Frontal Surface Area Versus Drag Area in Predicting Level Cycling Time Trial Performance

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In road cycling, performance time on level terrain is highly influenced by aerodynamic resistance (Ra). At a given velocity, drag area is the primary component of Ra. Drag area is influenced by the bicycle-rider system's frontal surface area (AP) and its drag coeffcient. Traditionally, level time trial performance has been predicted based upon the assessment of a riders' frontal surface area, which represents only one component of the bicycle-rider system's drag area. PURPOSE To compare how actual measures of the rider frontal surface area (AP), estimates of the rider frontal surface area (APest), and direct determinants of the aerodynamic resistance of the bicycle/rider system measured as the drag area (AD), predict level cycling time trial performance (LTT) in the field when normalized to the measured power output (P). METHODS Nineteen highly trained cyclists (Mean ± SD VO2 peak = 67.6 ± 6.4) performed an all out 22.1 km LTT in the aerodynamic position and bicycle of their choice. Power output (304 ± 26 W) measured with a rear hub power meter and finish time (1884 ± 106 sec) were determined from the LTT. Drag area (0.349 ± 0.059 m2) was determined by using rear hub power meters to measure the relationship between tractive resistance and velocity squared. Projected frontal area (AP) was determined directly via the digitized planimetry technique (0.338 ± 0.049 m2) or indirectly with various estimation techniques (APest) currently used in the literature (range: 0.306 ± 0.020 to 0.550 ± 0.028 m2). LTT power was normalized to all values of AD (P/AD), AP (P/AP) and APest (P/APest), then correlated to LTT finish time. RESULTS (mean ± SD): P/AD (r = −0.926) correlated significantly stronger than all other correlations with time, while P/AP (r = −0.746) was not significantly different from any of the P/APest (r = −0.517 to −0.725) correlations to time. CONCLUSIONS Measures of the AD of the bicycle-rider system, which can be accurately determined in the field with a cycle mounted power meter, is a better predictor of performance time compared to direct measures of the AP using digitized planimetry or estimates of AP.