Biomechanics of BJJ 3: Laws of Motion and Force

Basic Mechanical Principles

Motion

The three laws of motion, as taught by Newton, are as follows:

The Law of Inertia: The property of matter that makes an object continue its current state of motion until acted upon by an outside force.

The Law of Acceleration: The amount of force needed to accelerate an object is directly proportional to how much mass that object has, and the faster you want to accelerate an object the more force you have to utilize.

The Law of Reaction: For every action, there is an equal and opposite reaction.

These laws can be taken advantage of in BJJ by understanding a few key principles.

• The amount of force needed to change a body’s motion depends on both the mass and speed of the body. A heavier person will be harder to move than a lighter person, and a faster person will be harder to stop than a slower person. Inertia is the reason weight classes are required in competition, a heavier person will always have a mechanical advantage over a lighter person.

• A person who is moving will require a lot of force to stop their movement, however, it takes far less force in order to continue their movement. Some techniques in BJJ require using an opponent’s momentum to your advantage. Students should be taught to take advantage of their opponent’s momentum by pushing their partner along a path that they have already started on. This will overextend their opponent and allow for the student to take advantage of the overextension. Students should also be taught not to overextend themselves by forcefully moving with speed or they can be taken advantaged of. This can easily be seen in the elbow push side-mount escape, or the sacrifice throw in Judo.

• It takes a lot of force to start a person’s movement. Many sweeps take advantage of an opponent’s momentum. If an opponent is knowledgeable in BJJ techniques, they will be difficult or impossible to sweep or submit from a static position. Attacks must be set up by destabilizing an opponent in the direction opposite of the sweep. The forceful movement the opponent uses to return to their original position can be taken advantage of by providing the momentum needed to complete the sweep.

Force

            In the most basic sense, forces are simply the push or pull exerted onto an object or person – forces are any action or influence that moves an object. This could be the pull on a sleeve or lapel, or pushing on an opponent. This could also be the push or pull exerted on the ground. Forces can be split into two categories: internal and external forces. Internal forces are forces that are generated within one’s own body, and consists of muscular contractions, ligament restraint, and skeletal structure. External forces are the forces that act upon the body from outside of the body – these include gravity, friction, and an opponent’s body (Winter, 2009).

By teaching students to take advantage of all the forces available to them, both internal and external, they will be able to successfully and efficiently control both their own body and their opponent’s bodies. There are three ways to recruit force:

1. By using muscular force to push or pull directly on the opponent
2. By pushing off the floor, one can drive into their opponent
3. By using the force of gravity to apply weight on the opponent

The first two methods are available to both the top and bottom position, but since the force of gravity is always pulling downwards – only the person on top can take advantage of gravity. Students should be taught to drive off the ground to keep their base and to drive into an opponent. Keeping a strong posture while driving off the ground will allow for a stronger and more efficient transfer of force. The easiest production of force to teach students to utilize is the use of their own muscular force since most people already have an understanding of pushing and pulling on their opponent.

When instructing students to use force during a technique, they should be taught to take into consideration the following aspects:

1) Magnitude of force

Students should be taught how much force will be needed for a particular technique. Students should not use any more force than necessary in order to reduce energy expenditure and strain placed on the body. One consideration before attempting to use force is the understanding that to move a large amount of mass, a large amount of force will be needed. A larger opponent will always require more force to move than a smaller opponent.

2) Direction of application

Regardless of how much force a student can recruit, they will be unable to accomplish their movement goal if the direction of the force is incorrect. For example, many beginning students will bridge strongly with much effort in order to escape mount. However, if the direction of force is applied incorrectly, such as straight up towards the ceiling instead of over the practitioners shoulder, the bridge will serve no purpose except for expending energy.

3) Point of application

The point of contact when force is used will determine which movement will follow. If force is applied at an opponent’s center of mass then the movement created will be linear. However, the further away from the opponent’s center of mass the more torque will be created. If a technique’s purpose is to move the opponent directly forward or backward then the point of application should be at the opponent’s center of mass. If the technique’s purpose is to turn the opponent, the force should be applied further from the center of mass.

4) Distance and time of application

More force can be applied over a longer period of time and distance. This concept is important during top positions such as mount. The longer amount of time a student is mounted on their opponent, the more force they can exert on their opponent in order to sap their opponent’s stamina. The distance of application can be seen during the bridge defense against mount – the longer the distance of the bridge (the height of the bridge) the more force will be generated. Sometimes a large powerful movement through a short amount of time will be desired, but in some cases force over a long period of time will be more useful.

Read more about Biomechanics in the next chapter – Levers and moment arms