Please do us a favor … Go ahead and stand up. From this upright position, please jump. Oh yeah, there is one rule to your jumping assignment: you cannot lower (or even lean forward) any part of your body. Okay, now go ahead and jump!

Why didn’t you jump? Because you couldn’t load any part of the body in order to explode upward. Another way to put this is that you were unable to use gravity in order to defy gravity.

Think about how many motions of the body are loaded because of our best friend, gravity, and its best friend, Ground Reaction Force (GRF). Ankle Dorsiflexion, Knee Flexion, Hip, Flexion, Thoracic Flexion are motions that have been gifted (free of charge) to our body by gravity and GRF in the Sagittal Plane. Oh yeah, and there’s more news on this topic. Subtalar Eversion, Knee Abduction, Hip Adduction, and Thoracic Spine Lateral Flexion are motions have been gifted (free of charge) to our body by gravity and GRF in the Frontal Plane.

Our hamstrings did not orchestrate one such motion, knee flexion. You may be thinking, though, that the books state that the hamstrings flex the knee. In the scenario of jumping, the hamstrings actually decelerate knee flexion, becoming knee extensors. Therefore, muscle function (which react to joint motion) is context-dependent and is much more dynamic than what the books (traditional, one plane of motion, on the table) document.

It makes sense to then state that movement – functional movement – is context-dependent, too. Once such principle that provides context is Gravity and GRF. In fact, it is this principle that may force (pun intended) to rethink what we, as practitioners, are doing with our patients and clients in the areas of assessment, prevention, performance, and rehabilitation.

A question to ask oneself on this topic is simple: “Am I using Gravity and GRF authentically to the ultimate function / task or am I confusing the body?”

In this blog entry, we take a deep dive into the Functional Movement Spectrum. Specifically, we focus on Gravity and Ground Reaction Force, which is included as a principle / truth in the Physical Sciences. In “The Introduction” to this Functional Movement Spectrum Series, we identified the following descriptors for Gravity and Ground Reaction Force: Used (functional) vs. Confused (non-functional).

It is easy to accept the presence of gravity on earth, but not recognize how essential it is to functional movement. Gravity drives all matter towards the center of the planet, including human beings. When the acceleration of the center of mass interacts with the equal and opposite reaction force provided by the earth’s surface, an inevitable Chain Reaction® occurs. The Ground Reaction Force (GRF) is essential for normal movement. The joint motions created by Gravity and the GRF are given “for free.” These motions lengthen and load the muscles. The body must add active muscle force to complement the free forces to effectively and efficiently accomplish the intended task or activity.

There is no better representation of this “Truth” of movement (and criteria in the Functional Movement Spectrum) than watching a video of astronauts walking on the moon. Not only did they travel long distances with each step in the reduced gravity environment, but also their walking pattern was substantially different that how humans walk on earth. The reduced Gravity and GRF initially created a confused situation that required an adjustment. The astronaut’s conversation about walking on the moon describes conscious changes to walking that were required in order to use the modified forces given for free.

Another related example is when exercises / movements are performed in a water environment. The loading effects of Gravity are reduced, which often can prove beneficial in reducing pain while the water resistance can strengthen muscles. However, the movement specialist must recognize that walking in a pool without the normal Gravity and GRF interaction is not like walking on the surface of the earth. Research has shown different GRF and muscle activation patterns when walking in a pool compared to dry land ambulation. Water-based training provides a different proprioceptive input resulting in the altered muscle output. Too much water-based training can create a muscle activation synergy that is confused, creating a challenge for the body to use the normal Gravity and GRF forces.

These two examples demonstrate that a normal gravitational force and a commensurate GRF are essential to the movements that humans create on earth. It should also tell us that positioning our clients in an upright stance position, which then will create the appropriate reaction force, is a critical feature of functional examination / assessment, as well as functional training / treatment. If the functional activity our patients / clients want to perform occurs with the body in an upright stance position, then practitioners must create programs for training, rehabilitation, and injury prevention that are predominately upright and weight-bearing to produce the normal Chain Reaction®!

In the Functional Movement Spectrum, the Gravity and GRF criteria list the functional side as used. The paragraphs above focus on how these forces are used to create a Chain Reaction® of joint motions. In addition to creating motions that turn on muscles, the force created by Gravity has another critical role in normal movement. The force of Gravity creates rotational forces or torques. Muscles, in contracting to deal with these torques, becomes “sensors.” The study of this ability lies in the scientific realm of haptic perception. The motor output of muscles is “sensed” and is converted into “muscle based perception”. This ability to measure torques produced by Gravity was initially documented in hand-held objects while vision of the object was blocked. The same ability has now been documented in the lower extremity and the trunk. The body, through the muscles, is able to sense the authentic torques produced by Gravity and, thus, use this information to orchestrate the proper combination of muscle contractions to successfully execute the desired movement!

 1 Barela A. and Duarte M. Biomechanical characteristics of elderly individuals walking on land and in water. J. Electromyography and Kinesiology, 2008, 18: 446-454.

2  Hajnal A, et al. Haptic selective attention by foot and by hand. Neuroscience Letters, 2007, 419: 5-9.