THE BIOMECHANICS OF RUNNING


The Biomechanics of Running:

I wrote this review years ago for a class I taught at Westfield State, Analysis of Motion, the principles have not changed. It has been much revised recently. Provided here for your review.

Summary: Gluteal group produces force and the calf group applies force to the surface.
The flexion of the swing knee shortens the lever, and makes the thigh easier to swing conserving energy.
The fastest runners stay on the ground and in the air for the shortest time, Impulse.
What ever work the runner does on the surface, the surface does equal and opposite work on the runner.
To move forward the runner must produce and apply force and create disequilibrium, lean  forward, fall forward, stick out your foot.
Conservation of angular momentum is the key, flex the swing knee to move the thigh forward, this uses the pendulum action of swing without wasting force.
The higher you bound, the harder you crash, run over the surface while minimizing the vertical projection. Quick in the hips. Pick up your feet rather than pushing against the surface.  Avoid loping floating stride. Most runners over stride wasting energy that could be user for propulsion. If you improve your stride economy and efficiency, you will run faster  with less energy.
The action of the arms is a crossed extensor reflex, forward swing of one limb results in a backward swing of another.

The following video demonstrates some of the biomechanics of running. It reviews efficient running.

The Phases of running:

The single support phase and the swing phase. There is a phase of running where the body becomes a projectile.

Production of Force and Development of Linear Momentum.

The production of force in running occurs when the support leg moves from hip flexion to hip hyperextension, the gluteal group and the hamstrings pull/push the leg/body forward. This results in the support leg producing force at the hip which is eventually applied to the ground though plantar flexion of the support leg.
Application of force: As mentioned the movement of plantar flexion of the lower leg at the ankle results in the application of force to the ground.
Note: the gluteal group are the primary force producers in running. 

Newton’s Laws as applied to Running:

The body has inertia and tends to remain at rest, the force produced applied to the ground must be great enough to overcome the inertial forces.

Newton’s third law of linear motion suggests that what ever force is applied to the ground the ground applies to the person.

Newton’s second Law suggests that the acceleration of the person is directly proportional to the force applied but inversely proportional to the mass of the person.

(A 150 lb person falls 1.5 feet back to the treadmill with each footfall, there are approx.
180 footfalls in a every minute, 5400 footfalls in 30 minutes. How much work in ft lbs does the treadmill do on your body in 30 minutes of running? How about tons of work?


 Moment of Inertia and Running:

The principle of moment of inertia is very important to running. The principle of moment of inertia suggests that as the lever arm if reduced it takes less force to move the heavy thigh forward. During the swing phase of running, as the foot leaves the ground the swing  knee is flexed to lift the foot off the surface, this shortens the moment of inertia of the swing leg at the hip, making it easier to swing the thigh forward.

There is less resistance to the spinning of the thigh, therefore less force is used swing the heavy thigh forward.

In sprinting the ankle of the swing leg should pass higher than the knee of the support leg, this increases the spinning of the swing thigh by reducing the moment of inertia. The closer the heel is to the hip axis the greater the velocity there is in the swing thigh. The faster this swing thigh moves forward the more force is applied to the ground by the support leg. Note; the swing of the swing thigh generates the force that is applied to the surface; equal and opposite torque forces. see below. There is no need to push against the surface, as the swinging thigh creates an opposite against the surface.


Newton’s Law of equal and opposite torques forces.

This principle of moment of inertia alteration is the same principle that applies to the skater spinning, as the skater pulls her arms closer to her body, her rate of spinning increases, to slow her rate of spinning she moves her limbs away from her axis, she spreads out her arms and legs.

Principle of impulse applied to running;

The final momentum of an object is a product of the force applied and the time the force acts.  In running you have a choice, apply a great force with each foot plant for a very short time and get fatigued quickly, or apply a small force much more often (quick soft steps) and run for 30 minutes.   It would seem to this author that the fastest runners apply a great amount of force to the ground for a very short time, the fastest runners stay on the ground and in the air for the shortest time. See above; (equal and opposite torques) the swinging of the thigh increases the torque forces applied to the ground, there is no need for the runners to push against the ground but runners should concentrate on picking up the feet.  Newton’s law of angular motion, torque and counter torque. The greater the force is that is applied to the surface the more the body is propelled forward.

Conservation of Angular momentum, and Running:

The principle of conservation of momentum applies to the swing phase as well as to return the foot to the surface. At the end of the swing phase the lower leg is extended at the knee, this increases the moment of inertia causing the thigh to slow down transferring the momentum to lower leg and foot.


Landing, the coefficient of restitution and pressure:

The coefficient of restitution is the amount of energy retained by an elastic body to rebound from a surface. An Elastic body retains a degree of energy to rebound from a surface. The human body retains energy to rebound from the surface with each foot strike, the amount of energy that the foot has to rebound from the surface is related to the hardness/softness of the surface and of the foot or the shoe. The harder the surface the more energy the body retains, and the faster the person moves. As most of us realize hard surfaces increase the complications for injury. Modern shoe construction has provided protection to the foot and the body by including materials that absorb some of the shock of landing. Shoes also serve to increase the surface area of the foot which serves to reduce the pressure per  square inch.

Stability and mobility:

Mobility is created in the body as the center of gravity moves forward of the base of support, great disequilibrium is created by the center of gravity being well forward of the base of support. During the acceleration phase of running, the runner should lean forward creating great disequilibrium, in danger of falling on their face, the runner now has to move his/her feet quickly to prevent falling.


Drills to teach the Biomechanics of Running: Fast Feet Drills, Fast Skipping, and standing start accelerations. Quick in the hips, frequency is the key. Power drills; power skips, stair jump. Start with Marching drills, at a fast march and fast feet drills. Fall season is a great time to work on running technique.

http://www.brianmac.co.uk/middist/
[23] Daniels, Jack, 2nd edition, 2005. Daniels Running Formula, Ill. Human Kinetics

 

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