Will stretching make me slower? I have heard rumors that (excessive) stretching can hurt cycling performance. Specifically, stretching one's hamstrings and/or quadriceps supposedly reduces sprinting power. Is there any factual basis for the claim that increased flexibility leads to less power?  I would also be interested in evidence for the opposite: that loosening one's muscles leads to an increase in cycling performance. 

Note, I am not referring to over-stretching related injuries (like a strain), but rather to the results of long-term, correctly-practiced stretching that leads to increased flexibility.
The subject of stretching is controversial and there is ongoing debate within the scientific community. There is actually no absolute answer until there is further research. Here is what current and on-going research indicates as to the question, "Is there any factual basis for the claim that increased flexibility leads to less power?". 


Stretching and obtaining flexibility beyond your sport specific optimal range of motion is likely to decrease performance.
Stretching to optimize range of motion is beneficial, if tight muscles are the cause of sub-optimal range of motion.
Stretching may be necessary for individual cyclists in order to optimize aerodynamic positioning and/or to enhance riding comfort.
Stretching to prevent injury is not shown to be beneficial. 
The timing of static stretching is shown to influence muscle power and therefore performance. 
The benefits and drawbacks of stretching are sport specific.
Benefits of dynamic stretching/warm-up routines are inconclusive at this point.




More interesting studies from PubMed searches:


This one: "Overall, the evidence suggests that increasing range of motion beyond function through stretching is not beneficial and can actually cause injury and decrease performance."
And another: "...for sports with slow stretch-shortening cycle such as jogging or cycling, there is no scientific data showing a positive effect of stretching." However, negative effects are not showing up other than where one is increasing range of motion beyond what is necessary for a specific sport. 


I did some searching on PubMed and other biomedical databases, using the search terms: flexibility, stretching, cycling performance, muscle power, etc, in various combinations. 


There are no research papers coming up in searches that say a regular stretching program reduces sprinting power.
There is research saying that the timing of static stretching influences muscle power. See below.
There is also research saying that cycling performance can be enhanced by a flexibility program in cases where flexibility improves range-of-motion and/or cycling position. Such as time trials.


As a cycling and fitness enthusiast I feel compelled to figure this out. Since my last revision I found a recent, well documented book on the subject of stretching. The book is 'Dynamic Stretching' by Mark Kovacs, Phd. (The Phd is in exercise physiology).

The gist is:

Pre-event static stretching - Research shows that it can decrease performance in strength, speed and power activities. Some studies have shown decreases in performance by as much as 30%. (Pre-event meaning within 60 minutes of the event)

Static stretching as part of an ongoing training program - Improves range of motion and flexibility in the major muscles and joints of the body. Does not influence strength, speed or power except as to enhancing range of motion. However, there are performance benefits depending on the specialty of the competitive cyclist. For example, a time trialist needs to achieve and maintain an optimal aero position; a cyclocross competitor needs to get on and off the bike a lot; individual sprinters may need to improve range of motion, etc, etc...

Whew! A huge amount of misinformation and mythology on stretching is available. The book cited is focused on dynamic stretching, however the sections on the science of flexibility are well documented from numerous scholarly sources.