There is a lot to unpack here. So lets go through each
Are bikes with aggressive positions required to achieve high fitness levels when training on a bike?
There isn't a single answer because there are a lot of confounding factors
- Every bike has a fairly wide fit window now due to the wide array of components available. A bike with a more upright position can easily be made more aggressive by things such as negative rise stems
- What is "aggressive" differs greatly by each person. For example, a person with long legs and a short torso will find that a "regular" road bike geometry will fit more aggressively as the longer inseam effectively shortens the stack.
- If the position is too aggressive for your flexibility, body proportions or effort, it can impede gaining fitness due to discomfort and effects on biomechanics
Ideally to gain fitness the bike needs to be comfortable and put you in a position for good biomechanics. The bike fit required to get there can differ for each person.
Why do aggressive positions exist?
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 be 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.
That all said, an aggressive position also requires constant high power output to be sustainable. If you do not ride at a high power output (e.g., you are still working on fitness) then a more aggressive position may be inappropriate. When the position is appropriate for the effort you will find that your hands become unweighted, and you have a light touch on the handle bars. If you are too aggressive for your power output you will find that you place too much weight on your hands, and you may also be straining your back as you are likely not supporting your trunk weight from your hip and core, like you do when you have a light touch.
What is the best approach
The best approach is probably first to focus on just riding what ever you have, there is probably a big fit window you can explore with your existing bike. As you gain fitness try a slightly more aggressive, stretched out positions, see how that works for you, does it make it easier or harder to ride at a higher effort level. Once you run out of fit options, then start looking at a replacement bike. By this point you will have a much better idea of the type of fit you are looking for and therefore whether or not a particular bike will be suitable.