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A common instruction in forward folds in yoga is to "bend the knees to protect the hamstrings" (and specifically the hamstring tendon attachment up at the sitting bone).

The idea is that bending the knees will reduce the stretch on the hamstrings, and this will keep the hamstrings and their tendons safe.

As common as this instruction is, did you know that it’s not accurate from a movement science perspective?

Here are 3 reasons it’s time to retire this cue:

1) The hamstrings muscles cross two joints: the hip and knee joints.

It's true that if we bend our knees in a forward fold, this will effectively put some “slack” in the hamstrings muscles.

But in response, we tend to naturally take up that slack by angling our torso more forward because there’s suddenly more room for the hamstrings to lengthen from their sitting bone attachments instead.

The end result is hamstrings that are still stretched to the same degree! Because if we move less at one end of a two-joint muscle, we can simply move more at the other end.

By the same token, if we straighten our knees in a forward fold, this simply means that we might not angle our torso as far forward. Once again, in the end, the stretch on the hamstrings is the same!

So bending the knees is not a strategy that necessarily reduces the amount of stretch on the hamstrings.

2) In research studies, hamstring stretches with straight knees are regularly utilized.

Clearly, if these types of stretches are approved by research ethics committees (which exist to make sure that research study protocols are ethical and won’t harm subjects!), then the scientific community does not believe that straight-knee forward folds are harmful.

So why do we believe they're a problem in the yoga world? 🤔

3) Muscle contractions also stretch tendons!

In the yoga world, we tend to fear that passive stretches pull on tendons too much and that this can be injurious.

But did you know that muscle contractions effectively pull on tendons too?

Think about it: muscles attach to tendons and tendons attach to bones. When a muscle contracts, that muscle pulls in toward its center. This exerts a pulling force (a stretch!) on the muscle’s tendons, and the tendons transmit that force to the bones.

Yet we don’t hear fearmongering in the yoga world about active, engaged muscles, now do we?

In fact, we often hear the opposite: that activating our muscles is what supposedly “protects” our tendons from injury.

 Now how does this compute??

If anything, active muscle contractions and strengthening work apply more force to the tendons they pull on than passive stretches do.

Therefore, by this logic, passive stretches could be considered safer – not more injurious – than active, engaged muscles! 🤯

But in reality, neither passive stretches nor active muscle contractions are inherently injurious for tendons.

This is because tendons are tensile tissues that respond well to pulling loads (i.e., tension!).

In summary, it might be interesting to explore forward folds both with bent knees and straight knees to observe the differences in how your body feels in each one.

But neither is inherently safer or more injurious for the hamstrings or their tendons.

However, if we’re talking about protecting the hamstrings from injury, guess what factor is actually established by research to be protective against injury?

You guessed it: hamstring strengthening!

And hamstring strengthening happens to be very the focus of our 5 Weeks to Strong & Flexible Hamstrings program – designed specifically with yogis in mind! Order your copy today and make your hamstrings and their tendons happy and resilient!