Originally Posted by pedex
^^ what about the loading on the knees when pulling up on the pedals while seated and pedaling a freewheel bike? its no different than riding a fixed, you just tend to do it more often on a fixed gear
First of all i would like to say that the dynamics of pedaling a bike are quite complicated, and aren't going to be ironed out over an internet forum. people write doctoral theses on this subject! If its something you are curious about, there are a number of papers written on the topic. i might go try to dig some up in a bit if anyone is interested.
but for the sake of discussion and maybe making this thread productive, ill gander a bit of an explanation as to why i think resisting has a greater potential to hurt your knees.
for one thing, effective braking/resiting will consist of very high rates of deceleration. that means lots of force, more so than just casually pedaling. lets just estimate the amount of force that gets transmitted through your legs during a hard stop is similar to that of an all-out sprint. thats a lot of force.
Now, consider the way your legs work in a sprint relative to how they do while stopping. while in a sprint, the forward pedal (with the leg extended) is pushing down and the back pedal (with leg curled up) is pulling up. Whereas in stopping its reversed. the back pedal is pushing and the forward pedal is pulling.
your leg produces the most pushing force when it is almost fully extended (like when you jump up). you cant push very well with a curled leg (as anyone who has tried to ride fast with too low of a seat will know). so, a huge amount of force goes through the leg pulling up (front leg) to compensate. your leg evolved never having to pull up with any more force than needed to lift your leg (like to go up a stair, or on a freewheel bike). The muscles and the angles of the joints cant develop much force this way. So what i see happening is your hip muscles hold your thigh up (90* to the torso), and then the pedal pulls your calf down, with all the force between the two going into stretching (instead of compressing, like it was design for) the ligaments of your knee. Ouch!
I wont claim to be an expert on bio-dynamics, and again encourage people to find reading on the subject. but just look at your leg. it's made for pushing, not being pulled on. maybe that will lead you to your own ideas about the subject.