Force is a basic notion of dynamics defined according to the 2nd Newton’s Law of Motion, as a product of mass and acceleration (F=m·a, where m= mass, and a = acceleration).

If a body A of mass m is impressed with force F, then the body A accelerates in a directly proportional way to the impressed force and inversely proportional to the mass of body A

a=F/m, F=m·a

According to the above rule, force is directly proportional to both mass and acceleration. However when the mass is in motion many forces act at the same time e.g. force of gravity, forces of the environment resistance, force of displacement. This virtually means that force F in an equation F=m·a should be understood as a vector sum of all forces acting on human body and parameter as an acceleration of the centre of its mass.

When analysing force we should take into consideration the 1st and the 3rd Newton’s Laws of Motion in which the first characterises a situation in which the forces operating on the body stay in balance, that is to say:

A body will stay at rest or continue at a constant velocity unless acted upon by an external unbalanced force

In case of the balance of forces, we can assume, that human’s force will be equal to the external forces which are balanced, that is to say the human remains at rest (Fc = F2 where Fc represents the resultant of human’s forces and F2 the resultant of the external operating forces). Whereas the third law states that

Whenever a body A exerts a force on another body B, B simultaneously exerts a force on A with the same magnitude in the opposite direction. This law is often simplified into the sentence "Every action has an equal and opposite reaction".

This rule has substantial significance in understanding the training loads which occur during exercises because an exercising athlete affects different kinds of objects (starting blocks, benches, ground) and is exposed to the same reaction caused by objects. These reactions are received and accumulated by the locomotor system (muscles, tendons and ligaments).

Locomotor system transfers all forces into appropriate movements by means of a lever system which is composed of an active and non active motor apparatus which in turn makes the analysis of the force influencing the movement of a given limb on should apply the force momentum as in case of circular movements (movement of the limb in the joint) analogical value to force (in progressive movements) is force momentum – it equals to force product and its side length relative to rotation in joint (M = Fm · r, where M is force momentum and Fm is force, force side)

Muscular force – it is a feature specific to the abilities of locomotor system, defined as:

The ability to work against resistance or counteracting it at the expense of muscular effort or this is force product momentum developed by a muscle or groups of muscles in a single or maximal isometric contraction.

Second part of this definition describes a special case in which there is no muscle shortening and only during such contraction (isometric) muscle force momentum will be equal to the momentum of the external force which in practise is assumed to be the weight of a bar etc. (you have to remember that the weight of a bar = mass · coefficient of acceleration of gravity).

• Absolute force – the maximum force which can be achieved by a competitor regardless of their body mass
• Relative force – the maximum force which can be achieved by an athlete relative to their body mass. The force is defined as a quotient of the athlete’s absolute force and their body mass.
• Explosive force – it is notion typical for sport and used very often. It defines the ability to reach the maximum force (relative or absolute) in the shortest possible time. It is often related to as the speed of reaching the maximum force. The explosive force is important only when it comes to movements which last no longer than 300ms. In case of the movements which are longer the maximum force is what is taken into consideration. The speed-force index is often used to measure the explosive force (the speed to reaching the maximum force). The speed-force index defines the maximum value of the force divided by the amount of the time to reach it. (IdPS = Fmax/t).
• Dynamic force – it is the ability to overcome the external resistance with the greatest speed of muscle contraction. The dynamic force is equal to the power which is defined as the ratio of the work to the time used to perform it, or as the product of the force F and the velocity v (Fdyn = P = W/t; Fdyn – dynamic force, P – power, W – work, t – time; W = F·s so P=F·s/t = F·v)

Methodology of strength development in outline

The first step of strength development is determining its aspect (relative, absolute, explosive etc.) which will be the most suitable to a given discipline that is the one which is characterised by the strongest positive correlation to the result. Then according to the 2nd Newton’s Law force can be intensified by its influence on mass or speed or simultaneously on mass and speed. From the physiological point of view there are three main processes the development of which influence strength:

• Process of motor units recruitment increase – it is crucial in developing each aspect of strength but most of all it influences relative, absolute and explosive force. Method of exercise on short-term efforts and maximum resistance as well as relatively long time of rest.
• Process of intensification of anabolic structural changes – is of greatest importance in developing strength by emphasising the development of mass. Exercise methods consisting in multiple efforts repeated until complete muscular fatigue whose effect is intensification of processes of contractible proteins disintegration during training and its regeneration in excess (hyperregeneration rule) during rest
• Process of nervous and muscular coordination improvement affecting the movement speed – it is of greatest importance in developing strength emphasising acceleration. Exercise method consisting in short-term efforts performed with maximal and supramaximal speed whose effect is intensification of movement speed which results in the increase of strength.

In training practice the process described above can occur at the same time and the predominance of one of them depends on the applied training method and on the character of training.