Bicycle Motocross (BMX) is a new Olympic sport that involves covering a specific course as fast as possible in order to achieve the best final position. Taking a good position at the front of the group at the beginning of the race is crucial, since positions hardly change during the race. This is because races are conducted in mass start rounds of eight riders and riders must accelerate and take the best possible position during the first meters in the descending lane after the start, specifically until the first obstacle. Past the descending lane, front riders strategically occupy the best places to achieve maximum performance (Mateo-March et al., 2012a). Therefore, sprinting ability is one of the key skills during the initial acceleration phase after the start (Debraux and Bertucci, 2011; Mateo et al., 2011; Bertucci and Hourde, 2011). As a result, scientific literature examining BMX cycling has mainly focused on the initial acceleration phase (Bertucci et al., 2007; Debraux and Bertucci, 2011; Mateo et al., 2011; Zabala et al., 2009; Debraux et al., 2013).
In order to enhance power output and improve the pedalling mechanics in cycling, non-circular pedalling systems were first developed in 1977 (Henderson et al., 1977). They aimed to compensate for the changes and losses of force application throughout the pedal stroke on the basis that when the cranks are at either top or bottom dead centre, propulsive forces tend to be minimal (Ericson and Nisell, 1988). Briefly, non-circular pedalling systems can be divided into 1) non-circular chainrings with the major axis parallel to the cranks in order to help overcoming both dead centres (e.g, "Biopace" system; (Hull et al., 1992) and 2) non-circular chainrings with the major axis perpendicular to the cranks in order to increase the gear ratio during the downstroke e.g, "Harmonic Chainring"; (Ratel et al., 2004). Despite the large theoretical basis behind eccentric chainrings, however, their potential benefits are unclear. An early study found that the metabolic demand during an incremental test to exhaustion was unchanged with elliptical chainrings (Henderson et al., 1977). Other studies showed short-term power output improvements (Hue et al., 2001), or kinematic improvements, reduced angular decelerations in the knee joint and reduced range of movement in the ankle joint (Carpes et al., 2009). Thus, it appears that noncircular chainrings might enhance short duration and high power demand disciplines, like BMX and most track modalities (Mateo et al., 2010; Hue et al., 2007; 2008).
Recently, a new chainring called the "Q-ring" has been developed (Rotor Bike Components, Madrid, Spain). As with other elliptical chainrings, the Q-ring progressively increases the chainring diameter and gear ratio during the downstroke, aspect that leads to the theoretical assumption of increasing the time for force production at the downstroke phase, improving the torque production ability. In addition, the orientation of the major axis can be adjusted against the crank to match the individual technical characteristics of the cyclist. This option is called the optimum chainring position, which theoretically allows the cyclist to apply maximal force at an optimal point of the downstroke. This then adapts the applied torque to the requirements of the road (climbing or in flat terrain, etc.), or to the rider's biomechanical characteristics (Cordova et al., 2009). Important aspect because, as shown by Neptune and Herzog (2000), perpendicular orientation of an elliptical chainring produces slightly higher crank torque patterns during the downstroke when compared to a parallel orientation. This would then allow cyclists who are able to produce higher torque in the downstroke (i.e. better or stronger riders) to be advantaged when compared to weaker cyclists with this kind of non-circular chainring. This may also be the case in BMX disciplines.
On the other hand, it is well known that the weaker riders choose smaller gear ratios to improve their performance in cycling road modalities. Even more, due to the lack of strength, the rules limit these gear ratios through the young categories (i.e. for juniors, men and women), the maximum gear ratio authorized is that which gives a distance covered per pedal revolution of 7.93 m; (UCI, 2013). However, there is no gear ratio limitation in the BMX discipline.
We hypothesized that the non-circular chainring (Q-ring) could improve performance and maximal power output during the initial acceleration phase (3.95s) of a BMX race, but this improvement would be greater in the Elite riders compared to the Cadet riders. Therefore, the aim of the present study was to determine whether differences in performance, power and metabolic variables existed when using a non-circular chainring in comparison with a standard circular chainring.
Sixteen male BMX riders from the Spanish national team, aged 15-24 years, volunteered to participate in the study. None of these participants had any previous experience in riding with non-circular chainring systems however; they were familiar with all other testing procedures because they were based on habitual training and competition tasks. In order to analyse performance differences, cyclists were classified into two groups using a median split technique according to the overall average performance (total distance) obtained with both chainrings (noncircular and circular) during the two days of testing. These two samples separated by performance coincided with the different competing categories of age (Cadet vs. Elite, and would be named as Cadet the first two, and Elite the latest -15.8 vs. 23.3 years, respectively-). The best eight cyclists were included in the Elite group, while the worst eight cyclists were included in the Cadet group (Table 1). The study, approved by the Ethics Committee of the University of Granada (Spain), was conducted according to the principles of the Declaration of Helsinki and the cyclists gave written informed consent for participation. As the athletes were at a training camp for monitoring and assessing the national team during the experimental procedures, their food intake, hydration, legal drug use, physical activity patterns, recovery times and sleep times were accurately controlled. (Faulkner, 1968; Martin and Drinkwater, 1991)
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The non-circular chainring (Q-ring)
The Q-ring is a non-circular chainring approved by the UCI that aims to optimise pedalling mechanics without increasing the mass of the bicycle. For this study, we used a 38-tooth chainring, which was especially designed to be used in single sprocket bikes like track and BMX bicycles. Due to the eccentricity of the Q-ring, the equivalent gear ratio varied from 36 teeth along the major axis (and thus when the cranks are near top and bottom dead centres) to 40 teeth along the minor axis (when the cranks are near horizontal). This is in contrast to circular chainrings that provide a constant gear ratio throughout the whole pedal stroke. The above mentioned optimum chainring position system allows the rider to fix the minor axis of the Q-ring in the optimum individual position with respect to the cranks, i.e. the point at which the cyclist generates the largest torque during the downstroke. The five optimum chainring positions of the Q-ring were separated by 8[degrees] (0 = 103[degrees], 1 = 111[degrees], 2 = 119[degrees], 3 = 127[degrees] and 4 = 135[degrees]). In this study we used position 2 (119[degrees]) setup as recommended by the manufacturer for non-accustomed cyclists such as our subjects (Figure 1). This chainring setup has been used in previous research (RodriguezMarroyo...