Dooley Noted: 4/14/2015
I have a strong opinion about what makes a muscle a true hamstring.
One must meet all three criteria.
1. One must be innervated by the tibial nerve, the true oppositional nerve to the femoral nerve.
2. One must extend the femur, relative to the acetabulum (i.e., hip extension).
3. One must flex the leg, relative to the femur (i.e., knee flexion).
Gastrocnemius almost makes it. He has 1 and 3.
Even popliteus and plantaris almost make it. They also have 1 and 3.
Adductor magnus’ hamstring head nearly makes it. It has 1 and 2.
The short head of biceps femoris (BF) is generally accepted as a hamstring.
I have quite the problem with that.
Short head BF only meet criterion 3.
Short head BF is innervated by the common fibular nerve – unlike the true hamstrings.
Short head BF doesn’t cross the hip – unlike the true hamstrings.
Short head BF is not even as close as plantaris in regards to being a true hamstring!
I consider short head BF to be a major compensator for true hamstring function.
This muscle crosses one joint, unlike the true hamstrings. With a different innervation and one less joint to cross, short head BF can throw off the mechanics of the hamstrings. 
In the cadaver, the superior aspect of adductor magnus’ hamstring portion leaves off where short head BF begins. Once I saw this, I knew they must try to act together as a true hamstring.
But as my friend Jason Kapnick says, “Things that wire together fire together.”
And short head BF is wired differently than the rest of the posterior chain, below the gluteal region.
Therefore, short head BF should be assessed differently than the hamstrings.
Short head BF attaches to the linea aspera, a rough line on the femur. It is separated from the vastus lateralis via an intermuscular septum.
This septum can become so sensitive to the touch that I’ve had patients nearly jump off the table. As they misinterpret it as “IT band syndrome,” I explain there is much more anatomy in that area than just the IT band.
The short head biceps must be addressed in those who cannot obtain full knee extension.
Perhaps more importantly, short head BF can prevent full hip extension.
The short head BF joins with long head BF (a true hamstring) a few inches above the femoral head to which they jointly attach.
If short head BF is facilitated, its exertion so pull towards the femur can throw off long head BF mechanics at the hip.
These are often the people with a deeper knee bend on overhead teaching and bending, where they trade knee flexion for hip extension.
Short head BF can throw off the swing and strike phases of gait, where the muscle must eccentrically load to support those phases.
I have seen this result in true sciatica, since the long head biceps is tightly placed across the sciatic nerve. (Piriformis gets too much credit).
If long head BF gets pulled towards the femur and fibula, it stretches itself across the sciatic nerve, resulting in pins and needles down the back of the leg and into the foot.
I hope you start thinking of short and long head BF as separate muscles.
These are two muscles with different innervations, origins, and mechanics.
As for short head BF, I hope you add this one-joint muscle to the commonly facilitated list.
As always, it’s your call.
– Dr. Kathy Dooley