KBs might be the duration of the swing phase vs. the rest phase, the weight of the KB, the advanced level of other swing techniques (15), and the overall duration of the training session to produce optimal benefit.
Kettlebell swing exercises appear to activate muscle groups sufficiently to impart a strength training effect. The effect of inertia on the duration of muscle activation during a movement cycle (6) may need further exploration using electromyography to assess the degree of muscle strength improvement (6,15). Throughout the use of KB exercises, care must be emphasized to maintain proper form and spinal alignment (14), especially among novice lifters. Furthermore, the effect of different weights of KBs on muscle strength and power development should be explored using various measurement techniques to fully understand the impact of this form of training.
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