7.2 What is Force?
To understand how the human body moves, we need a working definition of force. An intuitive definition of force—that is, a push or a pull—is a good place to start. A force, which is measured in Newtons (N), is something that acts on or from within the body to either help or hinder movement, and it can come from a variety of sources, either in isolation or simultaneously.
Fitness Fact
Remember a time when you were trying to push something heavy. It could have been a dresser, a fridge, or maybe a cabinet. When you tried to push it initially, it likely required a lot of force from you. You may have pushed with your arms, leaned your body forward, dug your feet into the floor, or maybe you did all of that. Once the object finally started moving, suddenly you didn’t have to push so hard. Why is that? Why does the amount of force you need suddenly change? The object did not get lighter, and you did not get stronger, so what changed?
Well, there are probably more forces acting in this example than you might realize. The following section describes and differentiates between internal and external forces which influence how we move.
External forces
External forces include all forces that originate outside the body. This section will focus on three: gravity, wind, and friction.
Gravity
A constant downward force is exerted on all things on Earth. While other planets, or masses (e.g., stars), have their own gravitational pull, gravity is mentioned in this text, and it refers to Earth’s gravity because of its specific impact on human movement and objects.
Example
Wind
A force generated by air flow, sometimes referred to as air resistance. However, wind does not always act as a resisting force if it is happening in a favourable direction.
Example
Friction
A resisting force created by the contact between two surfaces, which, much like wind, can act in favour of or against a desired movement.
Example
External Forces: Help or Hinder?
In what ways do external forces (wind, gravity, or friction) help or hinder movement? Move the slider to learn more about wind, gravity, and friction.
Text Description
Slide 1 – Wind
- Hindrance: Driving a golf ball into the wind causes drag.
- Help: The wind propels the sail in a sailboat.
Slide 2 – Gravity
- Hindrance: High Jump.
- Help: Cycling down a hill.
Slide 3 – Friction
- Hindrance: Frictional drag while swimming.
- Help: Friction between the shoes and the rock surface.
Internal Forces
Internal forces include all forces that originate within the body. These forces are typically generated through skeletal muscle contraction. The following types of muscle contractions (as discussed in Chapter 5) act to generate internal forces:
- Concentric: A type of muscle contraction that generates force by shortening the muscle.
- Isometric: A type of muscle contraction that generates force without changing the length of the muscle.
- Eccentric: A type of muscle contraction that generates force while lengthening the muscle.
How do internal forces act within the body? Below are two examples:
Standing Hamstring Curl
Downward Movement:
- Eccentric contraction of the hamstring muscles
- Concentric contraction of the quadriceps muscles
- Knee extends, moving the foot forward
Upward Movement:
- Concentric contraction of the hamstrings muscles
- Eccentric contraction of the quadriceps muscles
- Knee flexes moving foot toward glutes
Standing Tricep Extension
Downward Movement:
- Concentric contraction of the triceps brachii muscles
- Eccentric contraction of the bicep brachii muscles
- Arms extend unilaterally downward and hand grip moves downward
Upward Movement:
- Eccentric contraction of tricep brachii muscles
- Hand grip moves upward
- Isometric contraction of the tricep brachii achieved by holding a partial tricep extension
“4.1 Forces” from Introduction to Biomechanics by Rob Pryce is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
