Yes, cliché saying, but using the force is what we are talking about today. A force is something that tends to cause motion – it’s a push or pull exerted by one object on another. When we talk about human movement – biomechanics – we often refer to motive and resistive forces. A motive force causes an increase in speed or a change in direction. A resistive force resists the motion of another external force.
So what does that look like? Well, consider if you were doing a triceps kickback exercise. The little guy below, Frank, and his chart will help me explain further.
MF = Motive Force, RF = Resistive Force
Part 1 is the action of moving the weight from Position 1 to Position 2 (the first part of the triceps kickback exercise). In Part 1, gravity is acting as the motive force – it is increasing the speed of the movement and dictating the direction. The biceps muscle is acting as the resistive force – it’s slowing the effect of gravity, allowing Frank to move the weight at a slow controlled pace.
In Part 2, the action is moving the weight from position 2 to position 3. In this part, gravity is acting as the resistive force. It’s resisting Frank’s attempt to move the weight to Position 3. The triceps muscle is acting as the motive force in Part 2 – it is working to increase the speed and move the weight back into position 3.
Now, you may not hear the terms motive and resistive force that often, but what you do hear is concentric and eccentric actions. Concentric action is what we see when the muscle acts as the motive force – the muscle shortens as it creates muscle tension. In our example above with Frank, the triceps is acting concentrically in Part 2.
Essentric action is what we see when the muscle acts as the resistive force – it lengthen as it creates muscle tension. In the example of Frank, the biceps is acting eccentrically in Part 1.
So what does this mean to you? As a fitness professional, it’s important to know the forces acting on the body during movement. An understanding of motive and resistive forces, or concentric and eccentric actions, can help you with teaching your clients appropriate technique and form.
You can read about Motive/Resistive forces on pg. 108 and 109 in the ACE Essentials of Exercise Science for Fitness Professionals.
Questions? Contact one of our ACE Education Consultants at 1-888-825-3636 x782.