Your hamstrings are not made to produce power by curling at the knee and instead are made to produce power at the hips while helping to stabilize the knee joint.
One of the most persistent objections I hear from riders who have never tried flats is that without being attached to the pedals you can not use your hamstrings properly which forces you to rely too much on the quads the power the pedal stroke. By not being able to curl the knee joint during the upstroke of the pedal stroke you create muscular imbalances and tire out the quads faster, or at least that is what these riders say. However, this understanding of which muscles are used and how they are used during a pedal stroke is completely wrong and potentially dangerous over the long run.
When I ask why someone thinks that the muscles are used this way during the pedal stroke I am invariably led to some variation of this picture/ chart:
According to this theoretical model of muscles used during the pedal stroke the hamstrings are used maximally from 8 o’clock to 10 o’clock position while the glutes and quads are responsible for the downstroke part of the pedal stroke. This paints a completely false picture of the situation and leads a lot riders to assume that the hamstrings are only there to flex the knee joint on the upstroke, which would be impossible to do if you weren’t attached to the pedals. This, of course, would mean that it would be impossible to optimally pedal without clipless pedals, which is where the faulty logic stems from.
The problem with this whole notion is that this chart is completely theoretical and based on how the muscles work in isolation from each other. Unfortunately, the reality of how the muscles work together to create the actual pedal stroke movement is much different than the what this chart tells us. The model this chart is based on also assumes that all muscles that cross a joint are there primarily to flex that joint, as if the muscles on the front side mirror the actions of the muscles on the backside.
The human body is not set up so that the muscles are mirror images of each other – the hamstrings are not the “backside” quads. The hamstrings are made to powerfully extend the hips while less powerfully flexing the knee, the quads are made to powerfully extend the knee while less powerfully flexing the hip. Together they both work with and counteract each other to produce lower body locomotion. Train the hamstrings to flex the hips and stabilize the knee and the quads to flex the knee and help stabilize the hip joint – that is how those muscles function in real life and how we should train them, not based on the old model of training each muscle that crosses a joint to powerfully flex it.
In fact, trying to have a rider curl their hamstring to produce force on the upstroke is unnatural and asks the knee to produce force in an unstable position. Your hamstrings are not made to produce power by curling at the knee and instead are made to produce power at the hips while helping to stabilize the knee joint. The idea that you need to curl your leg through the bottom of a pedal stroke is simply wrong and based on old and faulty logic – you want to flex the hip to push the leg through the bottom of the pedal stroke, not flex the knee.
Just like when running you don’t want to produce power by flexing the knee, you simply use knee flexion to get the leg back into position for the next “push”. The human body is made to push, not to pull, and trying to apply pulling (curling the knee is a pull) to lower body locomotion isn’t the most effective thing to do.
You want to produce your power at the hips, not the knee joint. The reason that cyclists have the knee issues they do is because the knee joint lacks stability, not strength. This is why I am an advocate for standing up more to pedal because it forces the knee and hips joints to act and stabilize more naturally than seated pedaling does.
As an interesting side note, I came across this chart of a pedal stroke while researching this article. It looks like it was based on actual EMG readings, not a theoretical model.
As you can see the Biceps Femoris (fancy talk to hamstring) is most active on the downstroke and least active on the upstroke. In fact, where first chart shows the hamstring to be most active is actually the place it is least active according to the EMG in the second picture. In other words, the first chart is flat out wrong and in no represents what is actually happening during a pedal stroke.
Take another look at the second picture and you’ll see how the downstroke finds all of the muscle groups lighting up and the upstroke sees very little activity by comparison. This also underscores the findings in the Mornieux and Korff studies, which was that a powerful downstroke with the lead leg and a more passive return of the trail leg was the most powerful and efficient way to pedal. You shouldn’t be worrying about trying to create power on the upstroke, which means that you can create the most powerful and efficient pedal stroke without being attached to your pedals.
The idea that you can not optimally use your hamstrings during a pedal stroke without clipless pedals is based on faulty logic and theoretical models. Now that we have a more accurate insight into what is actually happening we see that models like the first picture/ chart need to stop being used as a way to think about pedaling our bikes. The hamstrings are one of the more important muscles used during the pedal stroke but it is how they work in concert with the other muscles of the lower body on the downstroke – not by themselves on the upstroke – that form the reality of pedaling your bike.