Force

Forces Classification

Forces are divided into internal and external.

Internal Forces

Internal forces are forces exerted by one part of the observed system on another.

Despite the fact that, to make things simple, we imagine that human body does not change its shape and volume, in reality, of course, this is not the case. Human body is a system of internal structures: organs, bones, muscles, tendons, ligaments, cartilage, and other types of tissue. These structures exert forces on one another and deform one another. Muscles have effect on tendons and tendons pull bones. In joints the bones exert pressure on cartilage, which deforms and exerts pressure on other cartilage tissues and bones. If a traction force acts on the ends of a certain internal structure, it creates tension and often deformation. Internal forces hold objects (e.g. human body) together, even if the individual internal structures of such object are under stress or pressure. Sometimes the traction or pressure forces are higher than what the internal forces of the given structure can endure, the resulting deformation is too large and the given structure snaps or breaks. Among examples are strained femoral biceps, ruptured Achilles tendon, broken tibia, or ruptured cruciate ligament in the knee.

Muscles and tendons are considered to be the structures producing forces that cause changes to our kinematic state. Activity of muscles produces internal forces which cause movements of extremities and other body parts, but cannot cause a change to the motion of the centre of gravity of human body without external forces having effect on human body. Human body is only capable of changing its motion if it is in contact with other object. We can imagine a football goalkeeper preparing to catch a penalty kick. If he jumps before realizing that the ball is heading to the opposite side of the goal, he needs to change the direction of his original motion. But unfortunately he is not in contact with any other object, so no external forces have any effect on him, apart from gravitational force. Therefore he cannot catch the ball heading in the opposite direction to his own motion. When he lands again, he can push with his legs against the ground and change the direction of his motion. In this case the ground provides external forces, allowing the goalkeeper to change the direction of his motion. They can be understood as reaction forces to forces exerted by goalkeeper’s legs against the ground.

Study of internal forces allows to describe the motions of individual body parts, as well as the nature and the causes of injuries.

External forces cause changes to the motion of the centre of gravity of human body.

External Forces

External forces are forces resulting from the interaction between human body and its environment.

External forces can be divided into contact forces and non-contact forces. Most forces that biomechanics works with are contact forces. Contact forces are forces that act at the point of contact between two objects. Gravitational force, acting on all objects on the Earth, is a non-contact force. It affects objects even if they are not in direct contact with the ground. It acts through the Earth’s gravitational field.

In sport and physical exercise we will consider only one non-contact force – the gravitational force.

The force that is resultant of gravitational force and centrifugal force of the Earth’s rotation is force of gravity.

Weight is a pressure force exerted by an object (human body) on a surface, or a traction force exerted on suspension.

If we let go of any object, it will start falling to the ground and accelerate under the influence of gravity. If we neglect the air resistance it will start moving with the acceleration of 9,81⁶ m·s^-2 regardless of its weight or volume. This acceleration is called gravitational acceleration g. Gravitational force F_G (its unit is newton – N) acting on an object equals product of its weight m (kg) and gravitational acceleration g (m.s^-2):

F_G = m.g⁷

Contact forces are produced by mutual contacts of objects. Objects can be rigid or fluid. Contact forces in sport originate between athletes and other objects in their environment. For example in walking a reaction force of the Earth acts on the soles as reaction to the action force of the soles acting on Earth’s surface (Fig. 7)⁸.

Contact forces may be resolved into three components:

Component force acting perpendicular to the surface of contact and called normal force F_n⁹. In walking, when one foot or the other is in contact with the ground, normal force is the component acting downward on the ground (action) and up against the soles of the walker (reaction).
Component force acting parallel to the surface of contact in front to back direction, which is one of the components of friction force¹⁰.
3. Component force acting parallel to the surface of contact in left to right direction, which is one of the components of friction force¹¹.

The plane along which the friction force acts is parallel with surfaces that are in contact. Friction force acts against the direction of the motion. When a walker exerts force backward and downward, friction force is a component of the contact force that acts forward on the walker and backward on the ground. Friction force is a component of the contact force that is the cause of changes to horizontal motion and, as such, a major cause of human locomotion.

⁶ This value is valid for Prague.Zpět

⁷ Sometimes also used under symbol W as Weight, or G.Zpět

⁸ Abbreviation GRF – ground reaction force – is sometimes used in English.Zpět

⁹ In English this component of force of contact between the athlete and the ground is described as vertical V/GRF.Zpět

¹⁰ In English this component of force of contact between the athlete and the ground is described as anterior-posterior AP/GRF.Zpět

¹¹ In English this component of force of contact between the athlete and the ground is described as medio-lateral ML/GRF.Zpět