[Ti Designs]

In training I've used machines at the gym to work individual muscle groups within the same range of motion as they are used on the bike. The leg extension machine is great for working the quads, but it's not the same range of motion. If you fire your quad, your foot swings around an arc centered at your knee. The pedal on the other hand swings around an ark made by the bottom bracket - that's not a battle you can win.

Few people I've talked to have understood the destructive forces here, so I'll equate it to something some people get. There's this thing in engine tuning called detonation (also known as preignition or knock) where thermal expansion of the fuel mixture happens before top dead center. The combination of the gasses expanding and the piston still moving up creates huge forces against both piston and cylinder. The rise rate and magnitude of this force is where damage happens, the only saving grace is that it's very short.

OK, back to the bike example. I built a test device to do a bit of resistance training on the bike. I took a huge chainring, attached a chain to one point and ran it straight back to a pulley and weight system. When I turned the pedal it raised a stack of weights, but I only had half a pedal stroke before I ran out of chainring - this was fine, I was only interested in about 90 degrees of that. My first try was a bit of a surprise because I forgot my bike and trainer weigh far less than weights, and there was nothing holding it in place. With that problem fixed I got on the bike, clipped in with one side and tried moving the pedal from 11:00 to 2:00. I have pedaled a bike before, but this was really the first time the pedal was pulling back. Normally your bike is a system of inertia, your weight going forward divided by the gear is your cadence. This time is was just my muscles vs. the load, and I was shocked to find out that I couldn't smoothly get the pedal over the top. I tested a few other people and found that they couldn't do it either. Then I bet a bunch of people $5 that they couldn't do it - I had to make up the money from building the test device...

I brought this problem to one of the labs at Harvard 'cause they like this sort of thing. We discussed the motion involved and the muscle recruitment involved, along with the forces involved and the programmed response. We also discussed known reciprocal inhibitions as I was pretty sure it was going to come down to that. There are two hip flexors you can use for hip flexion, the rectus femoris or the iliopsoas, but the rectus femoris and the vastus lateralis don't like to fire at the same time, probably because of their opposing use of the patella. So, in trying to get the pedal over the top there's a little battle between two muscles within the quads, one shutting the other off which resulted in the bouncing motion I saw on the test device.

If you know exactly what you need to learn, you can teach your body to do almost anything. It took me about two weeks to learn to flex my hip with just the iliopsoas, then extend at the knee smoothly. Getting back on the bike I found I was faster on the flats as I felt my quads kicking forward with full force. After a few weeks I noted that my back problems went away - it's a wonderful trade-off.

Getting back to the idea of detonation, or pushing down before 12:00, the vastus lateralis is the dominant muscle, so while I thought I was getting the pedal over the top, the hip flexor was shut down as I started to push forward at 11:00. If you look at the knee angle at 11:00, the force generated by the quads firing is below 45 degrees, into the pedal circle. The opposing force pushes the hip and effects the SI joint. Reduce that sharp impulse and the back problems go away...