Pistons, Potatoes and Bullets: Energetics and Pedaling Technique
Cyclists’ muscles metabolize chemical energy, stored mostly as glycogen and fatty acids, and convert it to contraction forces that accelerate the legs around the pedal circle and drive the pedals. Metabolic capacity alone does not determine cycling success. The amount of oxygen one can consume and the amounts of glycogen or fat one can metabolize in a given time is just one of several important factors. Riders get different results from the same amount of energy metabolized, and even from the same muscle contraction forces. Aerodynamics, tactics, motivation, bike weight and position all affect success, as does pedaling style.
We’re going to focus on pedaling style, cadence and the movement pattern of the feet and legs. Muscles can only apply force to the bones to which they are attached. For instance, the quadriceps and hamstrings can forcefully change the angle of the knee, while the glutes and hip-flexors change the angle of the hip. (One of the quadriceps group, rectus femoris also flexes the hip.) Coordinated combinations of muscle contractions and bony levers transform these changes into force on the pedals. As with most elements of cycling skill, an overview of why pedaling style matters and how it can make you faster and more successful can motivate improvements.
Three Modes of Energy Transfer
In biomechanical terms, the goal of pedaling is to transfer energy from the legs through the pedals to the cranks. There are three modes of such energy transfer, each with implications for optimal pedaling style and optimal cadence, and each with a role in bike racing. The three modes are: 1) Piston mode – direct application of muscle force; 2) Potato mode – potential energy transfer; and 3) Rubber bullet mode – kinetic energy transfer. All three finally rely on pressure of the sole of the foot on the pedal.
Like pistons, muscles acting through bones, can push down on the pedals. The gluteus maximus (butt muscle) extends the hip, driving the knee down. The ilio-psoas (hip flexors) pulls the thigh back up. With no muscles below the knee, a rigid ankle, and heavy cranks one could, very awkwardly, make the pedals go round by alternately firing glutes and ilio-psoas. Fortunately for smooth pedaling we have the hamstrings to draw the lower leg back low in the stroke, and the quads to push the lower leg and foot forward over the top.
Increasing either pedaling force or cadence can increase the power available from direct muscle force. The higher the cadence, the less force is required to produce the same power. High forces lead to rapid muscle fatigue, so a high but not infinite cadence supports endurance. The force that muscles can produce decreases with contraction speed. You can’t produce as much force against the pedals when you are spinning madly as when you are mashing a big gear, so there is an optimal cadence and an upper limit to the range of cadences that can be used to generate more power.
Like sacks of potatoes, legs have weight. If you set a weight on a high pedal, the pedal is driven down, even if no additional muscle force is delivered. To get power you must lift the potatoes off the pedal at the bottom of the stroke and put them back on at the top. Hamstrings and hip flexors can lift leg weight off the rising pedal. The more frequently one can lift and drop a leg on the pedals and the heavier the leg, the more energy can be transferred. A typical rider pedaling at 60 rpm can develop about 50 Watts just by lifting the weight of the leg off the back pedal and dropping it on the front pedal without pushing down on the pedal with any muscular force at all. At 90 rpm the same rider delivers 75 Watts by this method. For a beginning rider, this can be more than half the possible continuous power. Even for an elite rider 75 Watts is around 20% of time-trialing power.
Higher cadences increase potential energy transfer but physiology again places a limit on effective cadence. Much of the work of lifting the leg is done by the small, low powered illio-psoas. As one pedals faster and faster the ability to bring the foot and pedal back up limits ability to pedal smoothly. The illio-psoas is trainable to produce more power, but it will never compete with the much larger quadriceps and gluteals.
If you fire rubber bullets at your pedals, the pedals will move. Kinetic energy, the energy of movement, is stored in the leg and then transferred to the pedal. As the pedal falls, the leg is moving downward as well as resting on the pedal. In the first quarter of the pedal stroke the thigh is accelerating and storing up kinetic energy. In the lower portion of the pedal stroke, it decelerates, so the energy of movement has gone somewhere. With an efficient pedal stroke, most of the energy has been transferred to the cranks. With an inefficient stroke, some is wasted. For our typical rider, kinetic energy transfer varies from about 12 Watts at 60 rpm to 27 Watts at 90 rpm, this is small compared to the other modes of energy transfer, but can be large compared to the differences in sustainable power between two racers in the same category. Note that roughly 100 Watts or one third of the output of a strong rider is available from potential and kinetic energy transfer with no direct application of muscle force to the pedals.
Implications for Pedaling Style
Watching the pros in races suggests that 90-110 rpm most of the time might be the ideal cadence. More than 90% of all hour records set during the last 80 years have been set between 101 and 106 rpm, suggesting this is an ideal cadence range for time-trialing for most riders. Biomechanical modeling indicates that ideal cadence will depend somewhat on body size and weight distribution and muscle type, so there will be no perfect cadence for all riders.
It is possible to pedal in such a way that the majority of the kinetic and potential energy of the legs gets transferred to the cranks, or in such a way that energy is wasted. Movement and muscle forces near the ankle make the difference. Earlier I mentioned a rigid ankle. This is good but not ideal. The largest and potentially most powerful muscle in the body is the butt muscle, the gluteus maximus. When the gluteus extends the hip, if the ankle is rigid, energy will be transferred to the pedals. If the ankle is slack so that the heel drops during any part of the pedal stroke in which the gluteus could be driving, that portion of the pedal stroke loses effectiveness. Allowing the heel to drop during the down-stroke can reduce energy transfer by as much as ten percent. Since the difference in average speed between first place and last can be a fraction of one percent, that’s an insurmountable disadvantage. Having the heel low at the top of the stroke and then driving the ball of the foot down, so that the ankle is somewhat extended by the bottom of the stroke not only prevents the energy drain, but allows some additional power from the calf to be transferred to the crank as well.
How can energy be wasted? The weight of the leg went down, the leg slowed, how could the energy not be transferred? Answering this question requires a small dose of 2-dimensional vector physics. You already have an intuitive grasp of the essential principle: Power is force times speed, but only to the extent that the force and speed are in the same direction. If you push from behind on a car that is rolling slowly, it speeds up. You’ve transferred energy to the car. If you push sideways on the same car, your arms and legs may get tired, but you won’t change the car’s speed and you won’t transfer energy to the car.
Pushing down when the pedals are moving down transfers energy to the pedals. Pushing down or standing on the pedals near the bottom of the stroke doesn’t. Optimal transfer of the potential, kinetic and muscle force energies of the legs to the pedals occurs when the force is in the same direction as pedal movement. This doesn’t mean that one should try to pedal around in circles, always following the pedal. Riders don’t have muscles that can push the pedals with large force throughout the cycle. They do have muscles and transferable energy that are very effective when pushing roughly down. Pedal force analyses of elite time-trialists show that they push down very hard on the pedals during the down-stroke, but apply barely any force at all during the rest of the pedal stroke. Elite time-trialists pull back, pull up and push over the top just enough that the pedals don’t have to push the riders’ feet around. Like them you should strive to deliver energy at points in the pedal stroke where your muscles can be used effectively, and just get the feet out of the way otherwise.
Three Useful Ways to Pedal
I count four modes of pedaling, three of them useful: 1) lazy-beginner pedaling, 2) effortless spinning, 3) hard pedaling, and 4) sprinting. The main differences are in amounts of pulling up and pushing down. Most riders can piston pedal, dropping the leg on the front pedal and pushing down. If that’s the only thing you do, the pedal that is driven by the front foot lifts the back foot. Try to go fast this way and you end up with the choppy pedal stroke typical of beginners, especially at higher cadences.
Experienced riders lose the ability not to lift the weight of the back foot at least a bit. If you pull up during the up-stroke just enough to unweight the rear pedal and then drop the weight of the leg on the front pedal without particularly pushing down, you get transfer of potential and kinetic energy and significant power with little feeling of effort. This is spinning. Spinning 90 rpm without pushing down on the pedals generates about 100 Watts for our typical rider, which is part of why experienced riders will find it so frustrating to ride with beginners. Experienced riders have to hold back or alternately coast and pedal to generate less power than this. Since the experienced rider feels that he or she is making no effort, it’s challenging to understand how anyone could need to go slower.
Starting from effortless spinning if you now add the transfer of energy by direct muscular force on the pedals, you get hard pedaling. Like beginner pedaling and effortless spinning, this type of pedaling can be sustained for a good while. Exactly how long depends on the effort and your fitness. This is the typical pedal stroke for chases and breakaways, and for entire road time-trials. Note that this pedaling style still involves unweighting the rear pedal rather than actually pulling up against it.
Only during low cadence climbs, hard accelerations and sprints do elite riders actually pull up against the pedal and deliver energy by direct muscular action during the upstroke. The muscles that pull up are small and fatigue quickly, so they are saved for moments when they can do the most good.
If you are not currently routinely pedaling in the ideal range of cadences, work towards that. On the majority of your aerobic rides, choose one gear easier than your most comfortable gear until your routine cadence is over 90 rpm and you can comfortably spin 100 rpm or a bit higher for an hour or more at a time. It takes several months or even years to develop a smooth, efficient pedal stroke that can be sustained in the optimal cadence range. Avoid spinning a gear so light that you feel that you are chasing the pedals. Be sure that you can in fact push on the pedals at your chosen cadence.
At the same time that you are working on cadence, or if you’re already spinning well, check ankle movement and whether it is optimized for energy transfer: a rigid ankle during the down stroke is okay, but a low heel at the top of the stroke with gradual ankle extension to the bottom of the stroke, driving the pedal with the ball of the foot, is better.
Efficient pedaling style really can make the difference between races won and lost, but if nothing else, it should give you something to think about while grinding away on the trainer or rollers during the dark, cold months of winter.
Head Coach Scott Saifer works with all levels and kinds of endurance athletes and loves new challenges. This article has been updated from its original publication in ROAD Magazine in 2007.