Physical inactivity and sedentary behavior have become a global scourge affecting more and more adults and children, especially in developed countries. The World Health Organisation (WHO) estimated in 2010 that almost a quarter of adults were physically inactive. Since then, the situation has not improved. And it is now clearly established that cardiovascular fitness (usually assessed by VO2MAX or VO2PIC) is linked to the risk of all-cause mortality and cardiovascular disease.
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One of the main reasons given for not being physically active is "lack of time". Official recommendations are for at least 150 minutes of moderate-intensity exercise (3-6 METs) or 75 minutes of high-intensity exercise (>6 METs) per week, in minimum 10-minute increments, plus at least two sessions of resistance training. This is one of the reasons why high-intensity interval training (HIIT) has been so successful in the world of sport and fitness over the last decade. Indeed, it makes it possible to greatly reduce the duration of a session (less than 30 minutes), to work at higher intensities and to obtain results similar or even superior to those obtained with classic "cardio" training (i.e. moderate and continuous intensity).
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However, it is important to note that in most scientific studies, the term HIIT and the associated training protocols consist of cardiovascular endurance activities at very high intensities close to or above VO2MAX, in running, cycling or rowing. However, over the last decade, the term HIIT has become more generalized and has been applied to circuit training involving a mix of exercises, with or without equipment, at tempos identical to those frequently used in the scientific literature. This form of training was an immediate success due to its non-monotonous nature, the pleasure felt by the participants and a better level of adherence. However, even though these HIIT circuits combine cardiovascular and neuromuscular demands, it is not yet clear what real benefits practitioners can derive in terms of fitness and health…
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To find out more, during 8 weeks of training, researchers studied the impact of a HIIT circuit on the cardiovascular and physical fitness of healthy adults. To do this, they recruited 13 people (8 women and 5 men) aged between 21 and 38, who had been practicing strength training and cardiovascular endurance 3 to 4 times a week for more than 6 months.
For 8 weeks, the experimental protocol consisted of 3 weekly training sessions. Each training session consisted of a basic 10-minute warm-up followed by 9 different exercises, linked in a circuit: 30s effort, 15s recovery. This circuit was repeated 4 times with 2 minutes of recovery between each set. The 9 exercises were: Sumo squat (65% 1RM), sit-ups with a 3 kg medicine ball, clean & press (65% 1RM), box jumps (45 cm), push-ups with TRX, wall-ball with a 4 kg medicine ball, burpees, sledgehammer with a 3-5 kg hammer and 10m sprint repetition.
Before and after the 8-week protocol, body composition, VO2MAX, knee extensors and flexors isokinetic strength, vertical jump height, bench press maximal strength and bench press muscular endurance (AMRAP @65% 1RM) were assessed in all participants. Finally, during the first and last week of training, from Monday to Friday, blood samples were taken each morning to measure blood concentrations of creatine kinase (CK), a marker of muscle damage, and C-reactive protein (CRP), a marker of inflammation.
The main results of this study show that 3 weekly HIIT circuit sessions for 8 weeks significantly improved VO2MAX by 4.6% (+ 1.9 ± 2.2 ml/kg/min). It is important to note that the average VO2MAX of the group was 44.2 ± 8.2 ml/kg/min at the beginning of the protocol and was measured at 46.1 ± 8.1 ml/kg/min at the end of the training protocol. In terms of body composition, only fat mass was significantly reduced by 0.64 ± 1.01 kg. The isokinetic strength of the knee extensors and flexors did not change. All other muscle performances tested were significantly improved.
Regarding creatine kinase concentrations, the increases observed following training were moderate and never exceeded the high limits recommended for non-athletes (491 U/L for men and 252 U/L for women). Moreover, the concentrations returned to normal values after 48 hours. However, when comparing the concentrations observed in the first and last weeks, the researchers noted a 32% drop in overall creatine kinase concentrations. No significant difference was observed in C-reactive protein concentrations.
Overall, the results obtained in this study are positive and show that it is possible to obtain significant gains at the cardiovascular and neuromuscular levels with 3 training sessions of less than 30 minutes per week, over just 8 weeks. However, the cardiovascular gains achieved are relatively small compared to those achievable with a "classic" HIIT program (running, cycling, rowing). On average, HIIT protocols report between 5 and 20% gains in VO2MAX. Of course, these gains are conditioned by the initial fitness level of the participants. Thus, high level exercisers will obtain lower gains because their baseline VO2MAX will be greater. And it is not uncommon to see gains close to 20% in sedentary people. Here, the participants have an average baseline level according to the American College of Sports Medicine criteria. They could therefore have gained more with a "classic" HIIT program.
As far as body fat is concerned, HIIT does not result in greater gains than continuous moderate intensity endurance training (MICT) for the same volume. And the results of this study are consistent with meta-analyses that have reviewed the currently available data on HIIT and body fat loss. Simply put, if you want to lose body fat, physical activity will help, but the bulk of the work will be done with your diet.
Of course, an interesting advantage of HIIT circuits is the involvement of more muscle groups and the diversity of the program. However, this advantage can also become a hindrance for some practitioners since the intrinsic difficulty of certain bodyweight exercises may be too great to be performed intensively. Finally, for health purposes (improvement of cardiovascular functions), simpler (but more repetitive) protocols using running or cycling will have a stronger clinical impact. And to obtain neuromuscular gains, adding 2 resistance training weekly sessions will be enough.
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