The main finding of this study was the greater effect on blood lipids improvement of a high intensity circuit compared to a lighter circuit or to an endurance training. Moreover different kinds of circuit showed different effects on blood pressure. A low intensity circuit training more improved SBP compared to high intensity training and endurance training, while the effect of exercise on DBP was greater in response to high intensity circuit training. Although resistance training has long been accepted as a means for developing and maintaining muscular strength, endurance, power, and muscle mass (hypertrophy) [34, 35], its beneficial relationship to health factors and chronic disease has only recently been recognized in the scientific literature [36, 37]. Prior to 1990, resistance training was not a part of the recommended guidelines for exercise training and rehabilitation for either the American Heart Association or the American College of Sports Medicine (ACSM). In 1990, the ACSM recognized resistance training as a significant component of a comprehensive fitness program for healthy adults of all ages , position subsequently confirmed few years after .
Regarding the role of the exercise’s intensity on health parameters despite the increasing interest on high intensity training there is still no accordance between researchers. Clinical trials generally reported greater improvements after vigorous (e.g. ≥ 60% aerobic power i.e. 60% HR res or 75% HR max) compared with moderate intensity (e.g. between 50 and 60% aerobic power) exercise for diastolic blood pressure, glucose control and aerobic capacity, but reported no intensity effect on improvements in systolic blood pressure, lipid profile, or body fat loss .
Starting from this last parameter, in the present study, we observed greater weight and fat loss in HICT group. To explain this result, we can argue that such kind of training could affect positively EPOC. As a matter of fact intensity and duration of exercise appears to have a key role to determine the increase in EPOC after a high intense training and multiple mechanisms potentially underlie this effects . As we have previously shown, they include, at least, an increase in resting energy expenditure (REE) and a reduction in respiratory ratio (RR)  even if other different mechanisms are supposed to be involved. The suggested other pathways include protein re-synthesis due to post-exercise muscle damage (that, has been calculated, contribute 20% increase in resting metabolism) [40, 41], glycogen metabolism , AMPK/ACC (AMP kinases/ Acetyl CoA Carboxylase) pathway , ANP (atrial natriuretic peptide) production [44, 45] and cytokines effects . All the above mentioned actors should be related to the intensity of exercise . The effects of different kind of exercise deserve some comments. Some research suggested that RT seems to be more effective on obesity and systolic blood pressure  but the precise amount and intensity of this multifaceted methodology of exercise are still not well known. The reason could be due to different modalities of resistance training execution. RT include a lot of variables, such as speed of movement both in eccentric and concentric phase, number of set and repetitions, recovery, different kind of exercise, etc., that make difficult to study this kind of training and could explain the contradictory results and guidelines indications [34, 35].
Recent results confirm that both dynamic RT and static RT have a beneficial effect both in subjects with optimal pressure and/or prehypertension. Indeed, a dynamic resistance exercise training significantly decreases BP, increases peak VO2, and decrease body fat and plasma triglycerides  Regarding potential risks, a recent paper by Moro et al.  showed that an high intensity resistance training has the same effect on arterial blood pressure than the commonly proposed protocols to cardiovascular patients and suggested by ACSM’s guidelines. As far as concerns lipids profile, in this work we observed that HICT group showed a significant decrease in CHOLt, TG and LDL-C and an increase in HDL-C compared to LICT and ET. It has been specifically demonstrated that cholesterol and triglyceride levels can be improved through regular resistance training in middle-aged subjects [50, 51] and training is associated with a decrease in total and abdominal fat as well as an increase in HDL-cholesterol in the elderly . As suggested for fat loss exercise intensity might be a key factor in blood lipids’ exercise response. There are indeed some evidences that total energy expenditure could be related to changes in lipids and lipoproteins in a dose–response manner; in other words changes in blood lipid and lipoprotein concentration seem to depend on total amount of calories expended . Our results are in agreement with those of Dalleck and colleagues , confirming the hypothesis [55, 56] of a positive effect of RT on blood lipids variables even if are in contrast with Wooten’s findings who found instead no changes in HDL-c after 12 weeks of resistance training . A recent systematic evidence suggests that RT could positively affects LDL- c, while combined exercise (as our circuit training) show improvements of both HDL and LDL cholesterol and high-intensity endurance training improves HDL-c.  but with conflicting results that could be attributed to the above mentioned variables of RT. During exercise, an increase in lipoprotein lipase activity, responsible for triglycerides breakdown, has been proposed as a molecular cause for HDL-C increase  PPAR-ϒ and PGC1-α could be also implied in the possible mechanisms that underlies the improvement in lipid profile  but the precise mechanism involved has been not, at the moment, clarified.
There are several strength and, obviously, limitations that should be considered with respect to our study’s outcomes. To our knowledge, this is the first study aimed to investigate the effects of a high-intensity interval circuit training on cardiovascular variables such as blood pressure and blood lipids. There are few studies that examined the effects of high intensity circuit training in young elderly overweight individuals. One recent paper by Romero-Arenas investigated the effects of an high-resistance circuit training in an elderly population, showed a decrease of fat mass and an improvement in cardiovascular-related performance index but the author does not report any data about blood parameters . The high-intensity interval circuit training is a technique that mix the concept of high-intensity interval training  with an high intensity resistance training technique . In our previous paper we have described the effects of this kind of training technique on some anthropometric and performance variables , the main point of strength of the HICT was the less time of effort and the greater effects on body composition and blood lipids whilst LICT seems to be more efficient in reducing DBP and ET in reducing total cholesterol. One caveat is that we have not measured the detraining effects to determine the duration the observed effects on blood lipids and blood pressure. It would therefore be appropriate to design also a detraining monitoring to assess the duration of the observed effects. Moreover it would be of interest to analyse blood lipids response to considered training methodologies in patients with severe hyperlipidaemia. Nevertheless our preliminary results are suggestive of a greater health effects promoted by an higher intensity circuit training.