A deep riding position reached its extreme in the early ‘80’s with new aerodynamic ideas. Even leisure riders had little option than to follow the trend to uncomfortable but efficient postures. Recently, more attention has been paid to the needs of some overlooked niches (women, older riders, etc.) and alternatives entered the market. As the industry ran out of customers who swallow fashion unquestioningly, it's been noticed that women outnumber men and mature riders are buying more bikes than youth.
An overreaction is typical of herd behavior and many very upright positions have become available on high performance bikes. From the competition point of view, this trend seems to have gone too far. Competitors are not changing the way the market does.
So, it's a good time to review the costs and benefits of back angle:
(1) The most natural posture is upright. An ergonomic office chair or standing provides this. Any departure needs some explanation.
(2) For pedaling in circles, upright posture favors quadriceps and disadvantages glutes/hamstrings. This is fine for low intensity, long duration because quads have the most bulk and best circulation. However, for peak efforts, the more muscle groups engaged, the more power. Leaning forward enables glutes/hamstrings to contribute, so riders feel stronger. So, leaning forward permits more power for bursts.
(3) Upright back puts more weight on the rear wheel; unless the wheelbase is stretched, which makes a long, slow, awkward bike. A minimal wheelbase is preferable. For handling, 2 wheel vehicles want fairly equal weight distribution. Sit upright (90°, measured from hips to shoulders, ignoring back arc) and it’s about 30:70 (front:rear). Lean forward to a comfortable 45° and it’s 40:60. Lean further forward to a sporty 30° and it will approach 47:53, thought by some to be a golden ratio. Here you have few speed wobbles and a front wheel that resists washing out in corners. Yet, there’s not so much weight forward to limit deceleration (lifting the rear wheel). You can easily determine your weight distribution by placing a scale under your front wheel with the rear at the same height. Divide this weight by the vehicle total and you have the front percentage. Play with back angle and watch the number change.
(4) Leaning forward reduces frontal area which makes cooling more of a challenge but lowers wind resistance, a serious factor at the high speeds of competition.
(5) Most healthy adults report few chronic pains with leaning angles of 35° - 45°. So, a posture with a good balance of benefits is available to most. This helps explain why leisure riding on racing bikes is popular.
I’ve raised my stem substantially over the past 10 years as I adjust to lower back stiffness. I have several disk herniations from accidents in the past. At this point, my bars (top surface) are only 2-3cm below saddle (top surface). If I were undamaged, that would be 5-8cm.
Higher stems favor folks like me and, conveniently, neophytes with more office chair than bike racing experience. Unfortunately, too many women’s specific bikes are offering these upright postures. Yet, women are generally more capable than men of low riding postures, even as novices. That's a trend that has gone much too far.
As far as the science behind all this, physics hasn’t changed. A 30°ish posture was typical for competition for the first 100 years; look closely at Coppi or Anquetil. Aerodynamic awareness has driven us to sacrifice comfort and muscle health a bit in favor of lower wind resistance. However, the speeds at which those postures really deliver are higher than most any training, endurance, or triathlon session. So, weekend warriors with such position are really following fashion and marketing.
Lower position has a tiny effect on handling, just as lower BB height, by lowering your center of gravity. A rider may notice such a trade off but, ultimately, races aren’t won with handling and good riders can out corner everyone with the worst handling bike. Handling, within the range found among most road bikes, is not the performance factor (as in the automotive world) that most suppose. It’s more a matter of preference, like clothing fabrics and fit.
Among pro team mechanics, the consensus is that this upright trend has gone too far especially with bikes whose super tall head tubes prevent low positions.
Every rider needs to be aware that his/her back angle is an option. Absent an orthopedic consideration, that angle matches a riding objective. For leisure riders, 60° may prove ideal. For athletic oriented training, 45° is probably enough. A competition minded rider will likely want 30°. And an actual competitor with high level aspirations will be in the 10° range. In the best of all worlds, fitting begins with this angle. Then, a bicycle is built below this posture, proportioned to support the contact points and deliver handling appropriate to the ride.