In conventional cycling seat position with a circular chainring and a traditional crank-pedal mechanism, the effective force is minimal when the crank is vertical at the top (near 0[degrees]) and at the bottom dead centres (near 180[degrees]) and maximal when the crank is near the horizontal forward position (90[degrees]) (Ericson and Nisell, 1988). The crank angular velocity remains nearly constant during one pedal crank revolution at a regular pedalling rate (Broker, 2003; Horvais et al., 2007). Different crank-pedal systems and chainring shapes have been proposed in an attempt to alter the pedalling motion by varying the crank arm length, the position of the chainring rotation axis, or the radius of the chainring during a pedal revolution (for a review, see Bini and Dagnese, 2012; Faria et al., 2005). Among these, non-circular chainrings were developed with the radius varying proportionally to the effective force applied to the crank as a function of the crank angle. Various chainring shapes and crank orientations relative to the minor and major axes of the chainring have been used to alter the phase of the crank angular velocity variation and the amount of variation, respectively (Hull et al., 1992). Noncircular chainrings (Osymetric, Stronglight, Monaco) have been introduced in which the shape is a skewed ellipse, the major and the minor axes are not perpendicular and the crank lever is mounted nearly perpendicular to the major axis. The lever arm of the force applied on the chain gets short when the crank is near the dead centres (vertical) but long when the crank is in the effective power phase (near horizontal). As a consequence, higher and lower instantaneous pedalling rates are achieved when the crank levers are near vertical and horizontal, respectively, when compared to a circular chainring (Hintzy et al., 2015; Horvais et al., 2007; Strutzenberger et al., 2014). Therefore, the time spent in the effective power phase increases, and inversely decreases around the top and bottom dead centres (Hintzy et al., 2015; Horvais et al., 2007; Neptune and Herzog, 2000). These kinematic alterations of the crank arm affected significantly the pedalling kinematic of the lower limbs: a reduction in sagittal knee joint power and an increase in sagittal hip joint power (Strutzenberger et al., 2014). Authors concluded that this joint-specific power generation might be beneficial for short distance races, especially as the effect sizes increased with higher cadences. Interestingly, both shape and orientation of this non-circular Osymetric chainring were similar to the theoretical optimal chainring shape maximizing the crank power proposed by Rankin and Neptune (2008). Authors showed that this theoretical optimal non-circular chainring could significantly increase the crank power (2.9%). The decrease of the crank velocity during the effective downstroke phase allowed the muscles to generate power longer and produce more external work.
When considering the above, the benefits of crankpedal designs and non-circular chainring systems should be apparent in short and maximal cycling trials. The noncircular chainring significantly improved performance during an all-out 1 km test (Pro-race: Hue et al., 2001; Rotor Q-Ring: O'Hara et al., 2012) and during a sprint BMX (Rotor Q-Ring: Mateo-March et al., 2010, 2014). The power output attained during intermittent 20 s maximal sprints was 2.4 to 6.7% greater with non-circular chainring (Rotor Q-Ring) than with circular, although this difference was not statistically significant (Cordova et al., 2014). In addition, the maximal power output calculated from the theoretical force-velocity relationship was significantly increased with a non-circular chainring (Osymetric: Hintzy et al., 1999a; Pro-race: Hue et al., 2008) over circular chainrings. Unfortunately, the mechanical explanations were not presented since intra-cycle pedalling kinetic was not measured.
Explaining the improvement of sprint performance with a non-circular chainring to understand the mechanisms involved requires instantaneous measures during the entire sprint. In addition, the upstroke and the downstroke phases of the pedal revolution should be analysed separately. Indeed, the radius variation of the present noncircular chainring slowed down the crank angular velocity when the cranks were near horizontal during the effective part of the downstroke phase for one crank as well as during the ineffective part of the upstroke phase for the opposite crank (Hintzy et al., 2015; Horvais et al., 2007; Neptune and Herzog, 2000). An experimental condition without clipless pedals will allow testing the effects of the non-circular chainring only during the downstroke phase with a pushing action. In contrast, the clipless condition will allow testing the effects of the non-circular chainring during the entire pedal revolution because clipless pedals can modify the force pattern during both upstroke and downstroke phases, by respectively pulling and pushing actions (Tate and Shierman, 1977).
Therefore, the purpose of this study was to assess the effects of a non-circular chainring on the maximal force, power and velocity attained during a sprint, as well as on the intra-cycle kinetic evolution. It was hypothesized that (a) the present non-circular chainring would improve the maximal power output during a sprint and that (b) the improvement would be due to a higher instantaneous force developed during the effective phase of the pedal revolution.
Twenty male cyclists (age: 24 [+ or -] 6 years, height: 1.78 [+ or -] 0.05 m, body mass: 68.0 [+ or -] 7.3 kg) participated in this study. Instructions were previously given to subjects: not to ingest caffeine the morning; not to perform an exhausting exercise during the previous 24 hours; to bed early and have a normal meal the night before the test. Regional road riders were selected, excluding track cycling specialists. All of the participants had regularly participated in regional level competitions for 5 years prior to the participation in this study and were in the competitive period of the year (from March to April) at the time of the study. Their weekly training volume covers approximately 350 [+ or -] 50 km. None of the participants had previous experience in using non-circular chainring. Since it has been reported that adaptation of muscle coordination on a non-circular chainring occurs over a short period of time (within the first 10-20 cycles for Neptune and Herzog, 2000), all participants were familiarized with the use of the noncircular chainring during two 10 min sessions. The Institutional Ethics Review Board of the University of Savoy approved the study and the participants gave written informed consent for participation.
Chainring and shoe-pedal interface