Maverick started by saying, “I feel the need.” Then Maverick and Goose, together, yelled, “The need for speed!”

The above is a quote from a classic. Our guess is that the majority of readers will automatically know that we are referring to Top Gun, a legendary movie that was produced back in 1986. While that may date some of us, the quotes within this movie may never be outdated.

“The need for speed” is a much-needed principle of human movement, as well as a descriptor of what makes movement “functional.” This dramatically affects and enhances how movement is looked at and applied, especially when it comes to prevention, performance, and rehabilitation.

In this blog entry, we take a deep dive into the Functional Movement Spectrum. Specifically, we focus on Mass and Momentum, 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 Mass and Momentum: Leveraged (functional) vs. Neglected (non-functional).

Mass is a physical property of an object, including the human body. We have all heard of the Center of Mass. It is the point of the entire body, or any of its individual segments, around which the mass is equally distributed. 

Mass creates inertia. Inertia is defined as the resistance to change. The more Mass, the greater the inertia. But Inertia is the resistance to moving when an object is stationary, but also the resistance to stopping when an object is moving. 

Mass in linear motion creates Momentum, while Mass in rotational motion creates angular Momentum. In both cases, the speed of the motion combines with the Mass to create Momentum. For a given Mass, the faster the motion, the greater the Momentum. For the same velocity of motion, the greater the Mass, the greater the Momentum. For this blog, the term Momentum will be used for both linear and angular types.

The practical question is: What does Mass and Momentum have to do with functional training and rehabilitation? In an article by Turvey, he summarizes the legendary Russian physiologist Nikolai Bernstein: “…inertia, reactive forces, and initial postural conditions combine with active forces in producing movement.” So, the internal muscle forces must be combined with the physical forces in the environment to successfully execute a movement. The body will leverage the environmental forces. Also, in many activities, the faster the movement can be accomplished, the more effective it is. Simply stated, the faster the movement, the greater the Momentum. As the body learns to leverage the physical forces including Momentum, the more efficient the movement becomes. Effective and efficient should be the goal of all our movement programs. Therefore, these programs must “teach” the utilization of Momentum. 

At Gray Institute®, we like to say that during gait, the quadriceps muscles do not extend the knee. This often produces an incredulous reaction in the movement industry. This statement refers to the time after the knee has flexed to absorb the motions created by gravity and ground reaction force. As the body passes over the weight-bearing foot, the knee goes through the motion of extension. The quadriceps are active to decelerate the knee flexion, but electromyography studies show that the quadriceps activity ceases very soon after the knee begins to extend. How does the knee extend if the quadriceps is not contracting?

That’s when the body’s Mass and the forward speed of walking create Momentum. The Momentum of the Center of Mass drives the trunk and the femur of the stance leg forward over the foot. The lower leg is also moving forward, but the calf muscles are slowing it down. Because the femur moves forward faster than the lower leg, the knee will extend. The body recognizes that Momentum will extend the knee, so why should unnecessary contraction of the quadriceps be used. Talk about being both effective and efficient! 

However, if our patients / clients (because of inactivity, injury, weakness, or poor balance) lack normal walking speed, then they will lack the Momentum needed for normal ambulation. This will require abnormal activation of the quadriceps and increased energy expenditure.

So, what is the “take home” message? In order to be functional, our programs must train the body to decelerate Momentum and use that energy to produce the desired movements. To make our training and rehabilitation functional, we must leverage Mass and Momentum by increasing speed and adding load. If we neglect, then the body – the patient / client – will not be able to feel the need for speed, nor control it!

[Side note #1: Muscle function is task- and context-dependent. The way the books describe muscle function is on the table and one-dimensional. Please feel free to take a look at Functional Muscle Function channel (1 of 21 channels of digital assets that further enhance your learning, understanding, and application of Applied Functional Science®) in Gray Institute’s Subscription Platform. For more info, please click here:]

[Side note #2: For movement – and the analysis / assessment – to be truly functional, Mass and Momentum need to be involved and emphasized. This differentiates 3DMAPS® (3D Movement Analysis & Performance System) from all other “functional” analyses / assessments. This is demonstrated in the Mobility and Stability foci within 3DMAPS®. Learn more and get certified by clicking on]

1 Turvey, MT. Coordination. American Psychologist, 1990, 45: 938-953.