Running Biomechanics Primer

Hi.  This is another training tips installment.  I am not attempting to teach everyone how to run. Well maybe I am.  This was a neat little study that I did in an attempt to learn different terms that are used to describe the physical aspects of running.  I wanted to improve my technique and a lot of online coaches use very technical terms to describe how to improve your form and technique.  So I did some research and compiled what I learned here.  Because I compiled this information from all over different places, I had to be very inventive to put it all together.

So let's start with some very basic terms in then get into the running part.

Biomechanics is the study of the mechanics (physical actions of forces) as it relates to the functional and anatomical analysis of biological systems (namely humans during running in this blog post). Kinematics is a description of motion in regards to time, space, and velocity.  Kinetics is the study of forces associated with the motion of the body. In other words, kinematics is focusing only on the motion itself and measuring the changes.  Kinetics also takes into account why those changes took place. So a lot of the terms I present in this blog post get's into the kinetics of running. Chock full of science.  You have been warned.

The biomechanics of running and walking are similar but are different in this way.  In walking, there is a point when both feet are touching the ground and that is when weight bearing is being shifted from one leg to the other.   In running, both feet never make contact with the ground at the same time and the leg absorbs the weight bearing just after the foot makes contact of the ground.

The Gait Cycle:

When we refer to the running gait, we are basically describing the biomechanics that are involved with human locomotion (or what your legs are doing as you are running).  It’s the repetitive sequence that’s common to all runners.  Running also involves the arms but we are going to look at first what your legs are doing.  When we describe the different phases and steps, we are going to focus on what one leg is doing at a time.  However, as you are running, both legs are involved in the gait cycle and are doing the same thing, but one leg is close in opposite phase of the other.  They overlap in the swing phase which creates two float stages (or the double flight).  (Note: In a walking gait, the phases overlap in the stance phase creating what is called a double support.)

There are two main phases to the running gait cycle: stance phase and swing phase. The stance phase begins at the moment your foot hits the ground until your foot leaves the ground.  The swing phase begins at the moment your foot leaves the ground until it makes contact with the ground again.  Within each phase is a number of steps or stages.  Within the stance phase you have the following steps (or stages): Contact, Absorption, Mid-stance, Terminal Stance, and Push Off.  Within the swing phase you have Pre-swing, Initial Swing, Mid-swing, Terminal Swing, and Approach.  There are also 2 float (or flight) stages when both feet are off the ground simultaneously.  The first one occurs during the approach step when the swing foot is just about the make contact with the ground and the other foot just left stance phase at push off and is just now entering swing phase during pre-swing.   Since we are looking at gait from the prospective of one leg at a time, the second float step occurs during pre-swing phase just after the foot pushed-off the ground while the other foot is still approaching.  

Phase: Stance Phase                                                     Swing Phase
Steps:   Contact Absorption Mid-stance Terminal Stance Push Off Pre-swing Initial Swing Mid swing Terminal Swing Approach 

And to compare with what each leg is doing together.

L Foot  App Contact Absorption Mid-stance Terminal Stance Push Off Pre-swing Initial Swing Mid swing Terminal Swing Approach 
R Foot Pre-swing Initial Swing Mid swing     Terminal Swing   Approach  Contact Absorption Mid-stance Terminal Stance Push Off
                       ^second float stage                                                                                 ^ first float stage

Before we look at each step of the running gait, let’s define some movements that our body makes. 

Flexion and Extension.  These are actions to the joints of our body and how the connected muscle causes the joint to flex or extend.  So if we say to flex our muscle, what we really mean is that we are bending (or flexing) the elbow joint so that our fist moves closer to our shoulder which makes our bicep muscle really pronounced.  When we flex our elbow, our bicep muscle contracts (shorten) and our triceps muscle lengthen.  If we extend the elbow joint, we move our fist away from our shoulder.  When we extend our elbow, the triceps shorten and our bicep muscle lengthen.  Likewise, say we are seated in a chair with our ankles dangling under us.  If we flex our knee we are pulling our heel closer to our butt which is caused by contracting of our hamstring muscle which also lengthens our quadriceps muscle.  Likewise, if we extend our knee, we are moving our foot away from our butt and stretching our leg way out in front of us.  When we extend our knee, this bending is caused by contracting our quadriceps muscle and lengthening our hamstring muscle.  Flexion refers to making the angle of the joint in question smaller while extension refers to making the joint angle wider.  Dorsiflexion is another way to say extension (opposite of flexion).

In the above figure, the knee is being flexed to perform the squat.
Also notice the ankle is flexed as well (plantar flexion).

In the above figure, the ankle is being demonstrated to perform both plantar flexion and plantar dorsiflexion (extension).
The angle of ankle flexion or extension is measured between the sole and the back of the leg.

Plantar flexion is referring to the way we are bending our ankle joint.  During plantar extension, just think of what happens when you keep your heel on the ground but lift your toes up off the ground.  It’s really an ankle flexion (the angle is getting wider from the sole of your foot and the back of the leg).  This is also sometimes called plantar dorsiflexion.   Plantar flexion is the movement which decreases the angle between the sole of the foot and the back of the leg which brings the toes back towards the ground.  

In the figure above, the left graphic demonstrates pronation of the foot (rolling inward)
while the graphic on the right demonstrates supination of the foot (rolling outward).

Supination of the foot occurs during contact and absorption stages of the running gait cycle when the foot doesn’t roll inward enough (under-pronation) or rolls outward.  Supination can be a result of high arches, tight calf muscles, and weak unstable ankles.  Supination can cause your foot to twist awkwardly and leading to ankle related injuries. A runner diagnosed with supination is recommended stabilizer running shoes which provides extra support and cushion which helps absorb the force from impact during the contact and absorption phases.   These shoes also have a straighter instep shape called a last to control inward motion that counters supination.   Shoes with supportive arches can also help. 

In the figure above, the quadriceps muscle is shown to contract in order to dorsiflex (or extend) the knee.

In the running gait cycle, hip flexion causes our thigh and knee to move in closer to our chest.  Think of coming into a fetal position while keeping our back straight. Extending our hip would bring the knee back down closer to the ground.  If we are standing straight with our knees fully extended, hip flexion would bring our foot off the ground in front of us while hip extension would bring it back down (think of the Nazi march).  Likewise, if we start in the same standing position with the knee fully extended, hip extension would move the leg behind us while hip flexion would bring it back to the standing position.

In the figure above, we see the hamstrings are contracting which results in the knee being flexed and consequently the quadriceps are also lengthening.  We also see that the hip is shown to be in an extended position.

In the figure above, we see three demonstrations of hip movement.
(Left graphic) medial rotation of the hip causes the leg and foot to turn inward while lateral rotation causes the leg and foot to turn outward. (Center graphic) Hip flexion raises the hip off the ground in front of the body while extension causes the leg to return to the ground and even pull slightly behind the body. (Right graphic) Hip abduction causes the leg to raise outward laterally (feet apart) while hip adduction brings the feet together.

In the figures above, we see two demonstrations of varus momentum on the knee (bending or twisting of the knee inward). 
This is also known as knee knocking. In the figure on the right, we compare varus rotation with valgus rotation.

Varus torque is created by applied forces that are exterior and below the knee joint which causes varus momentum or torque at the knee.  This causes a bending or twisting of the knee inward.  This is compared to valgus torque that is created by applied forces that are interior and below the knee joint. This causes valgus momentum or torque at the knee. 

Concentric contractions refer to a muscle that is actively shorting to generate a force.  When a muscle is activated and required to lift a load, the muscle begins to shorten.  An example of a concentric contraction is the bicep muscle shortening while raising weight during a bicep curl.  Eccentric contractions refer to a muscle actively lengthening usually in response to a greater opposing force.  When an opposing force is acting on the body and the muscle is activated to control or deaccelerate the motion of that body part, the muscle that is being elongated is said to be using eccentric contraction. An example of eccentric contraction is the hamstrings muscles when the body is lowered during a squat.  In this case, eccentric contraction is used to provide balanced tension to control the speed and range of motion during the decent.

Now that we understand the various motions related to biomechanics, let us look at each step in the running gait more closely.  Again, here is a graphic of the running gait.

Note: I attempted to combine different sources on the running gait and the specific movements within each step. There are different running styles that will diverge from what I write below.  And your particular running style may be a bit different as well.  This is to give you a general idea on the mechanics and to introduce terms used when describing the biomechanics.  This is also the style that I am trying to strive to implement into my running mechanics.

Contact: This is when the foot initially makes contact with the ground.  You will hear terms like heel striker, toe or forefront striker, or midfoot striker.  These terms refer to which part of the foot makes the initial contact during this step.  If the heal makes contact before any other part of the foot, then you are classified as a heel striker.  Likewise if the toes or the front of your foot makes the initial contact, then you can be described as a toe or forefoot striker.  Lastly, if the middle part or the ball of your foot makes the initial contact, then you are a mid-foot striker. 

Absorption: After the contact step we move into the absorption step or the absorption stage.  Sometimes this is referred to the loading response stage.  In the walking gait, loading response occurs when the foot makes contact and ends when the opposite foot enters toe off (remember both feet are on the ground simultaneously).  During loading response, weight is shifted from one leg onto the other.  In running, the feet are never on the ground simultaneously. We anticipate weight to shift from the weight bearing foot that undergoes push off to a neutral position during the float or flight stage and then to the new weight bearing foot during the absorption stage after initial contact has been made.  Sometimes this stage is also known as the braking stage because in effect, your body is performing a controlled landing.  Your feet, ankle joints, shin, knee joints, quadriceps, hips, and spine are all absorbing the shock that is caused by the ground during initial contact.  The knee joint and hip joint flexes while the foot pronates. During this step, a heel striker’s ankle will plantar flex as the rest of the foot makes contact with the ground while a toe striker’s ankle will plantar extend or dorsiflex.  Foot pronation will continue into the mid-stance and terminal stance stages all the way through push off.  During the absorption stage, the tendons and connective tissue within the muscles store elastic energy that will be used later in the stance phase. 

Important in the early stages of the stance phase is pronation (or in some cases- supination) of the foot.  Pronation of the foot begins to occur after contact during the absorption stage of the running gait cycle when the foot rolls inward.  Pronation helps your feet absorb shock and adapt to uneven surfaces.  If your foot rolls too far, then you over pronate and that can cause problems to feet, legs, knees, hips, and back.   

Loading is referred to the different forces and momentum felt by the body in reaction to the ground’s forces being applied to the body.  So when we enter the initial contact stage, there is a sudden increased amount of loading experienced by the weight bearing leg.  Remember Newton’s Third Law: For every action there is an equal and opposite reaction.  Now let’s look at Newton’s Second Law: Acceleration is produced when a force acts on a mass.  To put it a little different in prospective of what we are talking about, force is equal to mass times acceleration (or change to velocity).   The action and opposite reaction that we are looking at to loading and loading response has to do with the different forces being applied to the entire lower body and the opposite forces being applied back by our muscles in order to sustain a forward movement of the entire body.  The force being applied to the ground by our body during the stance phase of our running gait is directly related to our mass and the vertical acceleration towards the ground caused by gravity.  However, this is not the only ground reaction force acting on our body.  This would be true if we were jumping up and down on one leg.  However, with running, there is inertia that propels our body forward (horizontal movement) that has an effect on this loading reaction as well.  

The ground reaction forces change as we move from initial contact, during absorption, mid stance, and finally into terminal stance.  In the absorption step, our forward weight bearing leg is a bit out in front of our center of mass.  Ground reaction forces travel up the leg starting behind the ankle joint and travels posterior to the knee joint and anterior to the hip joint.  The angles produced on this leg (hip angle, knee angle, and ankle angle) are all as a result from the reaction to the ground forces.  During absorption, there is an initial loading response which causes a plantar flexion in our ankle joint combined with a pronation or supination due to a secondary reaction lateral to the subtalar axis.  Then there is a flexion movement at the knee joint combined with a varus torque due to a reaction that is medial to the knee joint. Lastly, the hip flexes combined an adduction momentum due to a secondary reaction that is medial to the hip joint.

During absorption, the body will control the movements with eccentric activity in the ankle dorsi flexors along with eccentric activity in the intrinsic foot muscles and other supinator muscles to control subtalar pronation. There is eccentric activity in the knee extensors along with passive tension in the lateral knee structures. Active force in the tensor fascia lata could contribute to knee stability in the frontal plane.  At the hip control is managed by isometric activity in the hip extensors and activity in hip abductor muscles.

Mid-stance: The braking action which is first experienced in the absorption stage continues but terminates into the mid-stance stage. At this stage, the foot is directly under the hip and the body is experiencing peak loading.  As we move into the mid-stance stage, the amount of loading on that leg is increased to its maximum as the full amount of weight is being supported by that leg. 

Also related to the shift of weight is that our center of mass is at its lowest during the mid-stance stage.  About as much as we hear about proper form to include having the foot land under your center of mass during initial contact, the foot will actually land slightly in front of it which also contributes to the braking action.  The closer the foot lands to the center of mass, the less force the body needs to apply to overcome loading.  The further the foot lands in front of center of mass, the stronger the loading forces being applied to the body and the more force the body has to apply to overcome these forces.   If loading forces is greater than what the physical body can sustain, injuries to the body can occur.

During mid-stance, the center of mass is at its lowest and the foot is directly under it and is now moving behind it. At mid-stance our knee joint no longer continues to flex, and our hip joint goes from a flexion action to an extension action caused by our hip flexors to switch from eccentric contraction to concentric contraction. Concentric contraction of the hamstring and hip flexors means that we are purposely using our muscle to move the leg backwards forcibly. 

Let’s look at how the reaction to ground forces change in the mid-stance step. The normal ground reaction force is now acting anterior to the ankle joint, anterior to the knee joint and posterior to the hip joint.  The change in the ground reaction forces causes a plantar dorsiflexion action (pronation continues), an extension of the knee (the varus movement continues), and an extension of the hip (the adduction movement continues).     

During mid-stance, the ankle will control the movements with eccentric activity in the plantar flexors and activity in the intrinsic foot muscles, soleus, and other supinator muscles (in order to supinate the subtalar joint to make the foot more rigid to bear the body's full weight). There is a passive force which develops in the posterior knee structures as they elongate during knee extension and passive tension in the lateral knee structures. Active force in the tensor fascia lata could contribute to knee stability in the frontal plane.  Lastly, there is eccentric activity in the hip flexors along with activity in hip abductor muscles. 

Terminal Stance: As our foot is moving behind our center of gravity, it will get to the point where the hip joint will extend back as much as possible.  You may hear this as the propulsion stage because from mid-stance through terminal stance until your foot leaves the ground, your leg is propelling you forward.  Our muscles also shifted from eccentric contraction to concentric contraction.  Eccentric contraction is found in the early parts of the stance phase during absorption and loading.  Those eccentric forces react against the ground forces to control our landing and prevents us from doing a face plant.  Concentric contraction forces are the forces that powers the run.  During the terminal stance stage, we are now purposely flexing the knee and ankle joints as well as extending the hip joint forcing the leg muscles to propel us forward and will help us push our foot off the ground.  We are also gathering potential energy in our leg that will eventually sling our leg forward during the swing phase. 

Let’s look at how the reaction to ground forces change in the terminal stance step.  The normal ground reaction force continues to act anterior to the ankle joint, anterior to the knee joint and posterior to the hip joint.  However, the normal ground reaction force has shifted from lateral to now medial to the axis of the subtalar joint.  This causes a shift of pronation to now a small supination movement at the subtalar joint.  The ankle responds by in the ankle plantar flexors which have acted eccentrically through mid-stance, now act isometrically. The continued advancement of the pelvis and lower extremity now move the tibia anteriorly, which causes the heel to rise at around 35 to 40 percent of the gait cycle. There is also activity in the intrinsic foot muscles, soleus, and other supinator muscles. These muscles augment supination in order to supinate the subtalar joint and produce an increasingly rigid foot.

There is a continued extension and varus movement in the knee. Therefore, the passive force in the posterior knee structures and passive tension in the lateral knee structures continue to stabilize the knee.  Likewise, there is continued extension and adduction in the hip.  The body responds with eccentric activity in the hip flexors and activity in hip abductor muscles.    

Push Off: Finally we come to the very end of the stance phase in the gait cycle and begin to enter into the swing phase.  Here the foot comes off the ground caused by concentric contraction of our hip flexors and hamstrings. The ankle is fully flexed with the toes pointed as back as they will go.  This is caused by concentric contraction of the plantar flexors which actually pushes our toes off the ground like a spring. 

Pre-swing: So now we are entering into the first stage of the swing phase of the gait cycle.  You should also be aware that we are also in the first float stage as the complimentary foot (the other one) is just about to leave the swing phase in the approach stage. This stage ends when the complimentary foot makes initial contact and the leg begins to sling forward.  This stage is also known as forward recovery stage.  During this stage, the knee joint continues to flex by concentric contraction of the hamstrings which forces the heel to rise up towards the buttocks.  You will also want to make a conscious effort to activate the glute muscles to provide extra power.  This stage is called the pre-swing stage because we haven’t yet slung the leg forward.  We are still using concentric contractions in our glutes and hamstrings to build up enough energy that will eventually sling the leg forward in the initial swing stage.  From mid-stance into the pre-swing stages, we are using concentric contractions to not only propel us forward, but to reserve the elastic energy that will sling the leg forward.  In pre-swing, we are following through with what we started in the late stages of the stance phase. 

Think of a wind-up toy car.  When you pull the car back on the ground, you are creating a lot of elastic potential energy. The gears inside the car cause tension to collect in the spring so that when you let the car go, the car flings forward automatically.  Likewise, from mid-stance all the way into pre-swing, we are applying the similar concentric contraction that will in effect cause passive motion throughout the remaining stages of the swing phase.  Although you hear about driving the knee forward, the most efficient way to move the swing leg forward is by letting it go and let it naturally move forward using passive motion.  Passive motion is the involuntary momentum after we reached the full range of motion as opposed to active motion caused by concentric contractions which drove our leg backwards during the later stages of the stance phase.  Another way of looking at it as pulling back on a rubber band then letting it go.  Active forces creates the tension in the elastic when we pull the rubber band apart, while passive forces fling it forward once you let go.   Although this is the first stage in the swing phase, it is the last stage we are using active motion, in this case the knee being fully flexed by raising our ankle up towards the buttocks. 

Initial Swing: As our complementary leg and foot begins to absorb the ground forces and load is being applied to it, the swing leg can relax and the initial stage of swinging the leg forward can now begin using passive motion.  At this point we will be executing eccentric forces on both legs.  On the complimentary leg, it is absorbing the shock and taking load in reaction to ground forces and bearing the weight to support our body.  In the swing leg, eccentric forces in our hip are used to assist our abductors to prevent pelvic drop on the swing side of our hips. We also want to keep our knee high and fully flexed at this point which will minimize the radius of the swing (compact movement) which will in return shorten the time it takes to swing the leg as well as preserve energy.  So our hips, glutes, lower back, and core are all being used to stabilize our body and keeping it upright. 

A thing to keep in mind is that passive momentum will drive the knee forward and not concentric contraction.  In other words, you do not force the knee to drive forward, it just happens naturally as a result of the steps prior to it. 

However, you can make a conscious movement to promote forward motion.  This is commonly known as the fall.  During initial swing, you will want to “fall forward” but in a way to keep your body upright and stable.  This will minimize the vertical oscillation (the head and body bouncing up and down as we move forward) which promotes a more graceful stride.  This more graceful stride will prevent the unnecessary vertical movements that results in wasted energy.

In the figures above, we see Meb demonstrate good upright posture on the left.  On the
right, Sage demonstrates a good forward lean that comes from the ankles and not the hips.

To “fall forward” correctly, you will want to slightly lean forward from the ankles (not the hips) whereby the upper body is straight and upright that continues from the hips to the top of the head.

Mid-swing: At mid-swing, our tibia bone is completely vertical to the ground and the weight of our body is completely supported by our complimentary leg.  We continue to stabilize the hip to prevent pelvic drop and maintain the high knee.

Terminal Swing: During this stage we continue to flex our hip using passive forces but we now relax the muscles which will allow our knee to dorsiflex.  This allows our leg to come forward to its full range of motion which influences our stride length.  With our knee dorsiflexing (extending), the foot swings out forward in front of our center of mass but the knee remains slightly bent.  The glutes and adductors help keep our pelvis stabilized.  We reach the end of this stage when the hamstrings decelerates hip flexion and the knee is brought as forward as far it will go. At this point our foot it in front of us with knee relaxed and ankle dorsiflexed which may cause our toes to be pointed upward and heel lowered. 

Approach: This is the final stage of the swing phase and just so happens to be the second float stage as our complimentary leg just pushed off the ground and is now entering into pre-swing. This stage will end when the foot finally comes into initial contact with the ground.  At the beginning of the approach, the knee is dorsiflexed with the foot further out in front of our center of mass.  As the foot is approaching contact, passive forces will actually flex the knee to prepare for the shock of the impact forces of the ground and loading reaction to bearing weight onto that leg.  The same passive forces will move the foot backwards and flex the ankle.  This will enable the foot to land more level and closer to the center of mass.  This passive force is sometimes referred to as the paw back.  While it is involuntary motion due to momentum, there are drills that can be performed that will reinforce muscle memory to make this action more pronounced.  This passive force is also the same force that will create momentum during the early part of the stance phase which will bring the leg backwards until mid-stance. 

A common misconception is that you can somehow use concentric contraction in the quadriceps to force this paw back motion.  The muscles are too weak to overcome gravity to assist the already natural motion of bringing your foot back closer to the center of mass prior to initial contact. 

Depending how much (or little) the ankle flexes will determine if you are a heel, midfoot, or toe striker. Keeping the knee relaxed during terminal swing stage will encourage a more forefoot landing as opposed to a heel strike.  When the knee is fully extended during terminal swing, the foot cannot come back far enough during the approach stage and thus encourages a heel strike that will cause injuries since reaction to ground forces is not being properly absorbed by the muscles that can most efficiently absorb them. 

Strides and Steps: The complete gait cycle occurs from the moment one foot makes initial contact with the ground until that same foot makes initial contact with the ground again.  You can also imagine that the complimentary foot has pushed off the ground twice.  The distance covered by the first leg when it completes the entire gait cycle is known as the stride.  The distance covered from the time one foot pushes off the ground until it makes initial contact with the ground is known as the step.  Each foot makes a single step in a complete stride.  Running cadence is usually measured by the number of steps taken by both feet per minute.  If we are measuring by strides, then it is the number of steps taken by only a single foot per minute.  For example, a particular cadence could be thought of as 180 steps per minute.  That same cadence can be thought of as 90 strides per minute. 

Arms: The arms should automatically be in synch with the opposite leg. So while the right leg is in the swing phase, the left arm should also be swinging forward. Likewise, while the left leg is in a stance phase, the right arm is pulling backwards.  Just as the swing phase of the leg is passive caused by the concentric contraction during the stance phase, likewise the arm is swung forward passively while we use concentric contraction to pull the arm backwards.  The movements of the legs and arms do not cross our hemisphere.  What that means, if you drew a line from the top of your head down the middle of your body, no body part crosses over from the left side to the right side (or vice versa). If the arms or hands cross over into the other hemisphere, that is making inefficient movement and wastes energy. That wasted energy adds up during the course of an entire long distance race which will degrade both pace and overall time. The following graphic depicts the concept of the runner’s hemisphere.

In the figure above, we draw a line down the middle of the runner’s body creating a left and right hemisphere.
The right arm, elbow, hand, foot, knee and leg remain in the right hemisphere at all times.
The left arm, elbow, hand, foot, knee, and leg remain in the left hemisphere at all times.

The arms should swing back and forth like pendulums. Think of a grandfather’s clock. The elbows are bent at 90 degrees and shoulders and neck relaxed and forward so the arms swing low and relaxed.  When the neck and shoulders tense, energy is required to tighten those muscles which adds no benefit to the running gait.  As a result, energy is wasted and those muscles will eventually tire.  Also, when the arms swing back and forth, the movement should be tight and compact to the body so the hands almost brush up against your hip. They should be moving parallel to the motion of our body moving forward.  You will want to eliminate as much as possible any horizontal motion either into or out of the hemisphere.  Also, do not make a tight fist as this also requires energy that does not benefit the running gait. Keep the fingers closed but relaxed. Think of holding a delicate piece of paper between your fingertips and thumb.