Knee Extension and Saddle Height
https://youtu.be/S7HE4ogNlbY
What about parallax and other sources of error? Couldn't this significantly affect your measurements?
This is no small issue but with a little work you can ensure that your measurements are accurate enough for amateur purposes -- in my experience within a degree or two in most cases. To do better than this often requires A LOT more work and for anyone other than a professional performing bike fittings, may not be worth the extra expense and effort.Sources of error:
Setting the vectors of the angles in the wrong place and not through the actual joints. This can be significantly improved by setting some adhesive dots over the appropriate joints to make setting the lines easier. It takes a little practice as well because the joint position changes slightly depending on the point in the pedal stroke -- i.e. the hip joint will be in a slightly different relative position when the foot is at the top of the pedal stroke (and the hip is flexed maximally) versus when the foot is at the bottom of the stroke and the hip is more open. It usually makes a difference of a degree or two and some may choose to ignore this and just always run the lines through the same part of the joint which can be marked with adhesive dots to make this joint identification easier.
Placing the adhesive dots in the right place. This indeed can be tricky, but with a little instruction and practice even a non-medical or non-clinical person can learn to get reasonably good at it. I have a couple of articles that go into this in detail and can instruct anyone how to place the dots. It goes into a lot of detail and you can find it here. There are additional articles that go into how to set up your camera as well as how to read the data etc.
Not having the camera precisely perpendicular to the rider. The line of sight of the camera is very important to making sure you get good data. Put simply, the measurements we take will be most accurate when they occur directly in front of the camera....the more the thing we're measuring goes to the edge of the view of the camera, the more error can be introduced due to parallax. To reduce this error I recommend that riders make two recordings -- of the upper body and the lower body separately, making it more likely that the joint(s) being measured can be done so more accurately. This may not be the most efficient way to do it for the professional fitter of course, but if you're just an amateur looking to learn a few things about your own fit, it can be fine.
In this video I had the benefit of also having infrared motion capture data for this rider so I was able to compare my measurements to the infrared's and even with a very basic setup of the camera I was within a degree or two at most for every measurement.
Liberal use of bubble levels and lasers (like a contractor would use to line up a kitchen cabinet install for example), if you have them, can help quite a bit. But some carefully placed tape lines and the 3-4-5 method can do wonders. (Google it...if you're unfamiliar the 3-4-5 method helps you make sure you have a 90° [perpendicular] angle).
What should the knee extension measurements be for different types of biking and bikes?
Each person will of course have their own acceptable ranges based on their mobility, fitness, flexibility, etc. So one racer might be best suited with a 42° knee extension angle and another at 34°.But in regards to other bikes, our road position will usually be the bike setup where we have the most knee extension. so let's assume a rider does best at 40° on their road bike, what might be their knee extension on other bikes?Mountain bike: likely to be anywhere from a couple to many degrees more into flexion -- so perhaps 42°-48° with a lower overall saddle height, depending on terrain -- however today with the pervasiveness of dropper seatposts, many riders can have their max seat height on their mountain bike be every bit as high as their road bike. They may only have it at this height for the occasional road jaunt to get to the trail, or on a fire road and the majority of the rest of the time their saddle may be bumped down a centimeter or two.Triathlon/Time Trial: Again, likely to be a little more knee flexion on the tri/TT bike (so our hypothetical 40° road rider may be in the 42°-43° range) , but there will even be some variance between a TT setup and a tri setup -- duration of the ride and whether we have to be prepared to run afterwards may influence things by a degree or two either way.Touring: I find that the balance of the loads placed on the bike make the largest difference here. A very light load or one that's exceptionally well distributed might do great with the same knee extension as the road bike. But more weight and/or more poorly balanced loads can benefit from 1°-3° more flexion in order to be able to shift weight around on the saddle depending on the conditions.