Introduction
In recent years, high-intensity interval training (HIIT) has become a popular modality for exercise training. According to the American College of Sports Medicine (ACSM), HIIT was reported to be the top fitness trend worldwide for 2014 (27 28 19 1–3,16 1–3,16 1–3,16 1–3,16 4,5,8,9,17,18,29,30 4,5,8,9,17,18,29,30 4,5,8,9,17,18,29,30 4,5,8,9,17,18,29,30 4,5,8,9,17,18,29,30 4,5,8,9,17,18,29,30 4,5,8,9,17,18,29,30 4,5,8,9,17,18,29,30 4,17,18 4,17,18 4,17,18 20 20
A modality often used by HIIT exercisers is sprint interval cycling (SIC). In a study by Freese et al. (2013), cardiorespiratory responsiveness was examined during SIC with participants performing 4 succeeding 30 second sprints, each followed by a 4 minute active recovery. Oxygen consumption (V[Combining Dot Above]O2 ) increased from the first sprint to the second, but was similar in the succeeding sprints and was above 80% of the estimated maximal value. Respiratory exchange ratio (RER) also was observed to decline from the first to the last sprint. The findings of this previous study demonstrate the extent to which aerobic metabolism is used during this type of HIIT and suggests that metabolic and cardiorespiratory adaptations can occur through SIC (8
Modes of resistance training are often integrated with interval-based exercise training. One popular mode is the use of kettlebells (KBs). Kettlebells not only have been shown to be effective in improving muscular strength (12,15,21,31 12,15,21,31 12,15,21,31 12,15,21,31 5,6,26 5,6,26 5,6,26 2 max) and an average of 86.8% of maximal heart rate (HRmax ) responses, which meet ACSM recommendations for optimal intensity for improving cardiorespiratory fitness (6 −1 ·min−1 or ∼6%) than circuit-weight training of the same training duration and frequency (5 5
The cadence of the movements with the added resistance of the KB makes it possible for specific KB exercises to elicit greater responsiveness of the cardiorespiratory system. It is even possible for KB training to be adjusted to be performed continuously, with rest periods and alternation of exercises integrated within the protocol to reduce fatigue. In a study by Thomas et al. (2014), a KB protocol at an intensity level of ∼60% of V[Combining Dot Above]O2 max was compared with graded-treadmill walking at roughly the same intensity. The protocol involved completing 10 repetitions of KB swings followed by 10 repetitions of KB sumo deadlifts at a set metronome cadence. This was performed continuously for 3 circuits of 10-minute duration with 3 minutes of rest in between. It was found that not only was the KB protocol comparable in intensity level to graded-treadmill walking, but also the KB protocol elicited a higher average HR and rate of perceived exertion (RPE) (26
A Tabata regimen is commonly used in HIIT and is a regimen that could be modified for use with KBs. This routine involves performing exercise intervals of 20 second durations with 10 seconds of recovery in between and is based on research by Tabata et al., who examined the acute cardiorespiratory and metabolic responses of high-intensity intermittent cycling and its training adaptations (24,25 24,25 Tabata routine in comparison to moderate-intensity aerobic exercise elicited a greater percentage of V[Combining Dot Above]O2 max (170% vs. 70%) and produced a 28% increase in anaerobic performance (25 Tabata training principles would be as effective as a standard SIC protocol used for HIIT. We did this by comparing the cardiorespiratory and metabolic responses of the respective protocols. It was hypothesized that the KB-HIIT protocol and SIC protocol would elicit similar cardiorespiratory and metabolic responses. If verified, the study could lead to the use of a new form of HIIT using KBs by athletes and recreational exercisers that would provide an inexpensive and time-saving form of training to improve health status and aerobic performance.
Methods
Experimental Approach to the Problem
Subjects first completed 2 preliminary sessions: an informational and familiarization session and a session to perform assessment of cardiorespiratory fitness and anthropometric measures. This was followed by completion of 2 experimental sessions in a counterbalanced, randomized fashion: a KB-HITT session and a SIC session. There were 5–7 days between these sessions. V[Combining Dot Above]O2 , RER, tidal volume (TV), breathing frequency (f), minute ventilation (VE ), caloric expenditure rate (kcal·min−1 ), HR, and total caloric expenditure were compared between the KB-HIIT and SIC protocols. A within-subjects' design over multiple time points within the exercise protocols was used with most of the statistical analyses to reduce the variance of the cardiorespiratory and metabolic measures. The outcomes of this study should provide insight into the acute responses of the novel KB-HIIT protocol and allow its effectiveness for training to be determined.
Subjects
Eight men between 20 and 23 years in age completed the study. Descriptive characteristics of the subjects' baseline anthropometric and cardiorespiratory fitness data are shown in Table 1 . All subjects were volunteers, and before their participation, they provided informed written consent after they were given a description of the study procedures and associated risks. The subjects had current or previous experience in some sports (e.g., football, pole vault, shot put, cross country, cross-fit, olympic lifting, and power lifting) and were considered “very active” (based on the average V[Combining Dot Above]O2 max of 52.16 ± 6.55 ml·kg−1 ·min−1 ; range = 41.2–60.5 ml·kg−1 ·min−1 ). Most of the subjects had some previous experience with KB exercise and cycling; however, they were not considered trained KB athletes or cyclists.
Table 1: Baseline anthropometric and cardiorespiratory fitness level data of subjects (men = 8).
Each perspective subject completed a medical history questionnaire and excluded if they had a current upper body or lower body musculoskeletal injury, musculoskeletal impairment, or had cardiovascular, pulmonary, or metabolic disease. The subjects were asked to (a) refrain from vigorous physical activity within 24 hours before each session, (b) maintain their normal dietary habits, (c) stay hydrated the night before and the day of each session, and (d) sleep an adequate amount of time (∼8 hours) the night before each session. The study was approved by the Institutional Review Board of Southeastern Louisiana University.
Procedures
This study was conducted in the spring time of the year. All sessions were performed in the afternoon and each session lasted approximately 1 hour. A certified exercise physiologist and a certified athletic trainer were present during each session to ensure safety and procedural effectiveness.
In the first preliminary session, a certified KB instructor familiarized the subjects with proper form and lifting techniques of the KB exercises that they would perform. The 4 KB exercises used and the order of the exercises in the KB-HIIT protocol were (a) sumo squat (single-handed or 2-handed), (b) 2-handed swings, (c) clean and press (using the dominant arm), and (d) sumo deadlift (single-handed or 2-handed). Subjects practiced each KB exercise in a group setting for ∼1 hour while the certified KB instructor evaluated their individual performance. During this session, the appropriate weight for each subject was determined for each KB exercise that would allow them to maintain a lifting rate without reaching volitional exhaustion quickly. The subjects informed the researchers of the weight that felt most appropriate for each KB exercise. Weight ranges for the KBs used in each exercise were sumo squat = 18–22 kg, 2-handed swing = 16–22 kg, clean and press = 10–22 kg, and sumo deadlift = 16–22 kg. Additionally, before or after the KB exercise familiarization, the subjects were familiarized with the specialized leg cycle ergometer used for the SIC protocol. Each subject performed 1 SIC interval to become accustomed to the intensity of the SIC protocol.
In the second preliminary session, body fat percentage was calculated by the 7-site skinfold measurement procedure and corresponding equation (11 2 max and HRmax were determined from a graded-exercise test to exhaustion on a treadmill (Cardiac Science Quinton Q-Stress TM65 treadmill; Mortara Instrument, Inc., Milwaukee, WI, USA) using the Kraemer protocol (14
During the SIC experimental session, the subjects completed the protocol on a specialized leg cycle ergometer for sprint cycling (Monark Ergomedic 894E Peak Bike; Monark Exercise AB, Vansbro, Sweden). Resistance (by a plate-loaded weight basket) was applied to the flywheel of the leg cycle ergometer during each sprint interval. This resistance was calculated using 8.8% of the participants' fat-free body mass, a method used by Freese et al. (2013) to determine appropriate flywheel resistance during SIC (8 Figure 1A for the specifics of the SIC protocol.
Figure 1: A) Sprint interval cycling (SIC) protocol, no repeat. B) Kettlebell high-intensity interval training (KB-HIIT) protocol, performed 3 times.
Before the KB-HIIT experimental session, the subjects were again familiarized with the KB exercises being used. For the protocol, participants passively rested for 5 minutes to record resting HR. After the resting period, the participants walked on the treadmill for 3 minutes at a speed of 1.5 mph and no grade, and then practiced the 4 exercises with the KBs for 7 minutes. Heart rate was measured during the last minute of the treadmill walk and after practicing the KB exercises. After warming up, the participants began the KB-HIIT 12-minute session. The pattern of 20-seconds of exercise followed by 10-seconds of rest was repeated for each exercise (Figure 1B ). A metronome was used to standardize the cadence of the lifting and lowering of the KBs. The metronome was set at 40–44 b·min−1 . For each of the exercises, except for the KB swings, the participants were instructed to move 2 beats for the lift phase and then 2 beats for the lowering phase of each exercise. For the KB swings, the cadence was 1 beat for the lift phase and 1 beat for the lowering phase of the exercise. Once the participant completed all 4 KB exercises, the whole process was repeated 2 more times for a total of 3 circuits of the 4 KB exercises.
During each experimental session, V[Combining Dot Above]O2 , RER, TV, f, VE , and kcal·min−1 were measured continuously by the metabolic analyzer. During the KB-HIIT session, HR was recorded immediately after the 20 second exercise and then at the end of the 10 second recovery period. During the SIC session, HR was recorded at the beginning and immediately after each sprint and also every minute of recovery.
Statistical Analyses
Intraclass correlation coefficients were calculated to assess the reliability of the V[Combining Dot Above]O2 , RER, TV, f, VE , kcal·min−1 , and HR measures in each exercise protocol (Table 2 ). To examine the main effects of V[Combining Dot Above]O2 , RER, TV, f, VE , kcal·min−1 , and HR across 12 minutes of exercise between the protocols, a 2 (Group) × 12 (Time point) repeated-measures analysis of variance was used. To examine the total caloric expenditure difference between the exercise protocols, an independent t -test was used. All statistical analyses were performed using the SPSS for Windows, version 20.0 (IBM Corp., Somers, NY, USA), statistical software program using an alpha level of p ≤ 0.05.
Table 2: Intraclass correlation coefficients.
Results
A significant (p ≤ 0.05) group effect, time effect, and group × time interaction were found for V[Combining Dot Above]O2 , RER, and TV (Figures 2–4 Figures 2–4 Figures 2–4 ). However, only a significant (p ≤ 0.05) time effect and group × time interaction were found for f, VE , kcal·min−1 , and HR (Figures 5–8 Figures 5–8 Figures 5–8 Figures 5–8 ). The group main effects for V[Combining Dot Above]O2 [F(1,14) = 4.886, p = 0.044, partial eta-squared = 0.259, η2 = 0.539], RER [F(1,14) = 133.151, p < 0.001, partial eta-squared = 0.905, η2 = 1.00], and TV [(F(1,14) = 10.416, p = 0.006, partial eta-squared = 0.427, η2 = 0.851] revealed that there were significant differences between the KB-HIIT and SIC exercise protocols.
Figure 2: Oxygen consumption across the 12 minutes of exercise between the protocols. Values are expressed as mean ± SE . KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Figure 3: Respiratory exchange ratio across the 12 minutes of exercise between the protocols. Values are expressed as mean ± SE . KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Figure 4: Tidal volume across the 12 minutes of exercise between the protocols. Values are expressed as mean ± SE . KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Figure 5: Breaths per minute across the 12 minutes of exercise between the protocols. Values are expressed as mean ± SE . KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Figure 6: Minute ventilation across the 12 minutes of exercise between the protocols bouts. Values are expressed as mean ± SE . KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Figure 7: Caloric expenditure per minute across the 12 minutes of exercise between the protocols. Values are expressed as mean ± SE . KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Figure 8: Heart rate across the 12 minutes of exercise between the protocols. Values are expressed as mean ± SE . KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
In the KB-HIIT protocol across time, there was a higher (mean ± SE ) V[Combining Dot Above]O2 (22.6 ± 1.48 vs. 19.9 ± 1.01 ml·kg−1 ·min−1 ) and lower RER (0.93 ± 0.02 vs. 1.22 ± 0.04) and TV (1.7 ± 0.07 vs. 2.2 ± 0.15 L) than for the SIC protocol. V[Combining Dot Above]O2 was highest at the peak of each SIC interval in comparison to the KB-HIIT, but it was the lowest during the recovery periods (Figure 2 ). During the SIC, the average of the 3 highest V[Combining Dot Above]O2 values (38.4 ± 0.87 ml·kg−1 ·min−1 ) equated to 73.6% of V[Combining Dot Above]O2 max, in comparison to the KB-HIIT (29.1 ± 0.09 ml·kg−1 ·min−1 ) which was 55.7% of V[Combining Dot Above]O2 max. With the KB-HIIT, V[Combining Dot Above]O2 fluctuated in a specific pattern throughout the 3 circuits of this bout, but remained higher in comparison to the SIC throughout most of the 12 minutes of exercise. Throughout a majority of the 12 minutes of exercise, RER was higher during the SIC than during the KB-HIIT, especially during the initial minutes (Figure 3 ). As time progressed during the SIC, RER gradually declined. During the KB-HIIT, RER remained fairly constant and fluctuated little. With TV, this measure also was highest during the SIC and declined during the recovery periods of this exercise protocol; however, even during the recovery periods, it remained higher in comparison to the KB-HIIT (Figure 4 ). During the KB-HIIT, TV was lower and it fluctuated less throughout the duration of the protocol in comparison with the SIC.
Average HR (±SE ) was higher during the KB-HIIT protocol than the SIC protocol (149.16 ± 7.4 vs. 139.69 ± 7.85 b·min−1 ). Peak HR however was higher at the end of each SIC interval (Figure 8 ). During the SIC, the average of the 3 highest HR values (168 ± 2.21 b·min−1 ) equated to 88.5% of HRmax , in comparison to the KB-HIIT (166 ± 1.41 b·min−1 ) which was 87.5% of HRmax .
Analysis of the resulting total caloric expenditure during each exercise protocol using an independent t -test revealed a significant difference between the protocols [t(14) = 2.323, p = 0.036]. There was a greater average in total caloric expenditure during the KB-HIIT (144.87 ± 6.56 kcals) in comparison to the SIC (122 ± 7.34 kcals) (Figure 9 ).
Figure 9: Total caloric expenditure between the protocols. Values are expressed as mean ± SE . *Denotes significant difference between the protocols, p ≤ 0.05. KB-HIIT = kettlebell high-intensity interval training; SIC = sprint interval cycling.
Discussion
Findings from this study revealed that the hypothesis was partially supported. This is the first study to demonstrate that a KB-HIIT protocol can produce similar kcal·min−1 , VE , f, and HR responses over time as a standard SIC protocol, which is a well-known form of HIIT. Different from what was hypothesized, the KB-HIIT produced higher V[Combining Dot Above]O2 across time compared with the SIC, and the SIC was shown to elicit greater RER and TV across time. A comparison of each of the cardiorespiratory and metabolic responses to the KB-HIIT and SIC protocols relative to findings from previous studies suggests that KB-HIIT is an effective form of HIIT.
Although the SIC produced peak values for V[Combining Dot Above]O2 at 3 time points that were higher than those produced in the KB-HIIT, the KB-HIIT still elicited greater V[Combining Dot Above]O2 across time. This further suggests that KB-HIIT can be used effectively to promote cardiorespiratory and metabolic adaptations. The average V[Combining Dot Above]O2 (22.6 ± 1.48 ml·kg−1 ·min−1 ) for the KB-HIIT across time in this study only averaged 43.3% of V[Combining Dot Above]O2 max, but this measure was affected by 10 seconds of passive recovery in between each exercise interval and including exercises that may not have been as metabolically demanding compared with the other exercises (i.e., sumo squat vs. swing). Thomas et al. (2014) reported an average value of ∼60% of V[Combining Dot Above]O2 max throughout 3 circuits of continuous 10 minute KB swinging and sumo lifting (26 2 values averaged 55.7% of V[Combining Dot Above]O2 max. This was a lower peak average %V[Combining Dot Above]O2 max than reported by Thomas et al. (2014), which is likely due to greater contribution of the phosphagen system and anaerobic glycolysis from the greater intensity of the protocol (i.e., more repetitions per minute) used in this study. This was indicated by higher and sustained RER values across time in this study than in the study by Thomas et al. (2014). RER, and also f, TV, and VE , are important as well to note from this study because these ventilatory measures provide perspective of the greater sustainability of the KB-HIIT protocol compared with the SIC.
Respiratory exchange ratio was significantly different between the KB-HIIT and SIC protocols. This was likely due to the difference in degree of glycolytic activity and buffering of hydrogen ions (23 23 8 13 2 of ∼34.1 ml·kg−1 ·min−1 ) during a 10 minute KB swinging bout, which included 35 second intervals of KB swings and 25 seconds of rest in between the intervals (10 E remained relatively stable during the KB-HIIT compared with the SIC (Figure 6 ). This was probably due to greater stability of TV and less fluctuation in f (Figures 4 and 5 Figures 4 and 5 ). Stable but elevated expired carbon dioxide (CO2 ) associated with elevated RER would contribute to increased but stable VE because of the relationship of arterial partial pressure of CO2 with VE (22
In addition to the ventilatory responses being more sustained during the KB-HIIT than the SIC, so was HR (Figure 8 ). There are few studies that have examined HR responses across time with KB exercise. Hulsey et al. (2010) reported that HR across 10 minutes of KB swinging averaged at 89.1% of age-predicted HRmax (10 max (HRmax value was determined through grade-exercise testing) (6 max , and KB swings were performed during the time points of these higher HR values. The percentage of HRmax during the KB-HIIT was similar to what was observed for the average of the 3 highest values of HR during the SIC (88.5% of HRmax ). The average HR for SIC in this study was comparable with previously reported HR responses to SIC. Freese et al. (2013) found the peak value for HR at each SIC interval to average around 83–89% of age-predicted HRmax (8 max in this study is important to note because unlike previous studies examining KB or SIC effects on cardiorespiratory response (i.e., Hulsey et al. [2012] and Freese et al. [2013]), in this study, HRmax was measured during preliminary graded-exercise testing and used as a basis for comparison. %HRmax provides a more accurate measure of the degree of aerobic exercise stress. Elevated HR, as expected, was concomitant with increased metabolic rate and caloric expenditure.
The kcal·min−1 elicited by the KB protocol from the study by Thomas et al. (2014) was found to be similar to graded-treadmill walking set at the same intensity level (26 −1 . The average values across time were 9.51 kcal·min−1 during the KB-HIIT protocol and 8.6 kcal·min−1 during the SIC protocol. The average of the peak kcal·min−1 of the SIC intervals was 15.78 kcal·min−1 . During the 3 highest peaks in the KB-HIIT (when KB swings were occurring), there was a 12.35 kcal·min−1 average. This is similar to the ∼12.5 kcal·min−1 average in the 10 minutes of KB swings observed by Hulsey et al. (10 −1 of the KB-HIIT and SIC protocols were not significantly different (although the work rates were different), the total expenditure elicited by the KB-HIIT was significantly greater than the SIC (144.87 ± 6.56 vs. 122 ± 7.34 kcals, p = 0.036). The reason for the difference in total caloric expenditure, but no group difference in kcal·min−1 , seems to be due to the 3 extended periods in which the KB-HIIT responses were ∼5–6 kcal·min−1 higher than the SIC (specifically during the SIC recovery periods), whereas there were only brief time periods in which the SIC responses were ∼7–8 kcal·min−1 greater than the KB-HIIT.
In conclusion, all of these physiological comparisons indicate that KB-HIIT can be an effective protocol for HIIT and may even be more attractive and sustainable than SIC. This study is 1 of 2 studies to date that has examined the use of a Tabata regimen with KB exercise. In a study by Fortner et al. (2014), a Tabata regimen with KB swinging was compared with a traditional regimen of KB swinging (included fewer sets and 90 seconds of rest between sets). It was found that not only could individuals complete the Tabata regimen significantly quicker (240.0 ± 0.0 vs. 521.5 ± 3.3 seconds), but also the routine elicited significantly higher average V[Combining Dot Above]O2 (33.1 ± 1.5 vs. 27.2 ± 1.6 ml·kg−1. min−1 ), percentage of V[Combining Dot Above]O2 peak (71.0 ± 0.3 vs. 58.4 ± 0.3%), and HR response (162.4 ± 4.6 vs. 145.6 ± 4.8 b·min−1 ) than the traditional KB swing routine (7 6,7,26 6,7,26 6,7,26 16
Practical Applications
We developed a new KB-HIIT protocol that used Tabata training principles and was designed to elicit cardiorespiratory and metabolic responses similar to those induced by SIC. Our data revealed that KB-HIIT can be used as a form of high-intensity interval training and also provided further insight into the sustainability of the KB-HIIT protocol compared with SIC. This is a new and time-efficient exercise protocol that can be used safely with less physical strain than SIC and can produce physiological responses that could improve cardiorespiratory fitness and metabolic function. This is a protocol that should be considered by exercise practitioners (i.e., personal trainers, strength and conditioning specialists) to incorporate into their training programs for athletes or fitness clients.
Acknowledgments
The authors are grateful for the technical assistance and safety assurance of certified athletic trainers Holly Parker, Matthew Reynolds, and Elizabeth Verplank, and also the kettlebell instruction of Casey Scalese.
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