Recent years have provided strength and conditioning professionals with a variety of athletic preparation tools and exercise modes. Among such tools is the kettlebell, an implement used in Russia for many years before its recent popularization in the United States (35). Currently, kettlebell exercises are used in various recreational fitness programs and in some collegiate and professional strength and conditioning programs. The offset center of gravity and unique construction of the kettlebell provide the user a tool that is suitable for ballistic full-body exercise (15). Only a few studies have investigated the efficacy of kettlebell training. Based on these studies, it seems that kettlebell training can improve maximal and explosive strength (20) and performance in Olympic weightlifting and powerlifting (22,26). Furthermore, kettlebell exercise involves an aerobic demand that is likely to lead to improvement in maximal oxygen uptake (8,14); thus, kettlebell training might provide for simultaneous endurance and resistance training.
The neuromuscular adaptations to resistance training are modulated by the endocrine milieu. Supraphysiological doses of testosterone (T) augment strength training–induced increases in muscle size and strength in healthy men (4). Conversely, when endogenous T production is suppressed, lean body mass and muscular strength improvements in response to resistance training are attenuated (19). Concentrations of circulating T, growth hormone (GH), and cortisol (C) are increased acutely in response to a bout of heavy-resistance exercise involving a large muscle mass (16,18,21,32). This acute hormonal response seems to be relevant for long-term adaptations because some studies have demonstrated greater increases in strength and muscle mass from training using exercises that elicit a greater acute hormonal response (12,29). Designing exercise training protocols that induce a large acute hormonal response might, therefore, be important in maximizing physiological training adaptations.
The magnitude of the acute hormonal response to resistance exercise depends in large part on the acute program variables, including intensity (27,30), volume (11,28,32), duration of the rest period (1,17), and exercise mode (34). Importantly, resistance exercises using greater amounts of muscle mass or resulting in greater volume of work elicit a larger hormonal response than those using a smaller amount of muscle mass (12,25,38). Resistance exercises using small amounts of muscle mass (e.g., upper body exercises) exhibit little to no hormonal response (25,33,41) and thus might not augment physiological adaptations in strength and muscle mass to the same extent of large muscle mass exercises (12,29). Appropriate exercise selection, particularly inclusion of resistance exercises that involve a large muscle mass, is important to maximize the acute hormonal response.
The kettlebell swing is a specific exercise in which the participant swings a kettlebell from between the legs to approximately eye level (35). This exercise is performed with neutral spinal alignment and relies on the muscles of the posterior chain (i.e., gluteal, hamstring, and back extensor muscles) as the prime movers. Thus, the kettlebell swing involves a large muscle mass and results in substantial muscular activation in the posterior chain and abdominal muscles (24). Because of this large muscle mass involvement (as compared with exercises with smaller muscle mass involvement; e.g., push-ups), the kettlebell swing exercise could induce a large acute increase in hormones that are involved in modulating neuromuscular adaptations to training (e.g., improvements in performance). Despite the widespread use of kettlebell training among strength and conditioning professionals, and the importance of the acute hormonal response to subsequent resistance exercise adaptations, the hormonal response to kettlebell exercise has not been examined. Therefore, the purpose of this study was to examine the acute T, GH, and C response to a kettlebell swing exercise session.
Experimental Approach to the Problem
This investigation evaluated the acute hormonal response to a kettlebell swing protocol that has been used in previous investigations of physiological responses and adaptations to the kettlebell swing exercise (15,20). Ten recreationally resistance trained men performed 12 rounds of 30 seconds of kettlebell swings alternated with 30 seconds of rest; Lake and Lauder (20) have previously demonstrated that training using this exercise protocol improves lower-body strength and power. A 16-kg kettlebell (Perform Better, Cranston, RI, USA) was used by all participants because it is the recommended starting weight for men (35) and has previously been used in evaluating the effect of kettlebell swings on strength training adaptations and acute aerobic responses (8,20). Fasted blood samples were collected using intravenous catheter before the warm-up (PRE), immediately postexercise (IP), 15 minutes postexercise (P15), and 30 minutes postexercise (P30) and were analyzed for T, GH, C, myoglobin, and lactate concentrations.
Ten apparently healthy men (19–30 years; mean ± SD: 24 ± 4 years, 175 ± 6 cm, 78.7 ± 9.9 kg) volunteered for this study. All participants were informed of the risks and benefits of participation in the study and subsequently provided written informed consent. The study and all procedures described in this article were approved by University's Institutional Review Board. Volunteers were screened with a medical and physical activity history questionnaire to help determine their eligibility for the study. To be considered for this study, participants had to have engaged in resistance exercise ≥2 times a week for at least the previous 3 months, have no history of anabolic steroid use, have no experience with the kettlebell swing exercise, and have no injuries or limitations that would prevent exercise involvement.
Session 1: Anthropometric Measurements and Familiarization
During session 1, height, body mass, and body composition were measured. Body composition was determined using dual-energy x-ray absorptiometry (Lunar Prodigy General Electric Company, Madison, WI, USA). Participants then performed a standardized 10-minute dynamic warm-up that consisted of lying knee to chest pulls, lying straight leg raises, quadruped scapulae retraction/protraction, quadruped thoracic rotations, quadruped hip circles, walking lunges with hip flexor stretch, lying hip abduction/external rotations, and glute bridges. After the warm-up, participants were familiarized with a proper kettlebell swing exercise technique and safety procedures. All swings were performed on a nonslip rubberized surface. Closed-toed shoes and athletic clothing were worn during all sessions.
Session 2: Experimental Kettlebell Swing Exercise Session
Approximately 7 days after session 1, participants arrived at the laboratory in a fasted condition in the early morning (0600–0700 hours) to complete the kettlebell swing exercise session. Leading up to the session, participants were refrained from (a) eating or drinking anything (except for water) for 12 hours, (b) ingesting any alcohol for 48 hours, (c) engaging in sexual activity for 24 hours, and (d) performing any resistance exercise or intense aerobic exercise for 72 hours prior. Additionally, participants were asked to drink 0.5–1 L of water the night before the session to help ensure adequate hydration upon arrival at the laboratory. Upon arrival to the laboratory, participants provided a urine sample for determination of hydration status using refractometry (Reichert, Depew, NY, USA). Participants who were not euhydrated (urine specific gravity >1.019 g·mL−1) were provided cold water to drink. After determination of hydration status, participants were provided a standard heart rate (HR) monitor chest strap to wear. A teflon catheter was subsequently inserted into an antecubital vein for the collection of blood samples (20 ml) using evacuated tubes at PRE, IP, P15, and P30. The catheter remained in the vein for the duration of the session and was kept patent with sterile saline. After the insertion of the catheter, the PRE blood sample was obtained and the participant performed the dynamic warm-up described under session 1. Immediately thereafter, the participant began the kettlebell exercise bout.
For the kettlebell exercise bout, participants completed 12 consecutive rounds of kettlebell swing exercise with each round consisting of 30 seconds of exercise followed by 30 seconds of rest. Investigators provided verbal encouragement to the participant to complete as many swings in the 12-minute period as possible. The number of repetitions performed in each round was counted and recorded using a handheld counting device (Fisher Scientific, Pittsburgh, PA, USA). Immediately after the completion of the exercise bout, the participant was seated and the IP blood sample was obtained. The participant remained seated comfortably for the remainder of the session, including during the collection of the P15 and P30 blood samples. Heart rate was measured before exercise and at the end of each 30-second round of swings (e.g., immediately after participant set the kettlebell down on the ground) through telemetry using a heart rate monitor (Polar, Lake Success, NY, USA). A rating of perceived exertion (RPE) was obtained using Borg's 6–20 category scale at the end of each 30-second round (5).
Serum samples were allowed to clot at room temperature. Samples were then centrifuged at 1,500g for 15 minutes at 4° C, and the resultant serum were aliquoted and stored at −80° C until analysis. Samples were analyzed in duplicate for T, GH, and C (Alpco, Salem, NH, USA) using commercially available enzyme-linked immunosorbent assays. Assay sensitivity was 0.08 nmol·L−1, 0.5 μg·L−1, and 11.0 nmol·L−1, and intra-assay variances (%CV) were 3.4, 8.1, and 4.5% for T, GH, and C, respectively. An automated analyzer (Lactate Plus; Nova Biomedical, Waltham, MA, USA) was used to measure lactate concentrations. Hart et al. (13) have previously validated the Lactate Plus analyzer against a standard bench top method.
Data were analyzed using 1-way ANOVAs with repeated measures on time (IBM SPSS Statistics version 20, Chicago, IL, USA). Fisher's least significant difference post hoc test was used where appropriate to determine pairwise differences. For T and GH, Mauchly's test indicated that the assumption of sphericity had been violated (T: χ2 (5) = 12.3, p = 0.032; GH: χ2 (5) = 11.5, p = 0.044); therefore, degrees of freedom were corrected using Greenhouse-Geisser estimates of sphericity (T: ε = 0.53; GH: ε = 0.54). For C, Mauchly's test indicated that the assumption of sphericity had been met (T: χ2 (5) = 8.05, p = 0.156). The alpha level was set at p ≤ 0.05. Data are presented as mean ± SD unless otherwise noted.
Participants completed a total of 227 ± 23 swings for an average of 19 ± 2 swings per round (30 seconds of exercise and 30 seconds of rest). The number of swings per round did not change significantly during the experimental exercise session. The HR and RPE data are presented in Table 1. After each round of swings, HR and RPE were significantly higher (p ≤ 0.05) than at the previous round (or PRE) and thus peaked for the final round of swings. The HR averaged across all rounds was 157 ± 13 b·min−1 (80 ± 7% of age-predicted HRmax [220—age]). Blood lactate concentrations (PRE: 1.1 ± 0.5; IP: 7.0 ± 3.0; P15: 4.0 ± 2.7; P30: 2.5 ± 1.8 mmol·L−1) were higher at each time point after exercise than at PRE; blood lactate concentrations at P15 were lower than at IP; and blood lactate concentrations at P30 were lower than at P15 and IP.
Results for T concentrations are shown in Figure 1. Compared with PRE, T concentrations were higher at IP (PRE: 27.5 ± 10.7; IP: 32.4 ± 11.8; P15: 28.5 ± 10.7; P30: 26.5 ± 10.4 nmol·L−1). The T concentrations were lower at P15 and P30 than at IP; T concentrations at P15 were higher than at P30.
Results for GH concentrations are shown in Figure 2. Compared with PRE, GH concentrations were higher at IP, P15, and P30 (PRE: 0.1 ± 0.1; IP: 1.8 ± 1.2; P15: 2.2 ± 1.2; P30: 1.5 ± 1.3 μg·L−1). The GH concentrations did not differ among IP, P15, and P30.
Results for C concentrations are shown in Figure 3. Compared with PRE, C concentrations were higher at IP and P15 (PRE: 617.2 ± 266.2; IP: 893.8 ± 354.2; P15: 874.6 ± 243.2; P30: 645.0 ± 285.3 nmol·L−1). Cortisol concentrations did not differ between IP and P15. Cortisol concentrations at P30 were lower than at IP and P15.
This is the first study to examine the acute hormonal response to the kettlebell swing exercise and provides unique physiological insight into this exercise mode. The major novel finding of this study was that serum T, GH, and C concentrations were elevated immediately after a kettlebell exercise session (12 rounds of 30 seconds of swings alternated with 30 seconds of rest). Furthermore, it was found that measures of metabolic demand (HR, RPE, and lactate concentration) were increased over the course of the kettlebell exercise bout. It seems that the kettlebell swing exercise, even when performed for a relatively short duration (11.5 minutes in total: 6 minutes of exercise and 5.5 minutes of rest), can induce a neuroendocrine response. Because no previous study has investigated the endocrine response to kettlebell exercise, direct comparisons to prior research on the response to this exercise mode is not possible; thus, we will make comparisons between the findings of this study and the results of studies that evaluated acute endocrine responses to resistance exercise.
Recently, there has been a debate concerning the physiological importance of the acute hormonal response to resistance exercise for long-term strength and hypertrophy adaptations. Some studies have demonstrated augmentations to training adaptations (i.e., greater increase in strength and hypertrophy) following resistance training programs that elicit a greater acute hormonal response (12,29); however, other studies have found no relation between the acute hormonal response and subsequent training adaptations (39,40). There are limitations to arguments on both sides of the debate such as small sample sizes and inherent weaknesses in study design (e.g., implementation of between vs. within-subject models) that make interpretation of findings difficult. Though it is beyond the scope of this discussion to continue the debate, it is important to recognize the potential validity of both perspectives. If the acute hormonal response to resistance exercise does indeed augment strength and hypertrophy adaptations to training as several studies have found (12,29), then determining how the acute program variables, such as exercise mode, affect the acute hormonal milieu is important. This knowledge would especially benefit the strength and conditioning practitioner implementing training programs. With the advance of medical science, it is becoming clear that many previously unexplained or disregarded physiological responses occur to serve specific biological purposes. Therefore, if the acute hormonal response, as some authors suggest, does not impact subsequent strength and hypertrophy training adaptations, then further research that explores other potential physiological impacts of this acute increase is warranted. So, regardless of the effect of the acute hormonal response to exercise on strength and hypertrophy adaptations, the findings of this study add important new knowledge by showing that the kettlebell swing exercise can induce an acute hormonal response. The molecular mechanisms that define the physiological response to resistance exercise are indeed complex and intertwined; no single mechanism accounts for the entirety of physiological adaptation. Though the acute hormonal response might not account for a substantial portion of the variance in phenotypic changes, it is unlikely that an acute hormonal response confers no physiological effects.
Among its biological functions, T serves as a potent anabolic-androgenic hormone that stimulates muscle protein synthesis and inhibits protein degradation (6,23,37). Deficiency of T in young men has been associated with decreased strength (23) and with reductions in strength outcomes from resistance training (19), whereas supraphysiological doses of T have been associated with increased muscle strength and hypertrophy (4). Circulating concentrations of T increase acutely in men after performing resistance exercise using sufficient intensity, volume, and muscle mass. In this study, the kettlebell swing exercise elicited an increase in T immediately after the exercise. The external resistance used in the kettlebell swing (16 kg) was much less, and the number of repetitions was higher, than that used in traditional lower-body free-weight resistance exercise. But the kettlebell swing still provided sufficient stimulus—a combination of intensity, volume, and muscle mass involvement—to induce a postexercise increase in serum T concentration. Although the magnitude of the increase in T concentrations in this study was similar to that found in previous studies investigating free-weight resistance exercise (12,21,29,36), the duration of this increase in T after exercise was shorter than that found in some studies (12,16). It is not currently known whether the duration of the acute T response is important for long-term training adaptations.
Human GH is a large family of polypeptides that supports a variety of physiological functions (including musculoskeletal development), and its concentration increases acutely in response to the metabolic demands of exercise. Although the 22 kDa is the most abundant isoform of GH (3), there are likely more than 100 GH isoforms in circulation (2) The biological activities of the various GH isoforms (e.g., GH aggregates such as dimers) seem to be greater than that of the 22 kDa GH isoform; however, the acute response of such GH aggregates to resistance exercise has not been fully elucidated; Most studies (including this study) investigate the 22 kDa isoform. Consistent with previous findings for the acute GH response to intense resistance exercise (11,21), serum GH was elevated immediately after the kettlebell exercise session used in this study. Though statistically significant, the magnitude of the increase in GH was relatively small in comparison with findings for heavy-resistance exercise protocols (29,32). It has been suggested that GH is secreted in response to the decreased blood pH that is associated with high blood lactate concentrations after intense anaerobic exercise (7,10). In this study, lactate was elevated immediately postexercise, but the increase was smaller than that commonly found for heavy-resistance exercise protocols (32). Thus, the smaller increase in lactate could help explain the smaller increase found for GH. Smilios et al. (32) compared the acute hormonal response of different resistance exercise protocols (different number of sets, number of repetitions, and relative intensities) and found the greatest magnitude of increase in GH and lactate with 4 sets of 15 repetitions at 60% of 1 repetition maximum (1RM) (high volume) and the smallest magnitude of increase with 2, 4, or 6 sets of 5 repetitions at 88% of 1RM (lower volume). Results for GH and lactate in this study are most similar in magnitude to the results of the groups completing sets of 5 repetitions in that study. Additionally, Ghanbari-Niaki et al. (9) examined the GH response to circuit resistance exercise in recreationally trained college men. Using a 16-minute full-body circuit training protocol (10 exercises at 35% of 1RM, 20 seconds for each exercise, 1-minute rest between circuits), they found a significant increase in GH lasting through the 30-minute postexercise period. In this study, GH concentrations after exercise were similar in pattern to, but lower in magnitude than, those found by Ghanbari-Niaki et al. (9). The lower GH response found in this study was likely because of the lower volume and anaerobic metabolic demand compared with that of studies using traditional free-weight exercise, as evident by the smaller peak circulating lactate concentrations (7.0 ± 3.0 mmol·L−1) for the present kettlebell exercise bout.
Released in response to stressors such as resistance exercise, C has potent anti-inflammatory, lipolytic, and catabolic effects. In muscle, C downregulates protein synthesis by reducing p70S6K phosphorylation and androgen receptor signaling (31). Similar to T and GH, C increases acutely after a bout of heavy-resistance exercise in relation to the total volume and relative intensity of work performed. In agreement with previous studies (11,32), C was elevated immediately after the kettlebell swing exercise session. However, C returned to PRE concentrations by P30, which was in contrast to other studies using high-intensity resistance exercise protocols, where C remained elevated at 30 minutes after the exercise; additionally, the magnitude of the increase in C concentrations was lower in this study (1,16). Again, this smaller hormonal increase was likely because of the lower metabolic demand during the kettlebell exercise.
In summary, the key findings of this study were that a single 12-minute session of the kettlebell swing exercise increased T, GH, and C for up to 30 minutes after exercise. Though the kettlebell swing exercise produced a significant increase in all hormones measured, the magnitude and duration of the increase were smaller than what others have reported with heavy–lower-body resistance exercise protocols. Given the role that hormonal responses play in the development of strength and muscular growth, the findings of this study suggest that kettlebell swing exercise might contribute to improvements in strength and muscle mass from resistance training. More research is needed to fully evaluate the use of kettlebell swing exercise as 1 component in a comprehensive resistance exercise program.
The kettlebell swing exercise is a ballistic full-body exercise, which if performed over time (i.e., training), can increase muscle strength and potentially cardiovascular endurance; thus, this exercise mode provides a somewhat unique physiological stimulus. Independently, traditional resistance training programs are likely to produce greater increases in maximal strength and hypertrophy than kettlebell swing exercise programs. However, the addition of kettlebell swing exercise to resistance training programs might augment strength and hypertrophy outcomes through additional acute increases in important neuroendocrine factors, especially if the kettlebell exercise is implemented on days for which the traditional training (e.g., “low repetition, long-rest days”) does not induce a substantial hormonal response. Furthermore, adaptations from a resistance exercise session occur not just on the day of exercise but also across the days after. Thus, acutely increased hormones by the use of the kettlebell swing exercise protocol on days after resistance exercise sessions could potentially augment the adaptations from resistance exercise. In addition, the aerobic demand of the kettlebell swing exercise might augment outcomes in strength and conditioning programs in which increased aerobic capacity is desired. In conclusion, the strength and conditioning professional might find the kettlebell swing exercise to be a useful addition to the toolbox for athletic development.
This study was supported in part by research grants from the Texas Chapter of the American College of Sports Medicine and the College of Education at the University of North Texas (RGB). The authors declare that they have no conflict of interest.
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