What does strength depend on? Muscular and nervous system factors

What does strength depend on? Muscular and nervous system factors

Have you ever wondered where the strength that helps you lift the weights in the gym comes from? Well, learn to know your body, and you’ll understand how strength is generated

What is Muscle Strength?

It is the maximum force that a muscle or group of muscles can generate.

Muscular Strength

For example, when doing a maximum bench press lift, it would mean the maximum weight a subject can lift to complete a single repetition (repetition maximum or 1RM).

How to Calculate Your Strength Using the RM Test RM Test

Muscular Factors

Muscle size

A larger muscle volume (found in a subject) does not come from a greater number of muscle fibres (in animals this possibility has been proven but in the case of humans, clear evidence hasn’t been found), but from the greater size of each one. These fibres are present in any athletes body from birth.

Muscle size

Muscle can increase at the expense of elements that can generate tension and others that are not capable of generating it, consequently, muscle volume is not proportional to the force that can be generated, although a certain volume is necessary to be able to generate a high force in the first place.

Type of activated units: Muscle contains four types of fibres with well-defined characteristics that all differ from each other. We have explored this topic before and you can check it out using this link to know more about that exist

Movement execution speed

It’s safe to say that this factor depends on muscle fibre quantity, since the number of fast fibres that an athlete possesses is essential when it comes to generating more strewngth for fast movements. The faster a movement is made, the fewer the number of fibres activated to work towards the overall strength, meaning that less strength is produced on the whole.


Two individuals could have the same maximal strength, yet one could be a professional shot putter and the other fail to even qualify for their local competitions. The reason for such a difference occurs due to the ability of the first individual to be able to apply a lot of force in the face of very fast movement; On the other hand, for the second case, although when the movement the person can apply a lot of force, when it is fast the application decreases considerably: because fewer fast fibres are activated, or because they are less developed.

The initial length of the muscle

When a muscle is stretched beyond the resting length (20% more), at that moment it has the maximum number of myosin bridges that can act on actin and consequently the maximum voluntary tension. As it stretches, actin-myosin bridges are lost and consequently the ability to produce force by the contractile mass decreases.

Muscle Length

This is one of the reasons sprinters start from a push up position.

The angle of the joint

Depending on the position of the joint, the muscle will have a degree of stretch that will allow it to have more or less potentially active bridges of actin and myosin, but in addition, the muscle will have an angle of traction on the bone at which optimum point you will obtain the maximum generation of force on the resistance that is mobilised.

angle of movement


If we hold a 20kg dumbbell, depending on its position, we will have to generate more or less force, even though the dumbbell always weighs 20kg.

Nerve Factors: Motor Unit Recruitment

According to most authors, motor unit recruitment follows Heneman’s law.

Principle of size

  • The smallest motoneurons, which are also the slowest, are recruited first; Thus, to mobilize a light load, the slow fibers or type I will be activated first, which are those that require a lower stimulation frequency.
  • If the load increases and, therefore, it is required to apply more force, we must activate, in addition to the slow fibres, the fast type IIa, which need a frequency of stimulus higher than the previous ones.
  • Finally if the load to be mobilised is very large, in order to move it, we must also activate the llb fibres that require a higher stimulus frequency.

Frequency of activation of motor units

All the motor units are not activated in unison, they only do so to achieve a maximum effort: in submaximal contractions, some motor units are at rest, while others produce the necessary force.

Dead weight

The active and resting motor units exchange their role frequently, so that fatigue of these units is avoided.

By increasing the frequency with which a certain number of units are stimulated, more can be activated at any given time, providing greater force of contraction.

Frequency of fibre activation

While greater force production can be achieved by increasing the number of motor units fired simultaneously, it is also possible to generate more force by having each motor unit fired produce more tension.


This is achieved by increasing the rate of discharge produced by the motor nerve.

Intermuscular coordination

It refers to what we know as coordination of movements, that is, of the adjusted participation in time and intensity of the agonist and antagonist muscles that intervene in a sporting gesture.

In untrained subjects, training with loads produces an increase in strength based not only on an increase in the activation of the agonist muscles, but also on the relationship between these and the antagonists.

The improvement of strength by this mechanism will always be a specific strength for a given movement, which is little or not transferable to others.

Muscle spindles

This proprioceptor sensory receptor has the function of inhibiting the muscles antagonistic to the movement that is produced and this is produced by relaxing it so that the movement can be carried out efficiently.

Strenght 1

The information sent by the muscle spindles to the central nervous system also stimulates the synergistic musculature of the activated muscle, helping to achieve better contraction

Myotatic Reflex and Muscle Elasticity

The force generated by a muscle is clearly greater when, prior to its contraction, it undergoes rapid stretching. Part of this increase in force can be caused by the myotatic reflex and another part by muscle elasticity.

Myotatic Reflex

When a muscle is suddenly extended and then contracts in times of less than 200 ms, with stretching, the muscle spindle fibres are also stretched and immediately send a contraction stimulus.

Thus, a sudden muscle stretching prior to a contraction can cause an increase and/or a more rapid application of force due to this activity of the muscle spindles.
Stregnht squat

Muscular Elasticity

It is given by the ability of the muscle to recover the initial degree of extension after a stretch.

When a muscle stretches, it stretches itself and its connective tissue, accumulating in it an elastic energy, which will be returned with the concentric contraction of the muscle, increasing the intensity of the force.

This potential energy that accumulates with muscle stretching will cause a plus of strength, as long as the period of passage between the eccentric and concentric phases is short, otherwise it will dissipate as heat. Furthermore, the amount of elastic energy stored depends on the force developed at the end of the stretch, the speed of the stretch, the length of the stretch, etc.

The use of the Myotatic Reflex and Muscular Elasticity, are extremely useful in most sports, since in almost all sports movements there is a pre-stretch.

Bibliographic sources

  1. Barbier, Miguel. La Fuerza y la musculación en el deporte. Sistemas de entrenamiento con cargas. Madrid: Librerías Deportivas Esteban Sanz, 2000. Print.

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