Did you know there is an external force named ground reaction force (GRF) that is crucial for running? It helps to understand your biomechanics, your injury potential and indicates how to alter your training all to aid propelling your running performance. It is an force exerted by the ground back onto your body – as a reaction to the forces propelled upon it by your body.

Ultimately, it arises as a result of Newton’s third law of action.

According to Newton’s third law, every action has an equal and opposite reaction. So, when we exert force on the ground during running, an equal and opposite reaction force is exerted on us by the ground. Scientifically, when we analyze forces behind any sport movement, it is known as kinetic study.

Within kinetic study, running GRF provides comprehensive information of the loads with each foot-to-ground contact. This information is crucial for both enhancing running performance as well as for predicting injury outcomes. So, lets get into this…

Running gait and GRF

Running cycle or gait involves two phases:

• Swing phase (aerial phase): a part of running movement when there is zero contact with the ground (both legs are in the air).
• Stance phase: is a part in running movement when one leg is in contact with the ground. Therefore, it is the stance phase in running, when you exert force on the ground and is the only phase that must be fine-tuned for enhancement of speed.

We often measure ground reaction forces in an athlete (such as yourself) using a force platform. A force platform is an excellent biomechanical piece of kit, (based on Newton’s third law of motion) for measurement of ground reaction forces.

Force platform

Force platforms are the “gold standard” for measurement of GRF. And this is how it works:

when you step on force platform during a running movement, force is exerted in two directions that is downward and backward. The ground produces exactly opposite forces in upward and forward direction, respectively and are measured by force platform as ground reaction forces.

There are different components of GRF and so is their role in running. The ground reaction force is a vector (has both magnitude and direction) that can be resolved into 3 components for better understanding and biomechanical interpretation, here are those three:

1. Vertical component of ground reaction force (VGRF) that opposes the downward motion of the body before propelling it upward for the next flight phase of running.
2. Anterior-posterior component of GRF, which acts in the direction of progression of running movement and can either reduce or increase running velocity.
3. Medio-lateral component that acts at right angles to the direction of running progression, thus being responsible for direction changes during running.

GRF components

Vertical Component of GRF in running

In terms of force magnitude, the Vertical component of GRF (VGRF) is largest and has been reported to be approximately 3 to 5 times of body weight during running.

The VGRF peak is directly related to the muscles that play role in internal force development for accelerating body off the ground during running. Hence, the VGRF component is explored quite frequently in biomechanical analysis as a performance enhancing parameter.
What about when you speed up? Well, as running speed increases, the vertical component (VGRF) rises.

Horizontal component (anterior-posterior component) of GRF in running

The anterior- posterior component of GRF has been reported to be 0.5 times body weight during running. And is a key component of high performance running. Research proves the importance of horizontal component of ground reaction forces in production of acceleration during initial phase of run regardless of running ability or skill level.

As high horizontal ground reaction forces generates the initial forward momentum of your run stride.

So not to confuse you so far we have… high horizontal component of GRF contributes to initial phase of your running initial acceleration, while high vertical component of GRF helps you maintain high velocity in the late phase.

Medio-lateral component of GRF in running

This component has the lowest magnitude in comparison to other two GRF components. It has been reported to be approximately 0.1 times body weight during running. It is the most variable component of GRF without any consistent pattern. Hence, not much to offer in the way of comprehensive information on this GRF component.

Improper use of GRF leads to sports injuries

An inappropriate posture and wrong use of body mechanics prevent proper utilization of ground reaction forces. The high impact of vertical component of GRF must be absorbed by correct, balanced body mechanics (proper ground striking pattern and posture) for safe running.

VGRF impacts can also cause serious injuries, if not handled properly. The sports injuries reported frequently in runners due to high impact VGRF include lower limbs injuries like knee joint injuries, Tibia stress fractures, Achilles’s tendonitis, gastrocnemius and thigh muscle strain.

Train biomechanically for proper utilization of GRF in running

The proven strategy to increase your running performance by utilization of GRF is to understand your both kinematic (video analysis) as well as kinetic parameters like loading of muscles, production of GRFs, ground striking patterns.

Recent technological advances now allow continuous analysis of GRF during a long run using a specialized treadmill. This treadmill is known as instrumented treadmill that allows you to calculate GRFs during a long run. Yet to get more technical there are even wireless devices also available for GRF calculation. These wireless sensors are accompanied with portable intelligent data acquisition systems. I suppose, the prime advantage of wearable technology is its capability to measure consecutive foot contacts during a run for more comprehensive and real time calculation of GRF.
But what would you prefer?

The impact of your running mechanics

The efficiency of your running mechanics will produce high horizontal component of ground reaction forces for good acceleration during initial phase of running stride, while keeping the vertical component of GRF moderate or low. On the other hand, the vertical component of GRF increases in the later phase of running for maintaining high velocity.

The part proper running mechanics play in regards to GRF, includes use of energy efficient ground striking patterns and the proper posture. Even elite runners analyze and exploit biomechanically efficient running mechanics to really benefit from GRF, but of course to aid peak performance (and an injury low-risk season).