This study investigated, for the first time, if the physiological response to 30 min kettlebell overhead non-stop routine (KT) defines this exercise for the prevalence of cardiopulmonary and metabolic resistance-type or endurance-type components. To this aim, we compared the KT response to that of treadmill running (TR), considered as a paradigm of aerobic exercise, at the same average VO2. Results indicated that KT determines much higher cardiopulmonary and metabolic responses than TR, identifying it as a resistance-type exercise, notwithstanding the hypothetical endurance effort due to long lasting duration.
Considering that in KT the HRmean set above 85% HRmax in our experimental conditions, the cardiopulmonary response to exercise was maintained close to maximal levels along the trial. The observed change in HR appeared similar to that found in previous studies analyzing the interval kettlebell swing and the kettlebell snatch routines. In particular, HRmean of around 87% HRmax was found during interval kettlebell swing (as many repetitions as possible in 12 min, 16 kg kettlebell) by Ferrar et al. [5] and during an interval session (20 s exercise-10 s rest for 12 min, kettlebell weight from 10 to 22 kg depending on the exercise) including four kettlebell exercises by Williams and Kraemer [16] (HRmean 86% HRmax in our study). Importantly, a sharp difference between VO2 and cardiac response was observed in KT compared to TR trial. This observation confirms previous findings obtained in studies comparing the interval kettlebell swing to endurance type exercises trials [4, 5, 11]. Systolic blood pressure (SBP) values measured in KT were similar to those reported by Freedson et al. [8] for dynamic efforts in resistance training (bench press at 50% of maximal isometric force), unlike diastolic blood pressure (DBP) (150 mmHg in Freedson et al. data and 100 mmHg in our). During KT, the SBP values assessed at 8 min and 15 min (210 and 220 mmHg respectively) resemble the upper acceptable values reported in the ACSM guidelines (≤ 210 mmHg in males) (2018) and confirm the physiological increase of 10 mmHg every MET. In fact, in our experimental condition, a total of 9.34 METs was reached corresponding to a theoretical SBP of 213 mmHg (9.34 × 10 mmHg + 120 mmHg at rest = ≈ 213 mmHg). These data disagree with values reported by Thomas et al. [15], observing a rise in SBP not above 150 mmHg and a slight decrease of the DBP, probably due to the low kettlebell weight (16 kg), not-overhead exercises (swing and sumo deadlift), and the interval type trial. Instead, the BP values observed in TR (SBP 160 mmHg and DBP 80 mmHg) correspond to values reported during an aerobic exercise of 4 METs in healthy subjects with a sufficient training experience (> 5 years) [12]. Importantly, although a linear correlation between VO2 and BP and HR has been proved during cranking and cycling trials [2], higher values are expected when exercise is performed with the upper limbs, and this response appears to be strictly related to the muscle mass involved. In our experimental condition, despite similar VO2, the observed KT SBP and DBP probably reflect higher peripheral resistance due to a change in sympathovagal balance towards increased sympathetic activity compared to TR. In fact, in physiological conditions, the acute control of blood flow occurs in few seconds or minutes as a result of the integration between vasoconstriction (chemoceptors) and vasodilator (baroceptors) signals. Overall, the exercise could influence the baroceptors activity [13], altering their role in the HR and BP regulation, by increasing BP levels (operating point) from which they start their regulatory activity [14]. Importantly, the sympathovagal balance resulting in the regulation of blood flow during exercise is closely related with the exercise intensity. In particular, at moderate intensities, a vasoconstriction should be appreciated mostly due to increased sympathetic activity [7], whereas at lower intensity levels HR and BP may increase as a consequence of the inhibition of the vagal output till maximal lactate steady state levels [7, 14], due to its quicker alteration compared to the sympathetic system [14]. Wong et al. [17] found that the sympathovagal balance was sharply altered after an interval kettlebell bout (30 s work, 30 s rest for 12 rounds) assessing the Heart Rate Variability and a similar change may be also hypothesized in a non-stop KT routine. As regards the change in BL, in KT measurements showed higher levels despite the lower VO2 in comparison with what observed in an interval kettlebell trial by Fortner et al. in 2014, which evaluated 1 min post-exercise BL. In our setting, BL levels exceeded those reported as anaerobic threshold and, probably, a major contribution to this result may be attributed to the robust involvement of the upper body as previously found [1]. This result was mirrored by a higher increased VE and f to dispose of BL excess and hypercapnia compared to TR. Finally, an interesting result concerned to the rating of perceived exertion that, in both trials, reported a lower value than that expected on the basis of the cardiorespiratory response (14 following KT and 10 following RT, corresponding to moderate/ somewhat hard and light exertion respectively). In KT the excess of the cardiorespiratory response, probably due to the change in sympathovagal balance, and/or the high technical skill in the execution of the overhead gesture may have contributed to this apparent discrepancy.