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Strength is one of the four basic physical capacities, however, some studies consider it to be the only true one, since movement depends exactly on the ability of muscles to generate tension. This idea arises from the fact that the rest of the basic physical abilities revolve around it: flexibility is the muscle’s ability to stretch and recover its natural position; speed is the muscle’s ability to contract and relax at maximum speed; and endurance is the body’s ability to provide energy to the muscles so that they contract and relax.

We can define strength as the muscle’s ability to generate tension. Understanding this is important, since we exert tension both when we contract muscles and when we stretch them; both when we push against a resistance, and when we resist an opposing strength. For example, we exert tension when we contract our muscles to jump, but we also exert tension when we contract our muscles to land on the ground after that jump.

Strength training allows various adaptations to be obtained such as hypertrophy (increase of the muscular size); increased energy consumption and improvement in the relationship between muscle mass and body fat; favors an increase in bone mineral content and makes it stronger and more resistant; increases the strength of tendons and ligaments; prevents bad postural habits; improves sports performance; and it is essential in any rehabilitation program.

To improve muscular strength, muscles must work with greater loads than they usually do. The weight of a mass is called load. Strength can be worked with two kinds of loads: Natural loads, relative to the weight of the body itself; and overloads, related to weights, machines, etc.


There are two types of factors that determine the strength and degree of muscular tension that a person is capable of performing:

  • Internal factors, within which we differentiate:
    • Neurophysiological factors: Such as muscle width (cross section), arrangement of muscle fibers, type of muscle fiber, muscle length, number of fibers used.
    • Biomechanical factors: Basically related to the bone system.
    • Emotional factors: That can increase the level of strength as they activate muscle fibers that are not normally stimulated. Among them we can find motivation, attention, fear, sacrifice capacity, concentration.
  • External factors, within which are among others temperature, food, training, climate, age and sex.


The strength doubles between the ages of 11 and 16. At the age of 16, strength reaches 80-85% of its maximum, and maximum strength is reached between the ages of 20 and 25, once muscle development has been completed. From the age of 30, if you do not work specifically, there is a slow but progressive decline. Between the ages of 50 and 60, a progressive degeneration of functional tissue begins to occur.


In general, men have more strength than women, due to their higher percentage of muscle tissue (36-44% in men vs. 25-29% in women). In addition, the man’s ability to develop musculature is greater than that of women. The difference in strength between men and women begins to be appreciated from adolescence, around 12-14 years old, when boys develop strength more quickly. Both, men and women, can increase their strength during puberty and adolescence, peaking between the ages of 20 and 25. From here on a person loses around 1% of their maximum strength each year. This loss is less in those who are more active or who perform strength training.


Depending on the resistance that opposes the performed strength, different types of muscular contraction can be carried out, depending on whether or not there is movement on the muscles.

  • Concentric contraction: When we lift, move or move an object, a concentric contraction occurs. The muscle generates tension and contracts producing a movement, it is the most common of muscle contractions. The insertion tendons and origin tendons are brought closer together by shortening the muscle belly. This produces movement in the bones where these tendons originate and terminate. Example: When we flex the arm, the biceps makes a concentric contraction, it shortens and brings the hand closer to the shoulder.
  • Eccentric contraction: When we stop the movement of an object. The muscle generates tension, but not enough and instead of shortening it lengthens. The muscle generates tension, but the strength is greater than the tension generated and the muscle “gives” by lengthening while continuing to generate tension. Initially the tendons shorten as in the concentric contraction and the muscle belly tries to shorten, when it reaches the limit it begins to lengthen and then the tendons also lengthen. Example: When we have to hold a weight with our arms slightly higher than we can, the muscles give way and lengthen.
  • Isometric contraction: When we hold an object without displacement. The strength we make is equal to the weight of the object. The muscle generates tension, but overall it does not shorten or lengthen. It does not change in length. But inside the muscle, the tendons lengthen and the muscle belly shortens, remaining the same length, but with greater tension. Example: When we are holding a heavy table that we keep in the air, but neither raise it nor lower it, we are doing an isometric contraction. Another example is when we push a wall, we are applying strength, but not displacement.


To differentiate the different kinds of strength, it is necessary to take into account the ways in which it manifests itself. From the point of view of training, three types can be distinguished:

  • Maximum strength. It is the ability of the muscle to develop the maximum possible tension, for this, large loads are mobilized regardless of the acceleration, as, for example, in weightlifting. The speed of the movement is minimal and the repetitions made are few.
  • Speed ​​strength. Also called explosive strength or power, it is the ability of the muscles to give a load the maximum possible acceleration, usually around 30% of 1RM. Movement speed tends to be maximum. This type of strength determines performance in activities that require explosive speed in their movements: volleyball when jumping and shooting, handball when shooting at the goal, athletics when sprinting, soccer when hitting a ball…
  • Endurance strength. It is the muscular capacity to withstand the fatigue caused by a prolonged effort in which many repeated muscular contractions are carried out. In this case, since neither the load nor the acceleration are maximum, the execution speed is not very great and a high number of repetitions can be done. It is the type of strength necessary for activities that require a long and continuous effort: long runs, rowing, swimming, cross-country skiing.


There are many different methods to develop strength. Among the most used systems are the following:

  • Weightlifting: It is directed, basically, to the work of maximum strength and mobilizes large loads. The load percentage is taken based on the maximum intensity for each specific exercise, through the 1RM test.
  • Isometry: Training system for maximum strength work. It is based on exercises of very short duration (4-12 seconds) that are studied so that the muscle performs an isometric contraction against immovable resistance. This system is widely used in the recovery of people who have been inactive for a while due to injury or illness.
  • Bodybuilding: Training system that allows the development of maximum strength and strength-speed through the use of weights and machines for strength work.
  • Isokinetic method: It uses special machines, and serves to jointly develop maximum strength and strength-endurance.
  • Plyometrics: It is a specific training system for the improvement of explosive strength. It consists of jumping from a minimum height that varies between 40 and 50 cm and after falling to the ground, jumping again as quickly as possible. It is based on the fact that a muscle that is subjected to an eccentric contraction later has a greater capacity to generate tension in a subsequent action due to the accumulation of energy in its elastic tissues, which adds to its concentric contraction strength. There are softer variations of plyometrics such as multi-jumps.
  • Circuits: They are used for the development of endurance-strength. A series of localized exercises (arms, trunk, legs) are performed using small loads, the weight of one’s own body, that of another person or light weights. The number of repetitions ranges between 10 and 15 and the speed of execution is moderate.


The use of strength training systems produces some effects on the body and has a direct influence on the muscular development of the subject:

  • Muscle hypertrophy: Increases muscle mass and the size of muscle fibers, but not their number.
  • Improvement of muscle metabolism: The muscular energy reserves increase, which facilitates its working capacity.
  • Improves neuromuscular coordination: The excitability and speed of nerve conduction increase, which allows a greater number of fibers to be activated simultaneously and therefore exert more tension.
  • Weight gain: As the muscle hypertrophies, its weight increases and the bone becomes denser, which also means an increase in its weight.



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