Let's consider two centers of rotation in a discussion of seat height: the hip and the knee. The force is produced by the gear pushed against on the bicycle, and the line of force is primarily affected by the direction of force application of the cleat on the pedal. The lever arm would be the distance from the hip and the knee joints to that line of force application.
In order to produce a greater torque at the hip or the knee joint, we can increase the force, which is easily done by shifting to a higher gear, climbing a hill, or simply pedaling harder. We can also increase the lever arm, which can be done by decreasing the seat height. With a decreased seat height, the knee and hip are in a greater amount of bend, or flexion, at both the top and the bottom of the pedal stroke, and both joints are thus farther away from the line of force. In other words, we look like Grandpa on a beach cruiser.
Unfortunately, this not only places an excessive muscular force requirements on the muscles surrounding the hip and knee joints, but it also places the muscles in a sub-optimal length-tension relationship. Therefore, a seat height that is too low can not only hurt your knees and hips, but also decrease your speed and place an increased metabolic requirement on your heart and lungs.
The knee should be just slightly flexed at the bottom of the pedal stroke, with the ankle extended, or pointed. By extending the ankle, you 1) allow the leg to move through a full range of motion without locking out the knee at full extension, where you are not capable of producing any torque, and 2) increase incorporation of assistive calf musculature.
Furthermore, by slightly dropping the heel, or flexing the ankle at the top of the pedal stroke, you can control the amount of flexion that occurs in the knee. A dropped heel at the top of the pedal stroke can improve the muscle’s length tension relationship at the knee joint, but caution should be taken not to "ankle" or excessively rotate at the heel throughout the pedal stroke, as this can lead to lower leg tension, and eventually chronic tightness or inflammation at the Achilles tendon.
Performance related suggestions are for a saddle height of about 109% of leg length for short term power output and about 105-107% of leg length for long term endurance activities. This will give you a "ballpark" correct range of motion.
The aero position is favored by most triathletes. In the aero position, the athlete is bent forward, with trunk flexed and the arms resting on aerobars. Typically, there is an increased seat tube angle (forward) to accommodate this position, as well as a slightly forward seat position. The overall effect of riding in this position is that the pelvic bone is tilted forward.
Because the hip extensors, the muscles responsible for "pushing out and back", are attached the pelvic bone, a forward pelvic tilt results in lengthened hip extensors and shortened hip flexors (the muscles responsible for pushing the leg up and forward). By lengthening the hip extensors, the force of the pedal stroke is redistributed among the hamstring and glute muscles, and slightly removed the from the quadriceps and knee extensor muscle groups. By shortening the hip flexors, the pull phase of the pedal stroke is quicker and the quadriceps become less fatigued. The end result is quicker cadence, increased incorporation of assistive hip musculature, and a slightly lower force production, due to the loss of the quadriceps muscle group.
By lowering force production at the individual muscle through redistribution of force requirements, time to fatigue is lengthened. Furthermore, the increase in cadence is believed to offset the loss in potential force production from the quadriceps, so power can still be maintained. And that's the beauty of the aero position: less fatigue with the same amount of power! Furthermore, it has been suggested that fresher quads and warmed-up hamstrings going into the second transition can accelerate the running cadence and speed.
Plus, riding upright on a bike equipped with aerobars just doesn't look that sexy. So if you're really serious about competing in the sport of triathlon, make sure you invest in a bike that biomechanically accommodates an aero position. It is probably the best "speed investment: you can make, when compared to lighter shoes, wetsuit, vented helmet, etc.
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Ben will be coaching the Iron speed Triathlon Camp in Coeur D' Alene from Saturday, May 9 - Thursday, May 14, 2009. Camp info is HERE.