Consider recommending installation of shorter crank arms on a bike when a cyclist complains of knee or hip pain. After years of bike-fitting, here’s why we’ve concluded that this modification is invaluable.
By Rick Schultz, MBA, DBA, and Amy Schultz, PT, DPT, CSCS
One of the biggest problems in bicycling, I’ve found, is that pain is considered normal.
I (RS) am a master bike fitter and an elite cycling coach. Roughly half the cyclists who seek my services complain of chronic knee pain—pain that has gotten so bad that, if they cannot get relief, they tell me, they are going to give up cycling.
Recently, I attended a cycling camp for 40 juniors, 10 to 17 years of age. These camps build team camaraderie on long, high-intensity rides, preparing cyclists for the upcoming racing season. I was asked to give a presentation on bike fit. Halfway through, the team director asked the campers: “How many of you have experienced knee pain when riding—not discomfort, but actual pain?”
Every one of those 40 juniors raised their hand.
Why do so many cyclists whom I see suffer from knee pain? Much of the problem exists because most training and certification courses on bike fit don’t educate the fitter on how to solve knee pain. Instead, the basic recommendation given by instructors is to raise the saddle.
What causes knee pain?
We hypothesize that cycling with crank arms that are too long places too much stress on the knee at maximum flexion—ie, at the top of the pedal stroke (Figure 1). During a bike fit, one of the main measurements is to place the knee at maximum extension (180°) into an accepted range of 142° to 152° (150° is optimal). To obtain the maximum extension measurement, the saddle height is adjusted either up or down. The leg at maximum flexion is never considered or measured by most bike fitters. What causes knee pain is that the knee at maximum flexion is in a hyperflexed position and, being at the top/start of the pedal stroke, this is the leg/knee to which the cyclist is starting to apply full power.
In bike-fitting, the belief today is that the saddle should be raised to relieve some knee pain. But the saddle cannot be raised too much or the leg at maximum extension will not be able to reach the pedals and, consequently, the cyclist will need to rock the hips to reach the bottom of the pedal stroke. The only way to adjust the knee angle at maximum flexion without affecting the knee at maximum extension is to replace the crankset with shorter crank arms. Today, the most popular crank-arm length on cranksets installed by bicycle manufacturers is 172.5 mm (roughly, 6.8 in).
Existing literature for this topic falls into 3 categories;
- Effect of crank-arm length on power output. Martin1 determined that, for chosen test subjects, a (shorter) 145-mm crank arm produced the most power, compared to 120-, 170-, 195-, or 220-mm crank arms (Figure 2).
- Crank-arm length versus torque. Johnston and colleagues2 looked at 14 studies (of which only 4 discussed knee pain) and found that 1) the knee is the most common joint affected by cycling overuse injury and 2) knee pain is reported by 40% to 60% of recreational cyclists and 36% to 62% of professional cyclists.3-6
- General cycling and knee pain. The solution recommended by researchers who discuss knee pain, in all other papers analyzed, is to raise the saddle.
As recreation bicyclists continue to cycle, they will get more fit and stronger; in getting stronger, they will tend to cycle farther and longer and push harder on the pedals. This will lead to overuse injury of the knee, however, and if the bike fit places the knees in a hyperflexed position, this will result in excessive shear forces on the knee. Professional cyclists ride more than 120 miles (193 km) a day while racing a grand tour; if their bicycle is set up with crank arms that are too long, this will also tend to cause knee pain, due to increased shear force on the knee at maximum flexion.
To alleviate knee pain, the literature that we reviewed recommends, again, only raising the saddle. True, raising the saddle diminishes the angle of flexion of the knees, but doing so risks other undesirable consequences:
- Knee fore-aft misalignment. Modern bicycles have a complex geometry; raising the saddle also forces it into a more rearward position, making another bike fit necessary.
- Maximum extension exceeds a tolerable range. Raising the saddle will likely cause the leg that is in the power-pushing part of the pedal stroke to overextend, which can cause pain at the rear of the knee.
- Pain at the rear of the knee. Having to reach for pedals can, in turn, cause hips and lower back to rock, which, likely, will lead to hip and lower-back pain.
- Power reduction force to the pedals because the cyclist’s knee angle will likely exceed the acceptable range (142° to 152°).
Testing the Hypothesis: The Real Problem
Every cyclist who comes for a bike fit should get checked for both maximum flexion and maximum extension angles (Figure 1). I find that every cyclist who comes in complaining of knee pain exceeds certain values for maximum flexion. Some knee pain is the result of a wrongly placed cleat or saddle (or both) but, for most cyclists, the main cause of knee pain is that the crank arms are too long, thus causing the cyclist’s knee to exceed certain angles. To describe the problem, I’ve coined the term, “Cyclists Knee Pain Syndrome,” in which crank-arm length is the largest contributor but other contributors (ie, syndrome) are ill-fitted cleats which usually place the foot into an unnatural position, placing excessive shear forces on the knees and hips. Once correct-length crank arms are placed, relief of pain is often immediate.
Biomechanics of the Knee When Cycling
During each pedal stroke, the knee goes through a revolution of both flexion and extension. Onset of knee pain is often coupled with overuse and exacerbated by overflexing the knee while applying a great amount of power. For example, at a minimum, a typical competitive amateur road cyclist trains at least 3 hours a ride, 5 days a week. Again, this is a minimum: Professional cyclists typically train 6 hours a day, 5 or 6 days a week.
Each week, therefore, a typical competitive amateur cyclist is on the bicycle 15 hours (3 hours × 5 days a week). When this cyclist is pedaling, cadence is typically 90 revolutions a minute. Each training ride, this cyclist’s knees are completing a full rotation of the crankset 16,200 times (90 × 60 × 3); each week, 81,000 revolutions; each year, 4,212,000 revolutions. If the knee is even slightly out of adjustment, the resulting pain can be severe and become chronic.
As the angle of knee flexion (KF) increases, the patella translates superiorly, increasing tension force on the patellar tendon. Not only does the tendon elongate as KF increases, compressive forces at the patellofemoral joint also increase. (Of note: Compressive forces rise from 50% of body weight during normal walking to 300% of body weight during stair-climbing to 700% of body weight during squatting, due to the increase in KF.) The more KF a cyclist has at the top of the pedal stroke, therefore, the greater their risk of repetitively overloading the patellofemoral joint.
The greater the loaded angle of the knee, the greater the shear forces (Figure 3); as the knee becomes more and more hyperflexed, greater force is applied to the patella—which, in turn, applies greater force to the articular cartilage. The more the knee is hyperflexed, the more strain there is on the anterior cruciate ligament, posterior cruciate ligament, patellar tendon, meniscus, medial collateral ligament, and lateral collateral ligament.
Entering the “Zones” of Flexion
Of the thousands of bike fits we have completed at Bike Fitness Coaching, roughly 50% of clients have sought relief from knee or hip pain, or both. For most cyclists who experience pain, the pain is in the knees; hip pain usually does not occur unless the cyclist also is experiencing knee pain.
As each cyclist is being fit, we record numerous metrics. The most important are:
- Presence of knee pain
- KF angle
- Crank-arm length.
For a client who reports knee pain, a new crankset, with shorter crank arms, is installed and the client is fit to their new crankset. The client is instructed to do their usual rides for the next 1 or 2 weeks, then to report back on how the new bike fit feels and whether they continue to have knee pain. For almost every client, knee pain disappears completely by Week 2. In the rare case in which knee pain is still present, the client is sent to a physical therapist or their orthopedist; most report back that they have an aggravating condition for persisting knee pain, including osteoarthritis, a meniscal tear, a ligament injury, or tendinitis.
So, what are the numbers we recorded? To make it easy for the client to understand, as well as to make a more dramatic explanation, I have come up with 3 ranges of angles of KF that I call “zones.”
These 3 zones of the angle of KF make it easy for a client to understand what is happening (or what will usually happen) if they continue to pedal within these zones:
- Cyclists in the RED zone (a hyperflexed knee) expose themselves to excessive stress, strain, and shear forces on tendons and ligaments of the knee; almost all experience knee pain in this zone.
- Cyclists in the YELLOW zone might experience knee pain.
- Cyclists in the GREEN zone do not experience knee pain.
What Is a Safe Crank-arm Length? Why Does Shorter Work Better?
Shorter crank arms have several advantages over longer crank arms. Consider the advantages created when we replace a crankset that has 172.5-mm crank arms with a crankset with 165-mm crank arms, thus reducing the crank arm by 7.5 mm on each side (Figure 4). Using this example:
- Pain is relieved. Just by installing shorter crank arms, without any other adjustment, the knee at maximum flexion is lowered by 7.5 mm. That difference provides approximately 3° to 4° of relief.
- The zone that the cyclist occupies changes for the better. A bike fitter will always measure the knee angle of the leg that is at maximum extension. After installing shorter crank arms, the cyclist will be sitting 7.5 mm too low in the saddle; the first thing the bike fitter will do is to raise the cyclist’s saddle by 7.5 mm. By raising the saddle, the knee that is at maximum flexion will also be raised, which will lessen the severity of hyperflexion by another 3° to 4°. Therefore, if a cyclist’s’ knee is hyperflexed, changing out a 172.5-mm crankset for a 165-mm crankset will provide approximately 6° to 8° of relief. This will usually move the cyclist’s knees from the RED zone to the GREEN zone.
- Hip flexion is also reduced. This reduction is especially important for triathletes, time trialists, track cyclists, and other cyclists who ride seated in an aggressive position, which can cause hip movement or impingement. If the closed hip–torso (hip closure) angle is too extreme (< 45°), the triathlete first experiences soft-tissue impingement. Going more extreme, bone-on-bone impingement follows. In even more aggressive positions and angles, the cyclist risks damaging the femoral and iliac arteries.
Case Studies: 6 Cyclists Who Were in the RED
All 6 cyclists (designated “A” to “F”) who are discussed below were in the RED zone prior to the bike fit that I provided for them; all were experiencing acute, severe knee pain. After the bike fit, shorter crank-arm length placed all 6 cyclists into their GREEN (safe) zone.
Cyclists A, B, C, and D below have responded to me that, even after longer than 2 years of continued cycling, their knee pain is completely gone. Cyclists E and F continue to race while still experiencing a slight level of knee pain. Before long crank arms on all the cyclists bikes were replaced with shorter crank arms, onset of knee pain was immediate for all 6 cyclists.
Note: Cyclists E and F require further medical investigation because permanent knee damage might be their problem.
- Cyclist A: 71-year-old male road cyclist, 5’6” (167.64 cm). He suffered from chronic knee pain resulting from an anterior cruciate ligament-related injury, sustained playing high school football, and subsequent surgery. He was sized for, and purchased, a 54-cm road frame assembled from the factory with 172.5-mm crank arms. After several weeks of riding, Cyclist A experienced increasing and chronic knee pain. He came for a bike fit and was evaluated using Vari-Cranks (a variable-length crank). Solution: 160-mm crank arms with a power meter. Pain resolved immediately and fully. Cyclist A rides his bike almost every day.
- Cyclist B: 38-year-old female triathlete, 5’0” (152.4 cm). She came for a bike fit complaining of increasing knee pain. Her bike was a 49-cm triathlon bicycle assembled with 175-mm crank arms. She was evaluated using Vari-Cranks. Solution: 150-mm crank arms. All pain subsided immediately. Cyclist B actively competes in triathlons.
- Cyclist C: 65-year-old female road cyclist, 4’10” (124 cm). She had ridden her new bike only twice (2.5 miles each ride) before giving up cycling because of severe knee pain. Three years later, Cyclist C decided to have a bike fit. She was evaluated using Vari-Cranks. Solution: Stock 170-mm crank arms were replaced with 145-mm crank arms. Most knee pain subsided immediately; all knee pain subsided within 2 weeks. Cyclist C rides her bike several times a week.
- Cyclist D: 52-year-old female road cyclist and racer, 5’4” (162.56 cm). She developed osteoarthritis in both knees. The 170-mm crank arms on her bike caused chronic pain in both knees. She was evaluated. Solution: A determination made that the correct crank-arm length for her should be 155 mm. After shorter crank arms were installed, all pain subsided immediately. Cyclist D rides her bike 5 times a week.
- Cyclist E: 24-year-old male professional cyclist. He raced when he was 22 and 23 years old but, because of debilitating chronic knee pain, hadn’t ridden his bike in a year. Solution: Stock 172.5-mm crank arms were replaced with 165-mm crank arms. Most pain subsided immediately. Cyclist E resumed racing. He still complains of slight knee pain; he and I are in discussion about switching to 160-mm crank arms.
- Cyclist F: 22 year old male professional triathlete and a Category 3 California state champion. He complained of chronic knee pain. His road bike and a triathlon bike had 172.5-mm crank arms. Solution: Evaluation showed that the road bike should be equipped with 165-mm crank arms and the triathlon bike with 160-mm crank arms. After the crank arms were replaced with shorter arms, he experienced an 85% reduction in knee pain over the course of 2 days and a 98% reduction over 10 days. Cyclist F continues training and racing.
When I started collecting metrics and looking at different industry crank-arm length standards, I quickly found that length calculators and formulas are from the 1970’s, which, by today’s standards, yield values that are too long.
For example: I am 6’0” (182.88 cm), with a 35” (89 cm) inseam. and I use 172.5-mm crank arms on one bike and 170-mm crank arms on my other bike. When I entered my height and inseam into several crank arm-length calculators, the results were that I should use crank arms between 173.85 mm and 192.24 mm. Because crank arms at those lengths are not manufactured, I would need to use between crank arms between 175 mm and 200 mm.
In the past, I did use used 175-mm crank arms but I now ride with 170-mm or 172.5-mm crank arms because of chronic knee pain.
New safe formulas
I’ve created Bike Fitness Coaching formulas (Table 1) that I believe are closer to ideal—to what a correct and “safe” crank-arm length should be. There are, in fact, 3 formulas:
- base (middle)
- upper (for a slightly longer crank arm)
- lower (for a slightly shorter crank arm).
The lower and base formulas were designed to place cyclists in the safe (GREEN) knee-angle zone. The upper formula is the longest a cyclist should consider.
How do these formulas stack up in real life? I tested 24 cyclists, whose inseam measurements ranged from 70 cm (~27.5 in) to 92 cm (~36.25 in), and found that the base and lower formulas placed all cyclists into their safe zone (Table 2). Offering myself again as an example: Taking my measurements (height, 6’0” [183.75 cm]; inseam, 35” [89 cm]), I use either 170 mm or 172.5 mm crank arms, which place me in the safe (GREEN) zone in all 3 crank lengths formulas (upper, base, lower). But, I’ve noticed that, if I do use 175-mm crank arms, I develop knee pain within the next 2 or 3 rides. So, armed with this information, I know never to purchase, or ride, a bicycle with crank arms longer than 172.5 mm.
Equating Bicycle Frame Size to Safe Crank-arm Length
Taking this analysis a step further, I’ve created a chart (Table 3) that shows cyclists who are purchasing a new bicycle with a frame size of xy-cm which crank-arm lengths I recommend they have installed on that bicycle to place them in the GREEN zone. Following this chart should keep a cyclist in the safe (GREEN) zone. Because triathletes ride in a more aggressive position, the crank arms that I place on a triathlon frame are usually 1 size shorter than the ones I place on the same size road frame.
Crank-arm Length and Generating Power
The common concern I hear from cyclists is “But if I go with shorter crank arms, I’ll lose power.” This seems to be the main reason cyclists stay with overly long crank arms.
The first study to refute this assertion was Martin,1 cited earlier, which concluded that crank-arm length did not statistically matter when it comes to power. (The only drawback to that study was that it considered average power only.) In 2005, Rinard, working at the bike designer and builder Cervélo, performed his own study,7 in which he concluded that 1) shorter crank arms are aerodynamically superior to longer ones and 2) shorter crank arms allow a triathlete or time trial-cyclist to maintain a more open hip angle that is closer to the hip angle of a road position. With shorter cranks, therefore, triathletes and time-trial cyclists can ride in a more aggressive (and yet more comfortable) position, and can stay in that aggressive position longer.
“No” to a Change in Crank-arm Length
What should the bike fitter do for cyclists who are in the RED zone but who don’t want to buy a new crankset? For a variety of reasons, there are always some clients who will not invest in a new crankset. For them, the bike fitter has 3 main choices to present; ultimately, the client will need to choose which option works best for them:
- Measure and adjust the saddle to the leg at maximum extension, leaving the other knee angle as is. The cyclist will be good at maximum extension, but “too low in the saddle” at maximum flexion, possibly causing even more flexion and even more knee pain.
- Measure and adjust the saddle to the leg at maximum flexion, leaving the other knee angle as is. The cyclist will be good at maximum flexion, but “too high in the saddle” at maximum extension, possibly causing the cyclist to “reach” for the pedals at maximum extension. This might cause severe hip rocking and shift pain to the lower back.
- Measure and “split the difference.” Most clients choose this option; they are “too high in the saddle” at maximum extension but still “too low in the saddle” at maximum flexion.
There appears to be a direct relationship between crank-arm length and knee pain among cyclists, based on the angle of KF and corresponding compressive/tension forces. There has been little field research on knee pain during cycling, despite customers’ often-reported knee pain. Our purpose here has been to close the gap in the literature on knee pain and crank-arm length. The key take-home points are:
- As KF increases, so do forces on the patellofemoral joint
- Shorter crank arms should, at the least, be considered when bike-fitting a cyclist who complains of knee pain.
Rick Schultz, MBA, DBA, is a master bike fitter and USA Cycling (USAC) Level 2 Coach and Certified Skills Instructor. He is the owner of Bike Fitness Coaching, San Juan Capistrano, California. Amy Schultz, PT, DPT, CSCS, is a bike fitting educator and is board-certified and licensed by the State of California in kinesiology and interdisciplinary studies.
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