Pedaling in Circles

posted Aug 16, 2012, 6:55 AM by Donald Vescio   [ updated Aug 16, 2012, 7:26 AM ]
It generally is assumed that the most efficient way to pedal is by applying maximum force on the downstroke of the pedal cycle, while simultaneously pulling upward during the upstroke.  The intent of this model is to establish a cycle in which variations are minimized, resulting in the smooth generation of power.  For years, cyclists have referred to this model--that of pushing down hard, while pulling up on the opposite site--as "pedaling in circles."  Riders would spend hours doing drills to maximize the upward pulling force in the pedal cycle (think of one-legged pedaling drills, for instance) to "smooth out their pedal stroke."

Interestingly, as early as as 1991, research documented that elite cyclists tend to focus primarily on the downward force component of the pedal stroke (and pulled up less) than non-elite riders during periods of submaximal and maximal efforts.  In simpler vernacular, what this means is that elite cyclists tend to stomp down on the pedals very powerfully during hard efforts, and that they demonstrate little upward pulling force during the other phases of their pedal strokes.  This finding runs contrary to the accepted wisdom that one needs to pedal smoothly and consistently if one wants to ride fast.  When looking at the data, it became apparent that riders who attempted to pull upward on the pedals rode at a higher metabolic cost and were less efficient than those who simply stomped down on their pedals, suggesting that the muscles that flex the leg on the upstroke are less efficient than those that extend the leg during the downstoke.  

To better understand this dynamic, the concepts of positive and negative forces need to be described:

  • Positive Force: The force that we apply to the pedal as it moves from the top of the pedal stroke to the bottom (in terms of a clock metaphor, think: from 12.00 to 6.00).  Positive force generates forward movement of the bicycle.  Positive force also can be described as a productive force.

  • Negative Force: The force on the pedal associated with our body weight as it moves from bottom of the pedal stroke to the top (think: from 6.00 to 12.00).  Negative force also can be described as a retardant force.  


Now, these two forces are closely linked.  As we press hard on a pedal during the downstroke of a pedal cycle, our bikes  move forward; but as we press down on one pedal, the other pedal ascends, and we have to overcome our body weight as the pedal rises.  The more energy that we reallocate from pressing down on the pedal to overcome the weight of our body on the opposite side of the crank, the less positive force is generated that contributes to the forward motion of our bike.  In other words, a portion of our energy is being used to lift out body upward, which lessens the efficient use of our power.  The more weight that we apply to the ascending pedal during the upstroke, the more energy we shift away from the productive force associated with the downstroke.  Efficiency, then can be seen in terms very different than "pedaling circles": efficiency could refer, instead, to maximizing the production of positive force during the pedal down stroke, while minimizing the impact of negative force associated with the pedal upstroke.


If we accept this characterization of pedaling efficiency, it then makes sense that we should do whatever we can to press down as hard (and as long) as we can on the pedals.  It also would make sense that we try to actively pull up on the pedals during the upstroke, as this would eliminate the negative force associated with having to lift our body weight.  The problem with this is two-fold: there is a higher metabolic cost associated with flexing the leg during the upstroke than there is in extending the leg during the down stroke; negative pedal force comprises a greater percentage of the overall pedal stroke than productive positive forces.    The following diagram illustrates the application of positive and negative pedal force:



Remember, positive forces are associate with pushing down hard on a pedal; negative forces are associated with the amount of weight the rider has to lift during the pedal upstroke.  Additionally, riders pay a greater metabolic cost if they pull up on their pedals, as opposed to pushing down on them.  The most efficient pedal stroke, then, relies on the high application of positive force during the downstroke, which comprises approximately less than 50% of the entire pedal stroke.  In practice, the most effective and efficient cyclists are those who stomp down hard on the pedals during the power phase of the pedal stroke, while unweighting their opposite feel during the upstroke, when little positive power is generated.  Unweighting the foot simply means making the foot and leg as light as possible on the pedal during the upstroke--top riders essentially are balancing their feet on their pedals during the non-productive phases of the pedal stroke.  This cycle of pressing hard on the pedal/unweighting the pedal is consistent, whether one rides with a rapid or slow cadence.

A simple way of understanding the pedal stroke is to view the phase between 12.00 and 6.00 (see diagram above) as centering on positive (productive) force; and the phase between 6.00 and 12.00 as centering on negative (retardant)  force.  This understanding, however, does not take into account the transition phases that occur at 6.00 and 12.00, when the rider is shifting from one force form to another.  Traditionally, the 6.00 and 12.00 positions of the pedal stroke are referred to as "bottom dead center" and "top dead center."    At these positions, the rider is able to generate very little positive force; on the other hand, negative force also is minimized in these positions, too.  In order to move the pedal more effectively through bottom dead center and top dead center, a rider can take advantage of vector geometry and the mechanics of elastics. When approaching the 12.00, top dead center position, riders can drop their heels as they begin to push down, thereby enabling an earlier application of positive force to the pedal stroke.  When passing through the 6.00, bottom dead center, position, riders can allow their heels to be snapped upward by the lower leg muscles, enabling them to take advantage of these muscles elastic properties by imparting a slight secondary acceleration of the pedal during the upstroke, which has the effect of unweighting the foot from the pedal.

What is interesting about the pedal stroke dynamic discussed above is that it also can help a rider understand why climbing while seated is more efficient than climbing while standing.  Many studies have demonstrated that climbing cadences tend to remain the same, whether one climbs seated or standing.    Climbing while standing is associated with greater peak positive forces (uo tp 130% greater than positive forces generated while seated), but it also is associated with much larger negative forces on the upstroke, which mitigates positive force gains.  So, for climbing at a given wattage, riders who stand will experience greater variations of power than those who climb seated.  Because riders shift their bodies forward in relationship to the center of the crank when the climb standing, the phase in which positive force is generated also is shifted forward by 15% to 30%.  Shifting the positive force phase in this manner results in little to no torque being applied to the pedals top and bottom dead center, which can create a  choppy pedaling motion.  Because riders who climb standing  necessarily are supporting a greater portion of their body weight than those who climb seated, and because the amount of torque generated top and bottom center center is less, standing climbers realize a higher metabolic and muscular cost than their seated competition.  In addition, because of greater peak power variations, riders who climb standing also experience additional metabolic and muscular cost as they experience greater accelerations and decelerations during their pedal strokes.

While all of this might sound complicated, the take-aways are quite simple.  

  1. Push down hard on the pedals during the down stroke;
  2. Make your foot as light as possible during the upstroke;
  3. And climb seated to minimize metabolic and muscular cost.

Activities such as one-legged pedaling drills are useful because they help riders habituate themselves to unweighting their pedals during the upstroke; such drills are not intended to build strength to enable riders to actively pull up on the pedals in normal cycling situations. Cycling efficiency has little to do with pedaling in circles in a narrowly prescribe cadence range.  Rather, effective cyclists understand the dynamics of the pedal stroke in such a way to maximize the application of positive force to the pedals, while eliminating sources of loss.





.
Comments