PA is defined as any bodily movement produced by skeletal muscles that require a significant increase of energy expenditure as compared to a resting phase [12]. We can consider leisure-time, exercise, sport, occupational work and transportation as all forms of PA. Exercise is a type of PA that is usually performed repeatedly over an extended period of time with the aim to improve fitness, physical performance or health. On the other end, Physical Fitness (PF) is defined by WHO as the ability to perform muscular work satisfactorily. Health-related fitness components are morphological, cardiorespiratory, muscular, motor and metabolic components. Commonly PF refers to aerobic endurance capacity as measured by maximal oxygen uptake (VO_2max) or peak oxygen uptake (VO_2peak). VO_2max/peak is strongly associated with physical efficiency and health. Studies have demonstrated that high level of endurance capacity results in a decrease of CVD [13,14]. In fact, longitudinal studies show that higher levels of cardiorespiratory fitness (VO_2max) are associated with a lower mortality rate [15].

Cardiorespiratory fitness depends on the integration of the cardiac and pulmonary systems, and it is defined as the ability of the body to transport and use oxygen. The fitness professionals should be aware that overweight and obese subjects should be poorly conditioned since the aerobic capacity of these subjects has been shown to be low, and in some instances, critically low [16-20]. De Souza et al. [21] reported that aerobic capacity of severe obese subjects (44 ≤ BMI ≤ 54.8) ranging from 16.1 to 34.7 ml∙min⁻¹∙kg⁻¹. Furthermore, Belli et al. [22] indicated that obese subjects affected by type 2 diabetes have a low aerobic capacity (VO_2max = 22 ml∙min⁻¹∙kg⁻¹). The poor aerobic capacity should be taken into consideration when determining exercise intensity in deconditioned subjects. Several recent published results showed that a moderate aerobic exercise program could improve cardiorespiratory condition in obese subjects [16,21-29]. For instance, Millen et al. [30] showed that peak oxygen consumption increased from 27.0 ± 5.1 to 28.8 ± 5.8 ml∙min⁻¹∙kg⁻¹ in overweight and obese subjects upon a 6 week of aerobic exercise training (≥3 days/week). The correlation between obesity and CVD is due to the potential presence of several risk factors in obese individuals such as hypertension, type 2 diabetes, and dyslipidemia [31]. Interestingly, it is important to make clear that the health benefits of weight reduction can be obtained with PA even without the achievement of individual optimal body weight. In fact, already two decades ago, some studies demonstrated that reductions of 5% to 10% in body weight could be already associated with health benefits for the improvement of risk factors, such as decreased blood pressure [32,33]. Among the parameters to be considered in the exercise program for unhealthy population, intensity has received special attention.

Early data of the literature suggested that the PA associated to caloric restriction resulted in the best treatment of obesity [34-37]. The studies showed that when the prescribed exercise program resulted in a significant negative energy balance, a more substantial weight loss was obtained [37-39]. Additionally, Ross et al. [38] showed that weight loss is positively associated with the volume of performed PA. The effects of exercise are maintained even when the exercise ends. In fact, the excess post exercise oxygen consumption results in a prolonged post exercise increase in caloric consumption. Moreover, fat-free mass is better maintained in individuals who performed weight training as part of their exercise program [40] and, therefore, exercise prescription should include aerobic and muscular fitness training. Exercise length is usually extended to increase caloric consumption and, upon that duration, is inversely correlated with the intensity and, thus, aerobic exercise (moderate intensity) is preferred to anaerobic exercise (high intensity). Lafortuna et al. [41] showed that although low and high intensity exercise groups obtain equal body weight loss, the higher intensity exercise program showed a better generalized improvement in muscle performance and physical fitness. Moreover, it has been shown that exercising in multiple bouts throughout the day can be as effective for weight loss as exercising for a single longer continuous bout [42,43]. Thus, the fitness professionals should individualize the intensity and the type of exercise upon the goals and overall health of the subjects.

The public health organizations guidelines indicate a moderate intensity exercise (aerobic) for overweight and obese subjects to be performed most of the day of the week [11,42]. More recent studies have drawn attention to the effects of moderate and vigorous intermittent exercise on weight loss [44,45].

ACSM Position Stand [34] emphasizes the importance to perform moderate intensity PA, between 150 and 250 min∙wk⁻¹, to significantly prevent weight gain. On the other hand, this moderate PA provides only modest weight loss, while greater amounts of PA (≥250 min∙wk⁻¹) have been associated with clinically significant weight loss. Further, moderate intensity PA (between 150 and 250 min∙wk⁻¹) has been shown to improve weight loss when associated to moderate caloric restriction but not severe caloric restriction.

Cross-sectional and prospective studies indicate that after appropriate weight loss, weight maintenance is improved by a regular PA ≥ 250 min∙wk⁻¹. World Health Organization [11] recommends to perform PA which would include leisure time PA to improve cardiorespiratory and muscular fitness, bone health and reduce the risk of NCDs and depression. Additionally, WHO suggests for adults, aged 18 - 64, to perform at least 150 minutes of moderate-intensity aerobic PA throughout the week or to perform at least 75 minutes of vigorous-intensity aerobic PA throughout the week or an equivalent combination of moderate and vigorous intensity activity. Generally, the exercise intensity suggested by public health recommendations is moderate. Fitness professionals should be aware about the terms “moderate” and “vigorous” when they are working with overweight and obese individuals. In fact, these subjects have low cardiorespiratory fitness, which is expressed by reduced oxygen uptake (VO₂), decreased daily activity and low exercise tolerance.

Percentage of maximal oxygen uptake (%VO_2max), peak oxygen uptake (%VO_2peak) or oxygen uptake reserve (VO_2R) could be used as exercise intensity prescription [34,42,62]. VO_2R is the difference between VO_2max and resting VO₂. When we use these variables to assess exercise intensity we should be aware that the relationship between the exercise intensity determined by the %VO_2R and the %VO_2peak mean regression seems to be influenced by the degree of obesity [48].

The energy expenditure during PA depends on the intensity, type and duration of activity. The method to assess the energy expenditure used worldwide is the metabolic equivalent unit (MET), which is defined as the ratio of work metabolic rate to a standard resting metabolic rate of 1.0 cal∙kg⁻¹∙h⁻¹. 1 MET is the resting metabolic rate obtained during rest and is equivalent to an oxygen uptake (O₂) of 3.5 ml∙kg⁻¹∙min⁻¹. Compendium of Physical Activities has been developed to facilitate comparison of coded PA intensity levels across observational studies [63,64]. Activity are listed in the Compendium as multiples of the resting MET level and range from 0.9 (sleeping) to 18 METs (running at 17.5 km∙h⁻¹). According to the compendium, exercise intensity is classified as lightintensity (1.6 - 2.9 METs), moderate-intensity (3 - 5.9 METs), and vigorous-intensity (≥6 METs) activities [63,64]. Considering that overweight and obese subjects have low cardiorespiratory fitness, individual exercise effort is not correlated with the MET value. For instance, De Souza showed that maximal exercise capacity of obese subjects ranging from 4.6 and 10 METs. Therefore, using 3 - 5.9 METs to prescribe moderate exercise intensity is not appropriated for this population.

Percentage of maximum heart rate (%HR_max), or heart rate reserve (HRR) could be used as a manner to monitor exercise intensity. While we are using HR data to monitor exercise intensity we must consider that:

• The linear relationship between HR and VO₂ depends on the mode of exercise.

• The equations to estimate maximal HR [65,66] have large standard deviations. Using these formulas, we could underestimate or overestimate the true level of physical stress imposed during exercise.

• HR could be influenced by a lot of physiological parameters, climate and environment conditions [67, 68].

Therefore, caution should be taken when using equations to estimate maximal HR [65,66] and when we use HR in order to prescribe exercise intensity.

Perceived exertion is how hard subjects feel their body is working and it is based on general fatigue. Previous studies have demonstrated that an increase in physiological parameters such as HR and VO_2, are associated with more intense perceptions of exertion [69]. The scale is valid in that it generally evidences a linear relation with both heart rate and oxygen uptake during exercise [70,71]. The use of RPE could be important in two situations: if the heart-rate measurement is for any reason difficult or if the individual is on medication(s) which could alter normal heart rate response to physical stress. When fitness trainer uses this scale to assess exercise intensity it should be consider that:

• perceived exertion is a method to determine the intensity of effort, strain, and/or discomfort felt during exercise;

• the range of sensations must correspond to the scale;

• either the RPE should be made specific to the overall body perception or the perception derived from a certain anatomical region of the body such as chest, arms and/or legs;

• careful explanation of the scale is necessary before using so that individual is able to understand the meaning of the descriptors.

Submaximal parameters such as lactate or gas exchange thresholds are better markers of the relative stress induced by exercise [53,60]. However, the use of the two submaximal gas exchange thresholds (aerobic, AerT_GE and anaerobic, AnT_GE) for exercise intensity prescription, have loss attention although their noninvasive nature make them an attractive tool [60]. Gas exchange thresholds allow to prescribe the exercise intensity based on the real subject's capacity to perform physical exercise. Considering that obese population has low cardiorespiratory fitness, it can be very important to use thresholds concept when exercise intensity has to be chosen. For overweight and obese subjects, the use of lactate parameters requires the continuously monitoring of plasma lactate levels that is not an easy practice to use for this population. On the contrary, gas exchanges parameters require only gas exchange analysis and it could be easier to apply for intensity exercise prescription. For all these reasons, we focused on gas exchange parameters and not on lactate thresholds. These methods to assess exercise intensity have been already used for subjects affected by various chronic diseases associated with abnormally low peak oxygen uptake (%VO_2peak) such as subjects with chronic heart failure [72,73] and obesity [59].

Aerobic Gas Exchange Threshold (AerT_GE).

The aerobic threshold marks the upper limit of an almost exclusively aerobic metabolism that permits exercise lasting for hours at a lactate level of approximately 2 mmol∙l^–1 [74]. This point is associated with optimal ventilatory efficiency [75,76]. The AerT_GE corresponds to the anaerobic threshold described by Wasserman et al. [61].

The AerT_GE is defined as the maximum exercise intensity fully supported by aerobic metabolism, representing in general a mild to moderate exercise intensity [77,78]. This landmark occurs at the time of the first rise in blood lactate concentration that leads to an increase of carbon dioxide output (CO₂) and a non linear increase of ventilation (V_E) occurs as a results.

Generally there are different procedures to determine the AerT_GE:

• V-slope method [79]: this method is based on the relationship between VO₂ and VCO₂. Before the ventilatory threshold is reached, VO₂ and VCO₂ tend to rise at roughly the same rate and a best-fit line through these points will have a slope close to 1. Once the ventilatory threshold is reached and a metabolic acidosis develops, VCO₂ rises at a faster rate than VO₂ and a best fit line through the points will now have a slope greater than 1. The point at which the two best-fit lines intersect is the AerT_GE.

• The first rise in the ventilatory equivalent for O₂ (VE/VO₂) without a concomitant rise in the ventilatory equivalent for CO₂ (VE/VCO₂) [60].

• Plotting the ventilatory equivalent (VE/VO₂) as a function of VO₂ in order to identify the point during exercise where this curve has its minimum value [75,76,80,81].

• The first overproportional increase in the respiratory exchange ratio RER =VCO₂/VO₂.

• The first increase in the expiratory fraction of O₂.

• The first upward bending changes in the VE versus HR slope during incremental exercise [82].

Anaerobic Gas Exchange Threshold (AnT_GE).

The AnT_GE represents the maximal workload where production and elimination of lactate are in equilibrium [83]. Thus, at intensities above the individual anaerobic threshold, there is a net and continued accumulation of lactate in muscle and in blood [80,83]. At this point, an overproportional increase of VE as related to VCO₂ occurs.

Generally AnT_GE is determinate:

• By plotting VE (y-axis) versus VCO₂ (x-axis). Two regression lines are fitted for the upper and lower part of the relation and their intersection represents the AnT_GE. Moreover, the first systematic increase in VE/VCO₂ parameter or the first decrease in the expiratory fraction of CO₂ could be used as alternative indicators.

• By the heart rate variability threshold during graded exercise [84]. The reliability of this method has not been assessed for overweight and obese population.

• From the second upward bending of the graph representing VE (y-axis) versus VO₂ (x-axis) [85].