By Dr. Stefano Casali
Muscle strength
From the physiological point of view, muscle strength is the ability possessed by the muscle to develop tension useful for overcoming or opposing external resistances. More in particular:
Force per square meter of section: 200 kN / m2.
A muscle with a square decimeter section can develop a force of 2000 N, equal to the weight of a mass of about 200 Kg.
In the sarcomere, any traction of the myosin head on the actin filament produces a force of 3-4 pN.
A billion myosin heads can exert, if they act simultaneously, a force of just 3-4 N, equal to the weight of a mass of about 0.3-0.4 Kg.
Each traction ("power stroke") of the myosin head displaces the actin filament by 10 nm.
The myosin head remains attached to the actin filament for about 2 ms.
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| From: www.sci.sdsu.edu/movies/actin_myosin.html | |
Eccentric contraction:
As the muscle lengthens, it exerts a force that opposes its stretching.
At the same tension, muscle injuries are more likely to occur during eccentric contractions (with elongation), rather than during isometric (static) or concentric (with shortening) contractions. We will try to understand what eccentric contraction is for and why it can damage muscle fibers.
Maximum voltage
The force-speed curve tells us that the muscle is able to exert a tension (traction force) more intense when activated while stretching (eccentric contraction).
Force-speed curve
Graph by J. Dapena, 1977, based on data by P. Komi, 1973
In many sporting techniques, but also in natural activities such as walking, an eccentric contraction is immediately followed by a concentric contraction ("lengthening-shortening cycle" or "stretch-shortening cycle"):
- The muscle stretches out against the lengthening (eccentric contraction)
- Immediately after the muscle shortens (concentric contraction).
This cycle can be used, for example, to increase the strength of the concentric contraction, as in the jump with counter movement.
Standstill:
- flexion
- Long pause
- Extension
The extenders:
- they get longer
- they stop
- they get shorter
Jump with counter-movement:
Stretching-shortening cycle of extensor muscles:
- flexion
- Immediate extension
The elevation is greater (practical demonstration of what has been said so far).
Example of lengthening-shortening cycle
(The jump with counter-movement)
1) From J. Dapena, 1977, modified.
- The joints flex and then extend.
- The extenders lengthen and then shorten;
2) Graph by J. Dapena, 1977, based on data from P. Komi 1973.
a) The extensors of the hips and knees are almost completely deactivated. They stretch almost passively, due to the force of gravity, which causes an acceleration of the body downwards with consequent flexion of the joints. The elongation rate increases rapidly.
3) From J. Da pena, 1977, modified.
b) When the elongation speed is high, the extenders are activated. Their tension is high and produces a thrust on the ground that is greater than the weight of the athlete. Therefore:
The fall of the body continues, but is abruptly stopped.
The elongation speed decreases rapidly.
4) From J. Da pena, 1977, modified.
c) Fall and elongation stop. The extenders are still activated, with a high percentage of fiber recruitment. For a moment, the extenders are still (isometric contraction).
5) From J. Da pena, 1977, modified.
d) The shortening of the extenders begins immediately. The recruitment percentage is maximum, but the voltage decreases as the speed of shortening increases.
e) The faster and faster shortening continues, with a consequent decrease in tension.
The force of the extensors is transmitted to the ground through the skeletal levers. The athlete pushes down and by reaction, according to Newton's first law, receives an upward thrust of equal intensity (constraining reaction of the soil).
From www.armin-kibele.de/oldpro_e.html, modified.
Note that the maximum thrust ( Force ) is produced at the lowest point of the CG (position c ), when the stretch ends and the shortening begins.
From J. Dapena, 1977, modified.
In positions a and b the extenders stretch quickly, but the force produced is less than the isometric one (position c ). According to the force-speed curve, in the stretch phase the muscle could exert a force much greater than that recorded in position c . Therefore, in the stretching phase the extenders do not activate to the maximum .
CONTINUE: Usefulness of the lengthening-shortening cycle ยป
