The worldwide trend of an increased sedentary state and obesity among youth calls for continued efforts to combat them. To this end, frequent and objective assessment of physical fitness, including cardiorespiratory fitness, should be promoted rather than solely relying on subjective assessment of exercise (Kyrolainen et al., 2010). However, although most exercise interventions for obese youth have been of aerobic type (for a review, see Paes et al., 2015), there is still a lack of appropriate running field tests allowing personalized training for cardiorespiratory fitness in this group.
Among the running field tests used to estimate cardiorespiratory fitness, the multistage 20-metre shuttle run developed by Leger et al. (1988) is the most commonly used among young people of normal weight (Olds et al., 2006; Voss and Sandercock, 2009) and adapted for overweight and obese youth with additional lower speed starting steps (Klijn et al., 2007; Quinart et al., 2014). However, this test presents some limiting factors that may not be counteracted when testing such a population. In particular, the 20-m shuttle run test includes 180 degree directional changes every 20 metres that require sudden braking and push-off muscle actions. In line with the high physiological demand of directional changes reported for ball-game players while running (Dellal et al., 2010; Hatamoto et al., 2014), the 20-m shuttle run test is reported as leading to earlier and higher peak blood lactate concentrations. Consequently, exhaustion is reached at lower maximal velocity than in a similar protocol with no directional changes (Ahmaidi et al., 1992). For normal-weight participants, this bias may be compensated for by the use of prediction equations for V[O.sub.2max] and maximal aerobic velocity (Leger et al., 1988). These equations have been repeatedly validated for adolescents (Boreham et al., 1990; Liu et al., 1992). Although obese youth require a greater proportion of their aerobic capacity to conduct weight-bearing physical activities (Ratel et al., 2006; Peyrot et al., 2009), and will be particularly affected by the directional changes due to their increased body mass, such equations are lacking for this population. To our knowledge, only the study of Quinart et al. (2014) has developed a prediction equation for the peak pulmonary oxygen uptake among obese adolescents taking into consideration sex and body mass index. Nevertheless, despite the use of this equation, maximal aerobic running velocity remains underestimated because running performance is considered as the final completed step (Cairney et al., 2008). This restricted performance evaluation and an early stop are often observed when this test is used with obese youth and the latter represent a clear methodological limitation for individual adjustments of training programme intensity (e.g., Bovet et al., 2007; Castro-Pineiro et al., 2011). Furthermore, it is expected that the early stop induced by a 20-m shuttle run test and the associated underestimation of the actual performance lead these adolescents to perceive themselves as physically less competent, especially regarding their objective physical condition. This is problematic as they are already characterized by significantly lower levels of physical self-perceptions than their normal-weight peers (e.g., Hau et al., 2005; Marsh et al., 2007). This is of major importance because physical self-perceptions are considered central for exercise motivation and, by extension, for adherence to exercise (Berger, 2004). If we consider the physical self-concept as a dynamic construct (i.e., sensitive to events or situation-specific), it can be hypothesized that the perceived physical condition of obese adolescents may be particularly influenced by their positive or negative experience (Ninot et al., 2004). In particular, they could perceive their physical abilities through their performances in a physical fitness test.
In order to circumvent the aforementioned limitation of a 20-m shuttle run among obese adolescents, an intermittent-type (15-15) of progressive and maximal running test has recently been developed (Rey et al., 2013). The intermittent type of run with intermediate passive rest periods of 15 seconds and the absence of sudden directional changes allowed the obese adolescents to reach significantly higher final velocities ([V.sub.peak]; 11.5 vs. 9.4 km x [h.sup.-1]) and net running times (8.7 vs. 5.3 min) than in the standard shuttle run test, while reaching similar peak heart-rate values and ratings of perceived exertion (RPE). To complete these initial findings, inter-test comparison of the direct pulmonary oxygen uptake measures remains to be assessed. An additional and promising advantage of the 15-15 test lies in the fact that it provides peak velocity values that should favour the individual setting of high intensity interval training (HIIT). Compared with traditional forms of continuous exercise, HIIT-type training has been recently highlighted as significantly efficient for improving cardiorespiratory fitness (Lau et al., 2015), perceived pleasure (Bartlett et al., 2011) and health perception (Shepherd et al., 2015) among overweight and obese youth. Based on this information it can be hypothesized that the aforementioned advantage of the 15-15 test will lead obese youth to perceive themselves as physically more competent.
The present study aims at investigating the combined physiological and psychological responses of obese adolescents to an intermittent progressive and maximal running test (15-15 test) compared with a 20-m shuttle run test. The 15-15 test is expected to generate similar peak heart rate, peak pulmonary oxygen uptake and RPE values to the 20-m shuttle run test, while resulting in significant improvements in running performance ([V.sub.peak]) and in perceived physical condition.
Participants and procedures
Eleven 14-15 year old obese adolescents (5 girls and 6 boys, male and female Tanner stage 3-4) schooled in a pediatric rehabilitation center participated in the present study. All of them were defined as obese according to the International Obesity Task Force's cut-off based on body mass and height (Cole et al., 2000). None of the participants had undergone medical treatment or possessed any metabolic disorder, cardiac or orthopedic pathology that might have interfered with the tests.
The protocol was constructed in accordance with the Declaration of Helsinki and met the French ethical requirements for human research. Permission to conduct the study was first obtained from the administration and medical staff of the pediatric rehabilitation center. An information letter was then sent to the adolescents' parent or legal representative, and only adolescents who returned the signed consent forms participated. All tests were conducted under the supervision of the medical staff of the pediatric rehabilitation center.
Running test protocol
The protocol included two progressive and maximal running tests, a continuous 20-m shuttle run test (Leger et al., 1988) and the intermittent 15-15 test (Rey et al., 2013). Both running tests were performed one month after the participants' arrival in the pediatric rehabilitation centre and were preceded by a test familiarization session with accompanying investigators at low velocity (warm-up and first stage). The actual running tests were randomly and individually performed at a 48-h interval and in the same conditions (e.g. no spectators except the investigation staff, same time of the day, same timed normative encouragement sentences for both tests).
The 20-m shuttle run test involves running continuously back and forth between two lines 20 metres apart. The runs are synchronized with a pre-recorded audio CD, which plays audio sounds ('beeps') at pre-set intervals. The starting speed is set for 1 min of warm-up at 8 km/h before being progressively incremented by 0.5 km x [h.sup.-1] per stage (1 min) until it was impossible for the runner to keep in synchrony with the beeps at the lines twice consecutively.
The 15-15 test differs from the 20-m shuttle run test by its intermittent rather than continuous form and by the absence of 180[degrees] directional changes. In this test, the warm-up stage is set at 8 km x [h.sup.-1] with successive speed increments of 1 km x [h.sup.-1] every 3 min. Each stage includes 3 back and forth runs performed in a 15/15 way, with 15 seconds to reach a given stage mark, 15 s to rest passively at the mark and 15 s to come back to the start line. As in the 20-m shuttle run test, the runners are stopped by the experimenter when they cannot keep in synchrony with the beeps at the stage marks twice in succession. As illustrated in the Appendix, this test requires a rectangular track of 75 x 10 m to be constructed using different marks set at the distances corresponding to each running...