Intensity and Duration of Physical Efforts in Physical Education Classes

OBJECTIVE: To evaluate the intensity and duration of physical efforts in Physical Education classes in primary and secondary school. METHODS: School-based cross-sectional study carried out by means of the observation of 218 Physical Education classes, including a total of 272 students (each one of the students was evaluated three times). The study was carried out in the city of Pelotas (Southern Brazil), between August and December 2009. In order to evaluate the intensity of the efforts, accelerometers were used and the following cutoff points were adopted (in counts per minute): RESULTS: The mean duration of the classes was 35.6 minutes (SD 6.0). The mean proportion of time spent in moderate to vigorous physical activity was 32.7% (SD 25.2). Boys (44.1%) were involved signifi cantly more in moderate to vigorous physical activity as compared to girls (21.0%; p<0.01). Students who practice physical activities outside the classes participated more in moderate to vigorous physical activity in the Physical Education classes. CONCLUSIONS: Besides the fact that Physical Education classes have a short duration, students practice moderate to vigorous physical activity during one third of the class, with a poor signifi cant contribution to students' level of physical activity.


INTRODUCTION
The world scenario regarding the main causes of morbidity and mortality has been changing for some decades.Infectious diseases have been progressively replaced by non-communicable diseases and injuries (NCDI). 16Public health studies have been increasingly aiming to answer questions that identify and characterize the reasons for such changes and to evaluate forms of mitigating their consequences.
According to the World Health Organization (WHO), a 4.9 million people die every year as a result of tobacco consumption, 2.6 million because they are overweight or obese, 4.4 million due to high total cholesterol levels and 7.1 million due to high blood pressure.Physical activity is related to at least three of these factors: overweight and obesity, high total cholesterol levels and blood pressure.
Health-related behaviors, such as the practice of physical activity, are largely incorporated in childhood and adolescence and tend to be maintained in adulthood, 1,15 which indicates that special attention should be given to this portion of the population in NCDI prevention.A recent study showed high prevalence of physical inactivity, inadequate food habits and alcoholism in Brazilian adolescents. 7e school confi gures an opportunity to approach children and adolescents in order to foster knowledge and habits that promote an active lifestyle, especially by means of the curricular Physical Education classes.b In a systematic review study in Latin America, the group of Physical Education classes was the only one that presented conclusive evidences of effectiveness among the studied interventions for the promotion of physical activity. 13e practice of physical education may be linked to diverse objectives, which vary among individuals.However, as a health modulation instrument, this practice causes physiological adaptations in the search for balance in the organism's functions and should be committed to intensity, duration and frequency. 19he recommendation is at least 60 minutes per day of moderate to vigorous activities in the majority of the weekdays for children and adolescents. 3Vigorous intensity can promote additional benefi ts compared to moderate intensity, 20 but the benefi ts of the latter are also recognized.Although with methodological differences, studies have shown high prevalence of physical inactivity in this population. 11,18,23Even with extensive literature presenting the benefi ts of physical activity for children's and adolescents' health, 10,24 the levels of fi tness and physical activity seem to decline in this stage. 14,25e approach to school programs regarding physical activity and its relation to health, how these programs are organized or whether these themes are tackled are little known in Brazil.Not much is known about whether Physical Education promotes activities with enough intensity, duration and frequency so as to bring benefi ts to students' health.The few studies that have been found indicate that Physical Education classes present short periods in which the intensity is suffi cient to cause physiological adaptations in students. 8,12e aim of this study was to analyze the intensity and duration of physical efforts and associated factors in Physical Education classes in primary and secondaryschool.

METHODS
School-based, cross-sectional study carried out in the city of Pelotas, Southern Brazil, from August to December 2009.The city is located in the south of the State of Rio Grande do Sul and has a population of approximately 320,000 inhabitants.The study was developed by means of observations of Physical Education classes, with evaluation of physical activity through accelerometry and interviews with students.
The largest sample size that was necessary was calculated based on an estimate of the time of the classes that was spent in vigorous physical efforts.The sample size was 777 observations (not individuals) through accelerometry, considering a margin of error of two percentage points, a level of confi dence of 95% and a 10% addition for possible losses and refusals.
The lists with all the city's schools were obtained for sampling.The urban schools with complete primary education and the secondary schools remained for the draw (n = 110).Eleven primary schools and eight secondary schools were drawn, stratifi ed by teaching network (municipal, state-run, federal and private).Three schools drawn for the secondary level coincided with schools drawn for the primary level, totaling 16.One class was drawn to each year of primary school (from the fi fth year onwards) and of secondary school.
Four students from each classroom were randomly drawn, two boys and two girls, and were observed during three classes.The evaluation of 68 classrooms, 272 students, 246 classes (classrooms separated according to gender) and 816 observations through accelerometry (39 more than what was necessary) was estimated.There were 21.3% of losses of observations through accelerometry due to the student's absence in the Physical Education class and to the accelerometer's failure during data collection.
A questionnaire was developed for this study and contained sociodemographic questions; teaching level, network and year; whether the student liked to participate in Physical Education classes, whether he/ she participated in guided physical activities outside the class period and, in case he/she did, how many times per week.The students' weight was measured through a digital scale (students without their shoes and wearing minimum clothing).Their height was measured by means of a metric tape fi xed on the wall one meter from the fl oor (students without their shoes and with their backs to the wall).
b World Health Organization.Promoting physical activity in schools: an important element of a health-promoting school.Geneva; 2007.(WHO Information Series on School Health, 12).
The classifi cation of students' body mass index (BMI) was performed according to the proposal by Cole et al, 5 considering the children's and adolescents' age and sex in order to determine overweight and obesity.
Physical activity was evaluated by means of Actigraph accelerometers, model GT1M, with fi ve-second epoch programming.The accelerometer was fi xed on the students' waist from the beginning to the end of the Physical Education class.In order to categorize the physical activity levels, the following cut-off points were adopted (in counts per minute): sedentary activities (0-100), light activities (101-2,000), moderate (2,001-4,999), vigorous (5,000-7,999), and very vigorous activities (≥ 8000).The categories moderate, vigorous and very vigorous were grouped to compose the outcome "moderate to vigorous physical activities (MVPA)". 21The total time spent in MVPA was recorded for each class.
The initial time of each class was written down when at least 51% of the students were present and the fi nal time, when at least 51% had left the space.This record was used to calculate the duration of each class.The measure of the outcome (mean proportion of class time spent in MVPA) was obtained by dividing the mean class time spent in MVPA by the mean total duration of the classes.
The instruments were administered by trained Physical Education students.The questionnaires were checked and doubly keyboarded in the program EpiInfo 6.0.After checking the keyboarded questionnaires, the data were transferred to the program STATA 10.0 for analysis.Descriptive analysis (absolute and percentage number for categorical variables, and mean with standard--deviation (SD) for numerical variables) was carried out.The mean proportion differences were evaluated with Student's t-test (variables with two categories) and one-way analysis of variance (variables with three or more categories).The level of signifi cance was 5%.
The study was conducted with the consent of the Municipal Education Department and Regional Education Coordination of each school, of the teachers and of the students' guardians.The study was approved (opinion no.040/2009) by the Research Ethics Committee of Escola Superior de Educação Física of Universidade Federal de Pelotas.

RESULTS
Of the 272 students, 6.2% did not obtain any valid accelerometry datum and were excluded of the other analyses.The sample contained 50.2% of female students, with mean age of 14.3 years (SD 2.8), predominantly with white skin color (72.6%).The majority of students studied at public schools (67.4%) and were enrolled in primary school (67.1%) (Table 1).
The mean proportion of students' participation in MVPA in the classes was 32.7% (SD 25.2).The mean duration was 35.6 minutes (SD 6.0) and the mean time spent in MVPA was 12.3 minutes (SD 9.7) (Figure 1).
Male students (44.1%) presented a mean proportion of class time in MVPA that was signifi cantly higher than that of female students (21.0%; p < 0.01).There was no statistically signifi cant difference between public and private schools, but the mean proportion of MVPA in the federal network was signifi cantly higher (49.3%;SD 27.6; p < 0.01) than that of the municipal (33.9%;SD

DISCUSSION
The low mean proportion of class time in MVPA (32.7%) in the present study was coherent with what was observed in other studies. 9,12Even with different methods of physical activity calculation, Hino et al 12 showed proportions of 26.3% for moderate activities and 8.7% for vigorous activities, totaling 35% of MVPA.In another study, the proportion of moderate activities was 16% and, despite the higher proportion of vigorous activities in relation to the other studies (15%), the total proportion in MVPA was similar. 9e mean duration of the Physical Education classes (35 minutes) indicates that some of the classes do not have their duration fully used.Similar mean times for classes' duration were found by other researchers in Brazil 9 and in the USA. 17Changing uniforms and moving from the classroom to the space where the classes take place may explain the difference between available time and the real time of development of the classes.
The low mean time effectively spent in MVPA occurs due to classes with short total duration and low proportions of MVPA.Two other studies 9,17 have shown similar results, with mean times of 17 minutes and 12 minutes for MVPA.So that health-related benefi ts occur as a result of the practice of physical activity, individuals must be submitted to moderate and/or intense efforts occurring during a determinate period. 19,22ysical Education classes have suffered a reduction in the USA, mainly in the more advanced years, in favor of disciplines that are present in academic knowledge evaluations and tests that are similar to vestibulares (university entrance tests). 26In Brazil, there is no scientifi c evidence of the reasons for the reduction in classes duration and frequency, although they are probably the same.
28.6), state-run (24.8%;SD 22.4) and private (35.3%;SD 21.0) networks.Practice of guided physical activities outside the Physical Education classes was reported by 50.7% of the boys and 31.1% of the girls, and was positively associated with the proportion of time spent in MVPA (Table 2).
The variable "likes to have Physical Education classes" was not associated with higher proportions of MVPA, A review has shown that, even with time reduction, the performance of physical activities acts positively over academic performance c by means of diverse mechanisms.Better academic performances are associated with higher intensities of physical activity. 4Higher levels of physical fi tness are related to better academic performance, suggesting that physically active children and adolescents are likely to achieve better results. 26ys presented a signifi cantly greater engagement in MVPA than girls, in agreement with other studies. 6,12he difference in physical activity practice between females and males can be explained by social and/or cultural differences. 27Nevertheless, in the school environment, it is expected that the offer of time, space and possibilities of engaging physical activities are the same to all individuals.This difference can be explained by students' motivation to attend the classes.
Enjoying the classes or not did not present signifi cance in relation to the outcome.However, the number of children who reported not to enjoy the Physical Education classes may have been underestimated.Even though this variable was collected without the presence of the teacher, the students may have been afraid of answering this question because they were inside the school, because they did not know the interviewer or because they believed that the information might be given to the teacher, and this may have infl uenced the result.Even with this limitation, the differences between sexes can be explained by the taste for the Physical Education classes: girls reported that they did not enjoy participating in the classes more than boys.
The level of physical activity decreases with the increase in age in adolescence, 2,18 although the physical efforts undertaken in the Physical Education classes did not differ according to age groups, which suggests that the reduction in the other studies is explained by a decrease in physical activity outside the school.This result may be explained by the obligatoriness of practice in the Physical Education classes, which leads to minimum participation in the developed activities.
The MVPA proportion was not different between the public and private networks.However, after the stratifi cation of the public network, the federal network presented a positive association with MVPA proportion.We have not found studies that evaluated the intensity of the Physical Education classes in different teaching networks, which prevented comparisons.No explicit relationship was found to explain such association, as the determinants of students' engagement and the development of Physical Education classes are little studied.
In addition, there are two federal schools in the city, and one of them was part of the sample.Thus, specifi c characteristics of the federal school may have infl uenced the results, perhaps more than the teaching network.
Students who practice guided physical activities outside the school period presented a higher proportion of MVPA during classes.This may be explained by the taste for the classes, higher among those who practice physical activity outside the school.
Students and teachers knew when data were being collected due to the observer's presence and the utilization of the accelerometer.Thus, the teacher might alter the classes' routine, and the students might present a different behavior because they were using the accelerometer.However, this behavior alteration, if it existed, may have occurred in the increase in the physical activities' duration and intensity, which makes the results be even more worrisome.
The low proportion of time spent in the classes in physical efforts whose intensity is enough to produce physiological adaptations, as well as the short duration of these stimuli are important, but not suffi cient, fi ndings.Knowledge about Physical Education classes and teaching programs is still limited.New studies that aim to identify the factors that infl uence the development of the classes, as well as the determinants of students' practice of physical activities in Physical Education classes should be conducted.
Students spend little time in physical activities in the school and the short periods occur with intensity that is at least moderate, but both should be higher.Although one of the responsibilities of Physical Education classes is the autonomy for the practice of physical activities, they could have a more signifi cant contribution to the students' level of physical activity.

Figure 2 .
Figure 2. Mean of class minutes spent in different intensities of physical activity.Pelotas, Southern Brazil, 2009.
Figure 1.Proportion of class time spent in different intensities of physical activity.Pelotas, Southern Brazil, 2009.

Table 2 .
Mean proportion of class time spent in moderate to vigorous physical activities according to independent variables.Pelotas, Southern Brazil, 2009.
a Cut-off points suggested by Cole et-al. 5c Trost SG.Active education: physical education, physical activity and academic performance.San Diego (CA): Robert Wood Johnson Foundation; 2009.