The Triathlete's Holy Grail: Part III
On to the bike! There are different biomechanical considerations on the bike when compared to the swim: 1) you are propelling an additional object attached to your body (the bicycle); 2) the drag component of water is no longer present; 3) the primary force application occurs over a smaller surface area (i.e., in swimming the force of the leg is applied over the entire surface area of the leg whereas in bicycling the force of the leg is applied via a single point on the foot). Furthermore, a bicycle is designed to be very speed efficient, meaning that in most situations you can go faster on a bicycle than off it, or at least I hope that is the case. This is because you are extending your leg’s lever arms (via the cranks), and harnessing the power generated in torque application to the rotational movement of the wheels. This power, referred to as "wattage" by some, when multiplied RPM (cadence) results in speed. In this situation, a longer lever arm and the presence of wheels means that small changes in biomechanical efficiency will multiply changes in potential power application and cadence, either for better or worse speed.
Because force application occurs through a smaller surface area, it is more possible to have "dead spots" in the pedal cycle, or areas of zero torque. This is because the distance between the center of rotation (the crank) and the line of force application (the point on the pedal where the foot connects) equals zero at two points in the pedaling motion: when the crank is at 12 o’ clock and at 6 o’ clock. In both these scenarios, the center of rotation (the crank) is directly above or below the point of force application (the foot on the pedal axis). Therefore, unless a forward or backward motion can be generated, neither the crank or the leg will move. In swimming, on the other hand, it's very difficult to have a torque of zero, since the line of force application occurs over the entire surface area of a limb (in addition to the fact that no matter where you push, unless you're swimming on ice, there is substanially less mechanical resistance).
To better understand, try this experiment. Place a pen point side down on a flat, solid surface. Now, push straight down through the pen with your index finger. If you push straight down through the pen with the point of your index finger, the pen will not move. This is because the center of rotation (the point where the pen connects with the table) is directly below the line of force application (your index finger). The only way the pen will move is if you introduce a horizontal movement by pushing diagonally across the pen with your finger. By doing so, you have changed the line of force application, and thus allowed for torque production at the point where the pen contacts the table. Swimming would be like taking the same pen, turning it horizontally, and placing it in jello. Yum. Also, very little chance of a "dead spot".
Using this same theory, when your foot is directly above the crank and pushing straight down or below the crank and pulling straight up, the point of force application lies directly in line with the center of rotation. The only way the crank will move is if a horizontal force is applied. This horizontal force can be generated by two movements: 1) pushing the foot forward at the top of the pedal stroke on one crank; 2) pulling back at the bottom of the pedal stroke on the opposite crank. This eliminates the dead spot and allows for torque production and enhanced productivity in the pedal stroke. It’s the same as pushing the pen forward, rather than straight down.
The unilateral pedaling drill is a perfect drill for improving efficiency in the pushing forward and pulling backward motion. Pedaling with just one leg, focus on "scraping" or pulling the foot back at the bottom of the pedal stroke, and pushing, or "toeing" the foot forward at the top of pedal stroke. The more circular your pedaling becomes, the more torque you are capable of producing.
So how else can torque be increased at the axis of rotation about the crank? As you now know, torque is a product of the force and the distance of the force from the center of rotation. Therefore, we can either change the force or change the distance. Do not forget, however, achieving maximal torque is not the The Holy Grail for triathletes - we instead want to achieve maximal torque with minimal fatigue and injury. It's like trying to determine how fast you can drive your car and still maintain optimal gas mileage.
There are, in general, two more ways to directly affect torque about the crank joint. The first is somewhat obvious - change the actual length of the crank arm itself. By increasing the crank length, we can increase the distance from the center of rotation to the point of force application and thus increase the lever arm and increase torque. However, an excessively long crank results in: 1) problems with cornering, since the crank could potentially touch the front wheel; 2) increased resistance to angular momentum, making fast cadences more difficult; 3) excessively flexed knees and hips at the top of the pedal stroke, which places the muscle at a disadvantage for contraction. The third problem is one that creeps up several times when considering torque on the bicycle - optimum muscle length for contraction. Basically, if your joints are excessively flexed prior to a contraction, the muscle is unable to produce as much power because the actual muscle fibers are at a biomechanical disadvantage, and also must fire over a longer distance, or longer period of time, which can decrease power (power is work accomplished over a period of time).
For a simple demonstration of this concept, try jumping from a deep squat position. Mentally note the height jumped. Now jump again, but start with your knees slightly more extended, in about a half squat position. You jumped higher the second time because the muscles were closer to their optimum contraction length, and had to generate the force over a shorter distance, resulting in more power and greater height. This concept is known as the muscle length-tension relationship and will be revisited when we discuss seat height, the aero position, and other considerations for running.
The crank length can also be decreased, thus decreasing torque at the crank joint, and increasing the force application required in the muscles to achieve a constant power. A crank that is too short will prematurely fatigue the muscles. Studies that examine optimal crank length are beyond the scope of this article, but a basic understanding of how crank length affects torque is helpful.
In addition, torque about the crank joint can be affected by the fore or aft position of the cleat, since this can change the actual point of force application, and thus increase the lever arm of the ankle muscles (a more forward position) or decrease the lever arm of the ankle muscles (a more backward position). If a bicycle has been fitted properly, the ball of the foot should be positioned directly over the pedal axle, which results in maximum torque production with minimum knee joint injury potential. A slightly forward cleat position, which places the ball of the foot ahead of the pedal axle, can allow for greater torque production, but makes higher cadences more difficult (increased resistance to angular momentum), while a slightly backward cleat position, which places the ball of the foot behind the pedal axle, can limit torque production by shortening the lever arm, but makes higher cadences more achievable.
There are a myriad of other biomechanical considerations for the bicycle, including seat height, seat fore/aft position, seat tube angle, and aerobar positioning. In order to keep these articles palatable for those with busy schedules or short attention spans, I've decided to elongate this series on the Triathlete's Holy Grail. Next week, I’ll discuss how seat height and the aero position can affect power on the bike, and, if time permits, continue in a discussion of biomechanical considerations for the run.
Until next time, train smart,
Ben Greenfield