Respecting the Two-Joint Muscle

Today’s article is the beginning of a mini-series inspired by questions I’ve fielded after two of my more recent posts.  My appreciation for the Hamstring Bridge Hold and dead-hang pull-up comes from a deep love of functional anatomy, and today we’ll take a look at why we sometimes feel specific exercises in seemingly random places.

Those “random” places are often due to where a muscle starts and ends, and today I’ll share some of the common moves I coach that benefit from some sexy pictures.  Let’s strike that balance between knowledge and execution, like Jacked Albert:

In my college kinesiology class, we used flashcards to learn our muscles.  It was rote memorization at it’s finest, and I loved it.  Learning the origin, insertion, and action of so many of our muscles isn’t everyone’s cup of tea, but a bit more knowledge of our inner workings goes a long way.

When it comes to two-joint muscles, our learning typically begins and ends with the quads and the hamstrings.  Specifically, it’s the rectus femoris, and biceps femoris, respectively.  The word “femoris” should tell you that they’re along the femur, or upper leg, but that’s not where they start or end.  Both of these muscles have origins, or starting points, on the pelvis and have ending points, or insertions, that give them control of the lower leg.  That means that they not only have control over what your hip joint does, but they also have control over what the knee joint does.

This can seem both simple and complicated at the same time, and my professors were inclined to stop there.  Fortunately, I’m lucky to have found countless books, workshops, and mentors who have shared the importance of these two joint muscles.

As a general rule to follow, if a muscle passes across two different joints, it will exert forces on each of them, but at different times.  Let’s look at the hamstrings of the model of the right.  They serve to extend the hip, which is what they do when we’re performing a deadlift.  They also serve to flex the knee, which they do when we’re performing a stability ball leg curl.

Typically, a two-joint muscle is strongest doing one of those at a time.  When the hamstrings are flexing the knee at their strongest, they’re weakest at extending the hip.  When they’re strongest at extending the hip, they’re weakest at extending the knee.  And when we ask them to do both at the same time, they aren’t the strongest at either.

It’s at those times, when we’re asking muscles to work from both ends at the same time, that things may feel a little funky.  In this series we’re going to focus on some of the two-joint muscles that we use most frequently, and how they can perplex us while we strive for strength.

Enter: Active Insufficiency

This mini-series is on Netflix is going to apply the biomechanical concept of active insufficiency.  A muscle that is actively contracting creates less force at it’s shortest or longest, so active insufficiency occurs when a multi-joint muscle reaches a length where it can no longer apply effective force.

The idea is typically used on models and in labs to help kinesiology students figure out what muscles are working at specific joint angles during movement.  I believe we’re all students of our body, so applying this concept is an important one for us all to understand.

You’ve experienced versions of Active Insufficiency if you have experienced the following:

• More calves than hamstrings on leg curl (or bridge) variations.
• More triceps than lats or abs on roll out variations.
• More lower back than lats on a pull-up.
• More lower back than quads or hamstrings while squatting or pedaling.

All of these things are normal to feel, and it doesn’t mean that we’re doing anything wrong.  In fact, sometimes feeling muscles other than the ones we’re expecting means we’re actually really nailing it.

The positions that our bones are in can have a big impact on how our muscles are working, and if we can control our position, we can control how our muscles function.  Before we do that, we must appreciate how moving one piece effects another, and that’s why it’s important to respect the two-joint muscle.