This is a cross-sectional prospective study with a descriptive and analytical aim. It was done from October 06, 2017 to May 25, 2018, i.e. over a period of eight (08) months.

Our study population took into account healthy adult subjects from Cotonou and Abomey cities. A healthy subject was defined as any person having no apparent orthopedic impairment objectified on examination and having no impairment of cardiorespiratory, osteoarticular, and neuromotor function. The sampling was carried out taking into account healthy adult subjects, aged 18 to 50 years, living in Cotonou or Abomey (or in one of their neighboring municipalities) and having consented to participate to the study. Pregnant women (female) were excluded.

The list of subjects was made by raising awareness in public places (churches, market, sports ground, schools, workshops, etc.). An appointment was made with the consenting subjects, either at the University Clinic of Physical Medicine and Rehabilitation of the National Hospital and University Center (CNHU-HKM) of Cotonou or the functional rehabilitation service of the Departmental Hospital of Abomey, to carry out their cardio-respiratory endurance tests.

For the evaluation of cardio-respiratory endurance, subjects had in two separate sessions of at least one week, three different tests: The six-minute walk test and the cycloergometer test at the first session and the step test during the second session.

- For the six-minute walk test (6 MWT), subjects did not perform vigorous exercise within two hours of testing. They seated and were at complete rest for ten minutes before the 6 MWT. They wore sportswear to be comfortable during the exercise. The test was done in a flat linear corridor, marked every 1 meter over a distance of 20 meters with a stud placed at each end. Instructions given to the subjects were: “The objective of this test is to walk as far as possible in six minutes. You will go back and forth by going around the studs and keeping a straight trajectory”. Encouragement given was standardized, every minute. Fifteen (15) seconds before the end of the test, subject was notified that the test was about to be completed.

- Cycloergometer test was performed after a minimum of thirty to forty-five minutes of rest after the six-minute walk test, after that the heart rate had returned to its resting value or very close to it and the subject felt ready to start. Therefore, we comfortably install subject on the bike (adjusting the height of the seat so as to avoid a flexion of more than 90˚ of the knee during pedaling). Subject is pedaling at free pace for three minutes as a warm-up. The test began really when the bike power is reached to 50 watts, that was increased by 15 watts every minute. Subject was allowed to pedal upright when the working power became greater. Instructions given to the subject were: “Pedal at a speed between 60 and 70 revolutions/minute; do not speak during the test; pedal as long as possible”. Standard encouragement was given during the test.

- For the step test, subject was first installed in a seated position on a chair so as to be put to rest for 2 minutes. After that, the resting heart rate (HR₀) was recorded and noted (step 0). Then, the subject started the test with the first board 0.1 m high corresponding to the first level. For 2 minutes he went up and down the board at a constant speed of 84 steps/min (step 1). At the end of this step, the heart rate (HR₁) was also recorded. The subject rested for 2 minutes, to repeat the same test, at the same pace with a board 0.2 m high, for 2 minutes after which the heart rate (HR2) was recorded. The same scenario was continued with boards of increasingly higher heights (incrementing 0.1 meter per step), until the subject was unable to follow the rhythm imposed by the metronome to ascend and descend.

On the 6-minute walk test, the total distance traveled and the heart rate immediately when stopped were recorded using the heart rate monitor worn by the subject. The maximum oxygen consumption (VO₂ max) was calculated, from the data of the cycloergometer test, according to the gender-specific formulas established by Thomas W. Storer:

VO₂ max (male) = 10.51 (w) + 6.35 (kg) − 10.49 (age) + 519.3 ml·min⁻¹,

VO₂ max (female) = 9.39 (w) + 7.7 (kg) − 5.88 (age) + 136.7 ml·min⁻¹.

Relative VO₂ max (ml/kg/min) was obtained by dividing these absolute values by body weight. Also, as soon as the cycloergometer test was stopped, the heart rate (HR) was taken on the heart rate monitor by the subject. During the step test, the developed power (watts) was calculated according to the formula used by Nielens H. and Plaghki L: Power = weight (in kg) × acceleration due to gravity (9.8 m/S²) × climb frequency/second (84 steps/minute) × height of the last plank with which the two minutes of ascent/descent were completed. Power relative to body weight was the ratio of absolute power to subject weight.

Encoding and processing of data were done in Microsoft office Excel version 2007 and SPSS software version 22, respectively. A descriptive analysis of the study variables was done. A correlation between VO₂ max and the distance covered on the six-minute walk test was sought. A linear regression model was used to establish the predictive equation for VO₂ max with the data from the six- minute walk test. The significance level chosen was 5%.

The extreme ages were 18 and 50 with a median of 28. The mean age was 30.0 ± 9.8 years. Subjects were especially men (54.20%), students (42.13%), with a normal corpulence (69.91%). According to their allegations, 23.61% of them consume alcohol occasionally and 21.30% have a fairly regular practice of sport. Table 1 shows other characteristics of the study population and Figure 1 shows data on the lifestyle of study subjects.

The average distance covered by subjects of our sample was 557.1 ± 93.6 m. This mean distance is greater than what was reported in the Arab population, by

Alameri et al. [18]. On the other hand, it is lower than what was found in the study of Chetta in 2001 and Prochaczek in 2007 [19] [20]. If we could evoke the question of age to explain the observed difference, it should be noted that our proportions remain lower than those of Chetta et al. in 2006, in a study population of identical age to ours (20 - 50 years) [19]. We can then simply make the assumption of the cultural question (sports practice, predominant mode of travel, etc.) to justify these noted differences. The average heart rate of subjects of our study was 136.7 ± 19.0 beats/minute, or 34.8% of the calculated theoretical maximum heart rate. This percentage shows that the test is submaximal. This average heart rate is higher than that found by some authors in the literature [20] [21] [22]. This difference may be related to reduced aerobic endurance in our cohort compared to these populations.

The average power of our subjects was 6.4 ± 1.8 watts/kg. This average exceeds those found in a study carried out by Nielens where the average power was 1.29 ± 0.39 W/kg and 1.01 ± 0.25 W/kg respectively for men and women with chronic pain. In healthy subjects, it was 1.74 ± 0.53 W/kg and 1.08 ± 0.40 W/kg respectively for men and women [23]. This higher value in our study would be related in the fact that our subjects went to exhaustion while in the study of Nielens the test was stopped at a heart rate corresponding to 65% of the theoretical maximum heart rate calculated. The average heart rate was 168.4 ± 15.4 beats per minute, or 80.4% of the theoretical maximum heart rate. This percentage of FMT achieved shows that the test is under maximal despite the fact that subjects were allowed to go to exhaustion. This would be due to the discontinuous nature of the protocol. Indeed the two minutes of rest after each level allows a recovery and therefore a drop in heart rate.

The mean maximum oxygen uptake (VO₂ max) of subjects of the study was 33.4 ± 9.3 ml/kg/min. Compared to other populations, the average VO₂ max of Beninese is lower than that of Japanese, Americans, Norwegians and Canadians [24] [25] [26] [27]. But these results are clearly superior to those reported by Shete et al. in subjects who are not known athletes [28]. The average maximum heart rate of our cohort was 179.4 ± 8.5 beats per minute, or 89.5% ± 7.1% of the calculated theoretical maximum heart rate. This percentage shows that this is a peak test. This average heart rate is similar to that obtained by Loe et al. in a Norwegian population [26].

The maximum distance traveled on the six-minute walk test was correlated with the maximum oxygen consumption estimated from the cycloergometer test (r = 0.65; p < 0.00). Burr et al. found a moderate correlation (r = 0.54) in healthy subjects aged 20 to 59 years [29]. In populations suffering from respiratory pathologies, a correlation between distance travelled during the 6-minute walk test and VO₂ max has also been found [30]. It is therefore possible to predict a subject’s maximum oxygen uptake from the six-minute walk test. The VO₂ max prediction equation shows that the latter evolves inversely with respect to body weight, age and gender. The influence of sex on VO₂ max is greater than that of others. Similar remarks were made by Burr et al. [29]. Also, Mänttäri et al. had noted that the predictors of the subject’s performance on the six-minute walk test varied by gender [15]. From their analysis, it emerged that for men, the best predictors for VO₂ max were walking distance, age, BMI, heart rate at the end of 6 MWT and height, and for women, walking distance, age and weight.

We can mention two main limitations to this study. This concerns the method of recruiting subjects and the taking into account of the heart rate as a parameter for assessing cardio-respiratory endurance. Indeed, a stratified cluster sampling would have been better. But, as we don’t have specific motivating factors for the subjects, such a recruitment method is difficult in our socio-cultural context. About the heart rate, it is well known that various other parameters such as tests examination conditions or stress, could lead to disturbances.