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? about seat tube angles and muscles(5 posts)

? about seat tube angles and musclesempacher6seat
Dec 30, 2002 7:45 PM
I hear lots of talk about steeper bikes, such as tri-bikes with 80 degree seat tube angles use different muscles then one with a slack 72 or 74 degree bike uses. Do these bikes actually use different muscles of the quadriceps group or are they the same muscles, just used in a different way (like training your pecs with bench press vs. seated fly)?

I posted this question on another board and only got one response from someone who seemed to know anything. Some of his more interesting points were that the difference between a 74 degree bike and a 78 only translate into a move of the seat around 2cm forward or backwards. He recommended this experiment: try getting out of your seat right now only using your quads. Now sit down again, keep your feet in the same place but scoot your but forward an inch and do it again. This is relatively the same difference you will experience by riding a steeper bike. That being said, is there really a huge impact on power output? How will a steeper bike negatively affect your "climbing muscles" as they claim to do?

He also mentioned something about the "hip-torso" angle, but it was unclear to me what he was talking about and how it can impact your cycling. Can someone elaborate on this?

Thanks!
Thats some can of worms you just opened! =)BigLeadOutGuy
Dec 31, 2002 6:32 AM
I think the general consensus is that when you are more forward on the bike, its a bit more aerodynamic and easier to spin at higher cadences but you dont produce as much power. When you are further back on the bike, you make more power but are less aero dynamic. You can notice the difference by sliding foward on your saddle all the way and spinning up then slide all the way back and spin up.
Somewhere in between those extremes is the golden position in which you produce the most power most efficiently. Trying to find it is the hard part =)
As far as using different muscles, I think that the further back you are the more you use your glutes and other large muscles in your legs thus producing more power.
re: ? about seat tube angles and musclescyclopathic
Dec 31, 2002 9:09 AM
you're using the same muscles, only in different proportion. Moving saddle back uses more hamstrings and gluts, moving forward more quads. This happens due to change in "hip-torso angle" (openning angle utilizes less gluts and hamstrings) and change in force vector direction relatively to seat tube. 8 deg change in STA is equiv to ~8% grade, think of it.

btw your friend isn't correct 1deg STA change translates in ~1/2" on 31-32" inseam, so 74 to 78 will do 2" not 2cm.

How it will impact power output? There's no simple answer; to many variables to take into consideration. Training, anatomic differences, metabolism, muscle make up etc etc etc. Quads usually ~50% stronger then hamstrings; however hamstrings/gluts have higher % of low twitch fiber, so they recover quicker and can maintain output longer. On top If you also take into consideration aerodrag, comfort, things get even more tricky. Talk about Cobb and Big Slam =)

With respect to Tri bikes keep in mind bike part is short/no draft; basically a rest stop btw swimming and running ;). Most races won running, and that's where they spent most training time. Not sure how this experience translates to cycling.
Steep anglesKerry
Dec 31, 2002 4:51 PM
The primary value in steep STAs is to allow you to get a better aero position - it opens the angle between leg and torso, so you can lower your torso. An early claim to the steeper STA was that made the transition to the run easier as the pedaling motion was "more like running." Not really true. Your example about rising out of the saddle doesn't really apply - it's better to think of the feet being anchored at the BB and your body rotating forward. Your back is closer to being flat because you've rotated forward, not because the angle between your torso and leg has changed. Also, you're looking at about 1 cm of forward saddle movement per degree of STA, so a 78 is going to put your saddle about 4 cm farther forward. This will obviously have a big effect on handling, muscle recruitment, etc. It's best to have a bike designed around this rather than accomplishing it with something like a reversed seat post. And you'll certainly have to get used to this kind of change - your body won't like it when you first change.
This guy knows what he's talking about...hayaku
Jan 1, 2003 1:19 AM
There is some good information about Hip/Torso angle and some realities about the pros and cons of steeper STAs.

It's hard to know which possition is best, you just have to experiment a lot and find what you like. Testing could help find you ideal possition but there are so many variables in racing that I think you should stick with what feels best.
http://www.slowtwitch.com/mainheadings/techctr/bikefit.html

M.