Running Biomechanics: The knee is NOT flexed by the hamstrings

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Published on: February 2, 2021

Audience: Therapists, Trainers & Runners

Main Point: The hamstrings do NOT significantly flex the knee at toe-off.  In other words, runners do not consciously flex their knee when they are running and training this is most likely folly.  I have read a number of chiropractic and physiotherapy running “experts” who advise people to actively flex their leg off the ground and keep it flexed so as to change the moment of inertia about the thigh when someone is running.  The idea is to get the weight of the leg closer to the hip joint so it is easier to swing the leg forward. The problem with this idea is that the hamstrings do not do this when you are running.  Knee flexion occurs passively. It is a result of the hip flexing rapidly and powerfully. While the knee is flexing the quadriceps are actually active.  They are acting to control the amount of knee flexion.  This is what puts strain on the rectus femoris.

The Evidence

We can look at two things:  a kinetic analysis of running (the forces that are produced by and acting on the body) and EMG studies.  One note,  there is some research showing the short head of the biceps femoris is active at early swing and late stance suggesting that flexes the knee.  However, this is one of the limitations of EMG - just because a muscle is on we can’t assume that we know what is doing.  The Kinetic analysis is what gives us insight into what the muscles are doing.  And it is the power and moment (i.e. joint torques) analysis that shows that the knee is not actively flexing but rather it is being passively flexed.  The EMG activity of the short head of the biceps femoris may be providing some other function rather than an as a primary mover. For example, it may be functioning to decelerate the forward rotation of the tibia while the tibia is on the ground.

Kinetic Analysis of the Knee

Knee function during swing and toe off

Ralph Mann and David Winter investigated this more than 30 years ago (click here) and more recently Anthony Schache did a wonderful paper looking at the 3D kinetic analysis of running at different speeds (click here).   What they and others (Cavanaugh) found was that at toe off power is primarily generated at the hips (flexion) and at the ankle (plantar flexion).

To quote a paper by Ralph Mann (1980):

“The majority of flexion is probably secondary to rapid acceleration of the thigh during the initial swing phase and the foot and tibia sort of follow along, bringing the knee into maximum flexion”

After toe off, a knee extension torque develops during the first half of swing (even though the knee is flexing, the quads are working to control this flexion) and power is being absorbed.  A knee flexor torque develops during the last half of swing and power is generated (the hamstrings work to flex the knee in preparation for ground contact).

Simplified version of Knee function during stance

At the knee, there is an extensor torque during most of stance with a slight flexor torque just prior to toe off.  Power is absorbed during the first half of stance (the quads work eccentrically for shock absorption) and power is generated during the latter half of stance. But…

Are the knees helping you push off during running?

Not as much as you think.  The knees really just absorb impact and stop you from collapsing and generate some power during late stance.  What Dr. Schache’s study strongly shows (as with previous work as well) is that with an increase in speed there is no greater work done at the knee joint rather  it is the increase in hip flexor power and to some extent plantar flexor power that is associated with an increase in running speed.  Dr Mann suggests that the knee extension during late stance is again a product of the opposite hip pulling the body forward and over the foot still on the ground causing knee extension.

During sprinting something different happens…the second period of knee extension during stance does not occur.  Rather there is a slow progressive amount of knee flexion during the stance phase.  The sprinter does not have sufficient time to absorb shock at the knee (the shock is absorbed at the ankle).

This finding was also supported by the McClay and Cavanaugh who investigated the extensor paradox.  These researchers (in the book “The Biomechanics of Distance Running” Chapter 6) found that knee extension was occurring more than 120 ms after the quad muscles had shut off.  This is more of time delay than can be expected from the electromechanical delay of muscles (this delay is the time it takes a muscle to turn on, take up the slack of the muscle and then exert force on its attachment).  Therefore, something else besides quadriceps force was creating knee extension during late stance.

EMG during Running

The graph below shows the average muscle activity during running and then sprinting.  Note how the extensor muscle groups are decreasing their activity by late stance and toe off (Source Mann et al 1986).  This has previously been called the extensor paradox.   Power analysis (inverse dynamics) tells us that the leg extension may be coming from another source which is mostly likely hip flexion of the opposite limb.

The bars below denote when the muscle is on and at what point in time in the gait cycle.  T.O. means toe off.

Running EMG

Sprinting EMG

The Bottom Line

The function of the hamstrings as a primary flexor of the knee joint during toe off and the swing phase is overrated.  This is a passive activity.  Kinetic analyses and EMG studies suggest that the hamstrings are more importantly functioning to flex the knee during late stance to bring the foot back to the ground.  This role is coupled with their role in extending the hip during late stance.  Another, important function that is highly related to increasing running speed.  A future post will explore the function of the hip during running.

Greg Lehman


1 Comment - Leave a comment
  1. Let’s put a clinical spin on this information. If the knee flexion is more of a passive motion resulting from powerful pelvic rotation and hip flexion, then what occurs in the runner with insufficient lumbopelvic rotation? That’s right. Greater work from the hip flexors, iliopsoas being a common culprit of low back and hip pain. And by extension perhaps greater effort from the HS to “shorten the pendulum”. That’s why one of the first things I’ll look for with a running eval is adequate lumbopelvic rotation and hip extension mobility. In ChiRunning it’s taught to think “knees down, ankles up” in an effort to reduce the rotational inertia (this recruiting HS more) but when it comes to changing gears (adding speed) the mantra becomes “swivel the pelvis”.

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