COVID-19 and Quarantine: Expanding Understanding of How to Stay Physically Active at Home

Citation: Sá Filho AS, Miranda TG, de Paula CC, Barsanulfo SR, Teixeira D, Monteiro D, Cid L, Imperatori C, Yamamoto T, Murillo-Rodriguez E, Amatriain Fernández S, Budde H and Machado S (2020) COVID-19 and Quarantine: Expanding Understanding of How to Stay Physically Active at Home. Front. Psychol. 11:566032. doi: 10.3389/fpsyg.2020.566032 COVID-19 and Quarantine: Expanding Understanding of How to Stay Physically Active at Home


Aerobic exercises Strength exercises
Conception Prolonged or short term exercises using large muscle groups Localized exercise with body weight, or free weight Exercises (include all major muscle)

Configuration
Merging one or more strength component with an aerobic component (see Table 2) Frequency 3-5 days/week (consecutive days for high levels of fitness) Time 10-30 min a day. This can be accumulated continuously or in shorter 10-min blocks Intensity Moderate effort (40-59% of heart rate-HR) for long workouts  or for lower levels of conditioning; Moderate to high effort (60-85% of HR)-for intermediate workout times ; High effort (>85% of HR)-for short workouts (RPE >7) Volume 150 or more min/week are required. 4-6 sets of 6-20 repetitions for selected exercise. 3-5 exercises for workout Workout form (a) Mobility and warm up (5-10 min); (b) core or strength (5-10 min); (c) multimodal workout (5-20 min)

EXPANDING RECOMMENDATIONS FOR PRACTICING EXERCISE AT HOME
The suggestions proposed by the ACSM (2020), WHO (2020), although more consistent, still do not offer a concrete logic to be applied and controlled. Moreover, the statement "Some activity is better than none" makes more sense when we talk about people practicing any physical activity at a beginner level, therefore merely minimally physically active (Chen et al., 2020). Thus, for practitioners of physical exercise at levels that require moderate to high metabolic and strength demand, or even athletes, these would be susceptible to detraining. According to the basic premises related to training, an ideal stimulus must be administered for the adaptation to occur properly, and that condition may not be prioritized with such positions. Considering the theoretical rationale prevalent in the literature, we believe that the suggestions proposed can be expanded in order to integrate groups of individuals who have moderate to high physical conditioning and not only sedentary individuals, providing better guidance on how to proceed during the quarantine period and offering the possibility of followup training even after the quarantine has ended. For such, the conviction that traditional strategies of aerobic endurance exercise may not be a suitable strategy for application in the residential environment (understanding that majority of the population does not have stationary bikes, arm or rowing ergometers at home), so the interval exercise pattern should be primarily stimulated, with or without the use of any viable resource of overload implementation. In combination with this proposal, the concomitant increase in intensity (vigorous to high intensity) is also essential to promote adaptive results independently of the initial fitness level.
The effects of high-intensity interval training (HIIT) are well-established in the literature for healthy people (Gormley et al., 2008) and those with some comorbidity (obesity, heart disease, diabetics) (Ballesta Garcia et al., 2019;Taylor et al., 2019). More recently, the effects of HIIT have been presented in sedentary individuals, suggesting that the application of interval exercises would be viable, consolidating itself as an important strategy for health promotion (Dorneles et al., 2019;Reljic et al., 2019). The literature shows significant physiological responses derived from different types of interval protocols (Paoli et al., 2012;Buckley et al., 2015;Box et al., 2019), and the improvement in performance seems to be related to the physiological mechanism of inducing mitochondrial biogenesis from the expression of the PGC-1 alpha transcription coactivator, as well as catalyzing enzymes of both the glycolytic and oxidative systems (Gibala et al., 2006;Gibala, 2009). Such adaptations promote greater efficiency metabolic rate in energy production and buffering capacity. In a new perspective, evidence indicates that HIIT performed with body weight can promote significant adjustments in strength, hypertrophy (Kikuchi and Nakazato, 2017), and the cardiorespiratory system. For instance, the Tabata protocol would fit as an interesting tool to be performed at home (Tabata et al., 1996). Basically, it consists of performing stimulus 8 × 20 s interspersed with 10-s recovery, a total of 4 min. The protocol is still performed more than once during an exercise session and with different exercise compositions. Emberts et al. (2013) reported mean values of 74% of VO 2max [rate perceived exertion (RPE) averaged 15.4 ± 1.3] and 86% of HR max (156 ± 13 bpm) during two types of Tabata workouts (e.g., mountain climbers, push-ups, split squat, box jumps, burpees, squats, lunges, Russian twist). This level of workout is a sufficient stimulus to generate adaptations to the cardiorespiratory component, and these data are superior to the recommendations proposed by the ACSM. Moreover, the increase in intensity seems to be the key to maintain the gains obtained before COVID-19 (Hickson et al., 1985).
To better target the perspective postulated here in our article, as well as to better interpret the designs positioned in Table 2, Buckley et al. (2015) proposed a high-intensity multimodal training format as a way to reduce the time required for multiple adaptations. For this, the authors compared the physiological responses of the traditional HIIT performed in a rowing ergometer versus multimodal training, involving analysis of different manifestations of strength, in addition to maximum aerobic power and anaerobic capacity. Thirty-two recreational trained participants performed 60 s "all out" and a 3-min recovery (total of 4 min per series). The multimodal HIIT protocol was configured as follows: a strength exercise for 4-6 repetitions, an accessory movement for 8-10 repetitions, and a metabolic component conducted all out for the remainder of the 60 s. The results were significantly promising, resulting in similar responses in aerobic and anaerobic performance tests; however, multimodal HIIT showed significant improvement in all parameters of different manifestations of strength.
It is suggested, therefore, that the configuration of multimodal workouts be constructed in a similar way to that reported in the literature, and the control of exercise overload (internal load) would be performed based on the RPE (0-10 in combination with session time (Foster et al., 2001). Table 2 shows a coherent exercise division format, and Table 3 shows examples of training session configurations.

CAN EXERCISE INTENSITY COMPROMISE THE IMMUNE SYSTEM?
Finally, establishing the relationship between the stresses generated from physical exercise at home and the immune system is an important point to be considered during this quarantine period (Amatriain-Fernandez et al., 2020a,b). Nieman (2007) proposes an open window of alteration of the immune system after physical exercise, and such manifestation would occur with significant magnitude in the face of long-lasting endurance, such as in a marathon, or also in the face of extremely heavy efforts. However, little is known about the immune responses to short interval exercise, but current evidence suggests that HIIT seems to be beneficial for the immune system (Bartlett et al., , 2018Born et al., 2017;Durrer et al., 2017;Dorneles et al., 2019;Steckling et al., 2019;Khammassi et al., 2020), although evidence still points to a higher increase in the percentage of leukocytosis after HIIT exercise (Jamurtas et al., 2018).
So,  investigated in 27 sedentary adult individuals the potential of immune response induced by continuous aerobic training of moderate intensity (MICT) and HIIT (volume 57% smaller). After 10 weeks, there was a significant improvement in the capacity of bacterial phagocytosis by neutrophils (+16 vs. +15%, respectively, for HIIT and MICT) and monocytes (14 vs. 19%, respectively, for HIIT and MICT) for both training groups. Also with a more recent perspective, Born et al. (2017) demonstrated that HIIT, in addition to the superior adaptive responses on the ability to perform exercise (time to 4 | Main positive and negative results from the perspective of HIIT and the changes resulting from this training model. Durrer et al. (2017) Objective: To determine the impact of a single session of HIIT on cellular, molecular, and circulating markers of inflammation in individuals with Type 2 Diabetes (T2D) Participants: Participants with T2D (n = 10) and healthy (HC) age-matched controls (HC; n = 9) Intervention: Acute bout of HIIT (7 × 1-min at 85% maximal aerobic power output), separated by 1-min recovery on a cycle ergometer    Outcome: HIIT = longer time-to-exhaustion and ↑VO 2peak compared to LSD, sIgA secretion rate was higher on the last day of training, as well as the area under the curve (AUCG) higher on the first and last day of training and follow-up compared to the LSD. The AUCG for cortisol remained unaffected on the first and last day of training but increased on the follow-up day with both, HIIT and LSD. sIgA secretion rate with the HIIT indicates no compromised mucosal immune function Bartlett et al. (2020) Objective: Determine if neutrophil functions could be improved in association with changes in fitness and metabolic parameters in older adults at risk for Type 2 Diabetes Mellitus using 10-weeks of low volume high-intensity interval exercise training (HIIT) Participants: Ten older sedentary adults with prediabetes completed 10 weeks of a supervised HIIT program Intervention: 10x 60 s intervals at 80-90% Heart rate reserve/50-60% HRR rest Measures: Before and after training, VO 2peak , glucose and insulin sensitivity, neutrophil chemotaxis, bacterial phagocytosis, reactive oxygen species (ROS) production, and mitochondrial functions were assessed (VO 2peak and neutrophil functions were compared to six young (23 ± 1 years) healthy adults) Outcome: Significant ↓ in fasting glucose and insulin were accompanied by ↑ glucose control and insulin sensitivity; VO 2Peak ↑ 16 ± 11%; Following training, chemotaxis phagocytosis and stimulated ROS ↑ while basal ROS ↓ similar to levels observed in the young controls; mitochondrial functions ↑ toward those observed in young controls, ↓ the deficit of the young controls between Treadmill running 3× per week, for 12 weeks. 4× 4 min intervals at 90% HR max , with 3 min active recovery at 70% HR max Measures: Body composition, VO 2max , serum plasma levels of cytokines (levels of IL-1b, IL-6, IL10, IL-18, TNF-a, interferon-gamma-IFN-c), nitrate and nitrite (NOx) levels, and adiponectin, resistin, leptin, and ghrelin were determined along the intervention Outcome: VO 2max and anthropometric parameters were ↑ after HIIT, while ↓ levels of proinflammatory markers and ↑ levels of interleukin-10 (IL-10) were also found. Adipokines were also modulated after 12 weeks or training. The mRNA expression of the studied genes was unchanged after HIIT Kaspar et al. (2016) Objective: To compare effect of single-bout endurance (ET) and HIIT on the plasma levels of 4 inflammatory cytokines and C-reactive protein and insulin-like growth factor Participants: Seven healthy untrained volunteers Intervention: HIIT: 6 sets of 30 s of all-out supramaximal intensity cycling; ET: 45 min of ergometer cycling at a moderate intensity, which was calculated at 62.5% of Max HR Measures: Plasma samples for the interleukins (IL), IL-1β, IL-6, and IL-10, monocyte protein-1 (MCP-1), insulin growth factor 1 (IGF-1), and C-reactive protein (CRP) Outcome: ET: significant acute and long-term inflammatory response with ↓ decrease at 30 min after exercise in the IL-6/IL-10 ratio (−20%) and a ↓ of MCP-1 (−17.9%); There were no significant changes in the plasma levels of CRP, IL-1, and IGF-1 from baseline to either 30 min or 2 days after the intervention exhaustion-p = 0.02; VO 2Max -p = 0.01), induced functional immunoglobin-A adaptations following 4 days of training in recreational adult runners. Furthermore, HIIT promotes similar inflammatory responses after exercise compared to traditional endurance training, suggesting its viability as a training strategy (Kaspar et al., 2016;. However, an adequate progression of intensity is suggested to avoid deleterious effects due to high doses of exercise. In the workout model recommended here, despite the fact that it is called high-intensity interval exercise, the effective physiological impact (product of volume vs. intensity) is reasonably small (main workout). Moreover, such proposals mainly focus on recreational trained people. In line with this, several studies have shown significant findings in favor of HIIT protocols when compared to moderateintensity exercise, showing how the immunological system responds to vigorous to high-intensity training with very short duration ( Table 4).

FUTURE PERSPECTIVES
It is reasonable to think that HIIT can also be adjusted to improve physical fitness and health in individuals with low levels of fitness (Gormley et al., 2008), as well as for overweight and obese people, according to the trend facing this pandemic (Wewege et al., 2017). First, it is important to understand that the term high intensity should not necessarily be interpreted as a high effort (that would generate limiting condition), since the effort depends on the ratio between intensity and time. In the case of protocols with neuromuscular characteristics, it is possible to establish a suitable threshold for each fitness pattern, mainly controlling the pace with which the movements are performed or the time spent in each stimulus. For cyclic aerobic exercise, the external load, related to the percentage level of VO 2Max required by the coach, is in high physical demand, while the internal load, referring to internal perceptions and changes, can modulate a perceived effort to tolerable levels (Foster et al., 2001). Thus, considering the nonprolonged exposure to high-intensity stimuli, we were able to produce significant results for the cardiorespiratory component (Buchheit and Laursen, 2013), as well as important functional adaptations to the immune system , and promote greater adherence to exercise by individuals with a lower level of fitness (Hartman et al., 2019). Therefore, HIIT is expected to be recognized from a safe and effective doseresponse perspective (Taylor et al., 2019) as a potential tool for the improvement of the immune system and consequently for the prevention of respiratory diseases.

AUTHOR CONTRIBUTIONS
AS participated in the conception of the idea and complete writing of the article, along with SB, CdP, and TM. SM, DT, DM, LC, and CI participated in numerous reviews of this study. CI, TY, and SA participated in the suggestions and the final writing of the article and the adequacy and submission of the study. HB, EM-R, and SM were the main advisers and tutors of all trajectory of studies and designing all phases of the study. All authors contributed to the article and approved the submitted version.