Why is power not a lab indicator of the correct position on the bicycle?

The reasons are varying and the unaware bicycle user, who associates power to the performance, often underestimates them. 

Kinematics concepts applied to cycling are the first and the most important reason.
The cyclist’s performance is determined by the average speed that the athlete kept along a route that is, the distance traveled divided by the time spent. Since the distance is a certain data, it is necessary to analyse all the variables that could influence the time spent.

As for auto racing, the time spent is conditioned by the resistances reduction due to frictions (kinetic, rolling, and aerodynamic) and the increase of engine’s power. However, in cycling we find two other essential elements to keep the average speed high: the comfort and the fatigue index.

If we analyze these elements one by one, it is easy to understand that the power stated in the laboratory could not be and indicator of the cyclist’s optimal position. Excluding kinetic and rolling frictions, which depends on the vehicle and the road’s characteristics, the aerodynamic friction is an important element to reduce parasitic resistances that steal Watt to the cyclist’s performance. However, it is easy to understand that they do not influence the expression of power on a simulator in the laboratory.

Comfort is another element that, together with the fatigue index, most influences the long-lasting performance. Indeed, it is on the duration of the performance that the biomechanical test in the laboratory, with the sole analysis of the power, is not reliable.  It is proved that a better position, compared to a less-performing one, does not modify the short or medium-term power, but reduces (sometimes in a very substantial way) the decay of the performance during the last part of the test. For example, if a subject switch from a less performing position to a more performing one (where the main changes concern the aerodynamic aspect or comfort), he will find that the average speed in a 4-hours test is certainly higher, but will not find differences of the average speed in a 1-hour test.

As for auto racing, the winner is the one who has the higher average speed, independently from the peak speed reached, so, as time went on, it is important not to have a performance decay.
In order to evaluate this important element of the performance, it is necessary to borrow from many studies, which state the best range of motion of each joint, and direct or indirect aerodynamic studies, which could give useful indications on the cyclist’s correct position, remembering, however, that the cycling operating speed is clearly lower than that of auto racing.