Understanding Bike Fit – KOPS (knee over pedal spindle)
Over the last 20+ years of performing bike fits, no single bike fitting axiom seems to be more of an issue than the concept of Knee Over Pedal Spindle or (KOPS). To illustrate a common issue with the concept I offer this rather typical question from a client and my response.
Question: I have a somewhat technical bike fitting question. I have heard from many sources that give the following information: “In proper neutral fore-aft saddle position, with the pedal at 90 degrees [3 O’clock], a plumb line placed on the front edge of the kneecap should fall to the end of the crankarm.”
On the other hand, an equal number of others say that’s just not correct. I’m more likely to agree that this method is faulty, because people have different femur lengths with regards to their total inseam. Also, different bikes are made with different seatpost angles. I find that even if I were to put my seat all the way forward, my knee still wouldn’t be over the spindle! Is there a better rule-of-thumb?
Answer: This is an excellent question and one that many people have had trouble rectifying when reading from the various sources that offer their opinions. For this discussion we will assume we are discussing a standard road geometry where this type of positioning would apply. Triathlon, Time Trial, and Mountain bikes would only in the rarest of instances match to this discription.
The exact quote ” in the proper neutral fore-aft saddle position, with the pedal at 90 degrees (3 O’clock), a plumb line placed at the front edge of the kneecap should fall to the end of the crankarm”, is generally incorrect as it assumes too much. I have seen this description listed in many an article over the years and it is a tough one to justify since the front edge of the knee cap is also not the bio-mechanical marker for the knee. This position would place the riders knee from 2-5 mm forward of the ball of the foot relative to the common bio marker of the Tibial Tuberosity (TT or bump just below knee cap)depending on cleat position, length of crank arm, etc… , however since they also use the end of crank arm instead of spindle center the general relative distance between TT and pedal spindle is about the same. In either case the fact still remains, it can be a poor position for delivering effecient power to the pedal, a cause for generating knee strain, vastus medialis soreness, hot spots in the foot and a number of other issues. It does not mean it is the incorrect final position and can many times be the resulting position, however it is faulty logic to say that every rider should be in this position from the outset. KOPS is not the ultimate end goal, more over it is on average very close to the final position.
KOPS is used, even though it has no real biomechanical basis because it does seem to work quite well and is an ideal starting point when setting a bike up from nothing. One of the reasons it works is really very simple. A diamond frame bike could theoretically have a much steeper or shallower seat-tube and be as biomechanically efficient at either extreme, but the inclination of the saddle would have to be tilted up or down and friction would be insufficient to stop you sliding off it. Similarly, the pedal forces in the KOPS position are predominantly upward with a small rearward component. In normal pedalling efforts these will be reacted by gravitational and saddle frictional forces.
If one is to follow strict KOPS rules then the neutral postion the plumb line or laser should generate from the tibial tuberosity to the center of the pedal spindle. This would be at or just behind the ball of the foot (or 5th metatarsal). This position tends to greatly reduce the strain on the patellar tendon during the donwstroke, specifically when the most force is being generated, on a typical road geometry. Having the seat too far back tends to over straighten the leg creating essentially a seat that is too high. From the neutral postion (once found) you can tweak the seat postion forward or back depending on your riding style (high cadence spinner, big ring masher or something inbetween).
All this is still not much help without taking into account all the other factors that effect a properly positioned knee, but those are for another discussion. So here is in a nut shell how saddle positioning brakes down: a higher seat assuming nuetral positioning transmits more power, while a lower seat conserves more muscle energy. The seat height of course also affects the bio mechanics of the knee itself. The more you bend your knee the greater the stress placed upon the patellar tendon, if the seat is too high then the strain tends to go to the iliotibial band. Assessment – you feel pain in the front of the knee – raise the seat slightly. You feel tightness or pain in the back or side of the thigh – lower your seat. If you have a functional discrepency causing the knees to be two different angles that is where shims, wedges, orthotics or other devices can help.
To further illustrate my point that KOPS is not what we as fitters are actually looking for here is an exerpt from an article in the mid 90″s by Keith Bontrager.
The KOPS rule seems sensible enough; it puts the knee in line with the pedal at maximum pedaling force, which must help, right? Wrong. The KOPS rule of thumb has no biomechanical basis at all. It is, at best, a coincidental relationship that puts the rider somewhere near his or her correct position. It probably grew out of someone’s observations that may successful riders sit on their bicycles with their knees somewhere over the pedal spindle. In fact, there has been little comprehensive work done in the field of cycling biomechanics that has studied rider position on the bicycle in order to maximize power input or minimize fatigue. Most builders and fitting specialists rely on customer feedback to tell them whether a change in position feels better or worse. This information doesn’t pertain to power output; it is a result of physiological response called perceived exertion, only one of the several important variables that are related to a rider’s muscular effort. In short, there is no scientific evidence to support the KOPS method.
To properly assess these structures in the dynamic environment of pedaling it is advisable too have a profesional bike fit expert work you through the process and make changes based on trained objective experience. A proper bike fit is the only way to not guess at the proper location for your knee.
It should be noted that this Q and A was from a conversation in 2005. Today, nearly 6 years later this question still comes up in some form nearly every fitting. Today we have adopted tools such as Retul to provide true 3D motion capture that is accurate to less than a millimeter in an active dynamic environment. This allows us too see exactly what is the best relationship of the foot, cleat, pedal, crank length, Femur, Tibia, saddle, saddle position and more in real time to achieve the correct position for the type of rider and type of bike. Listed below is a quick quide as too some normative standards relative to our prefered standard of (knee forward of foot, where knee = Tibial Tuberosity and Foot = Fifth Metatarsal) for different types of bikes.
- Road Bike (0 mm to -10 mm)
- TT Bike (+50 mm to +100 mm)
- Tri Bike (+50 mm to +100 mm)
- MTB Bike (-20mm to -10 mm)