In the study of the mechanics of muscular contraction we define:
load: the force exerted by the weight of an object, for example a handlebar, on a muscle;
muscle tension: the force exerted on the object in question by the muscle that contracts.
Muscle tension and load are therefore opposing forces, which as such are opposed to each other:
to overcome a load, the muscular tension must be greater than the force (weight) that it exercises.
Muscle contraction is that active process by which a force is generated within the muscle.
The types of contractions that can be had are two:
DYNAMIC CONTRACTS and STATIC CONTRACTIONS
Dynamic muscle contractions ←
DYNAMIC contractions can be
ISOTONIC, ISOCINETIC, AUXOTONIC and PLIOMETRIC.
Isotonic contraction
The contraction commonly called ISOTONICA (at constant tension) occurs when a muscle is shortened by moving a load that remains constant for the entire duration of the shortening period; it can be divided into two phases:
CONCENTRIC or POSITIVE phase when the muscle shortens and develops tension (eg by lifting a weight)
ECCENTRIC or NEGATIVE phase when the muscle lengthens developing tension (for example by slowly lowering the same weight)
Isokinetic contraction
The ISOCINETIC contraction occurs when the muscle develops the maximum effort for all the amplitude of the movement, shortening at a constant speed (variable tension); it is obtained only with particular machines, defined as isokinetic.
Auxotonic contraction
The AUXOTONIC contraction increases progressively with muscle shortening (eg elastic).
Plyometric contraction
The PLIOMETRIC contraction is an explosive concentric contraction, immediately preceded by eccentric contraction; in this way the energy accumulated in the elastic structures of the muscle is exploited in the previous eccentric phase.
Static muscle contractions ←
STATIC contractions are the isometric ones (which occur at constant muscle length) and are obtained when muscle shortening is prevented by a load equal to muscle tension, or when a load is supported in a fixed position by the tension of the muscle.
Isometric contraction occurs when the muscle contracts without changing its length (without therefore shifting the load).
Now let's see a summary table:
STATIC or ISOMETRIC CONTRACTIONS The muscle develops tension but does not change its length and does not produce work. | |
MAXIMUM CONTRACTS Applied voltage with immovable load. | PARKING CONTRACTS Voluntarily interrupted movement. |
The developed tension is equal to the applied resistance, the muscle does not change its length and the distance between the muscular insertions remains unchanged . | |
DYNAMIC OR ANISOMETRIC CONTRACTIONS The muscle develops tension and changes its length producing work. The distance between the insertions varies during the contraction. | |
CONCENTRIC CONTRACTS (POSITIVE) The developed voltage is such as to allow the overcoming of the applied resistance. The muscle shortens leading to the approach of the insertions. | ECCENTRIC CONTRACTS (NEGATIVE) The developed tension is lower than the applied resistance and the muscle lengthens, leading to the removal of the insertions. |
ISOTONIC CONTRACTS The muscle shortens, developing a tension that remains constant for the entire duration of the shortening period. In reality there are no isotonic contractions in vivo, as the developed tension varies with the variation of the lever. Approaching isotonic contraction by performing exercises using cam-equipped equipment. | |
ISOCINETIC CONTRACTS The muscle develops maximum tension throughout the entire range of motion by shortening at a constant speed (special isokinetic equipment is used). | |
AUXOTONIC OR AUXOMETRIC CONTRACTS The developed tension increases progressively with muscle shortening (eg elastic). | |
PLIOMETRIC CONTRACTS These are explosive concentric contractions immediately preceded by eccentric contractions; in this way the energy accumulated in the elastic structures of the muscle is exploited in the previous eccentric phase. | |
Hill chart
Hill has mathematically demonstrated that speed is inversely proportional to force. Consequently at the maximum speed the force is equal to zero, while at zero (or negative) speed the force is very high. The concept, summarized in the graph to the side, can also be expressed in other terms:
the force expressed is maximum during eccentric contractions (negative repetitions), is reduced in isometric and even more in concentric ones.
