I believe there are two guiding principles, one is aerodynamics, which is more important for racing against other riders, than general fitness. The other is biomechanics, which is important for fitness. As you become more fit you will find you can sustain higher power outputs. Under sustained higher outputs you will find that you will naturally want to lower your body trunk so you can use gravity to resist each pedal stroke rather than just core musculature. If you take a very upright position and try to pedal as hard as you can you will find you have to hold tightly onto the bars to resist the pedal strokes. You will also find you may start to round your back which can affect how you pedal. None of these matter on a short-term basis. However, if you want to sustain a higher power output this can quickly becomebe less efficient as you need to recruit more of your muscle tissues (in this case more core) to resist the pedal strokes, which can be come unsustainable over longer duration of high output.
From Myth 5: An Upright Position is Always More Comfortable:
What is important is that our positions match our power outputs. A cyclist’s upper body acts as a counterweight to the forces of pedaling. The harder we pedal, the more inclined our upper bodies should be.
That is why racing bikes have low handlebars and stretched-out positions, while on cyclotouring bikes, the bars are higher, and the riders sit more upright. The extreme are some European city bikes where the riders sit bolt-upright. On those bikes, the riders’ power output is limited, and you won’t often see them in hilly towns…
The more aggressive position allows you to cheat by using gravity to resist your pedal strokes, meaning you are using less effort to pedal and can focus more of your concentration on effective pedal strokes. This also requires good flexibility at the hips and in the thoracic region of the back, which may not always be there or may need to be worked on. Also if you looking at pro racers, their low stretched out positions are on a much more extreme side than what I am envisioning.