The orientation of the muscle fibers inside the muscle determines its strength and the amplitude of contraction. With respect to its longitudinal axis, the cells can be arranged parallel (following the course of the tendon fibers) or obliquely. All this, as we said before. , has an enormous importance in muscle mechanics.
The fibers parallel to the longitudinal axis of the fleshy belly have a similar length to it and allow the muscle to shorten more, thus generating a large and fast movement (greater articular excursion). The muscles that contain them are defined as "bundle muscles" parallels ".
The fibers with an oblique direction, on the other hand, have a length much shorter than that of the belly and can develop an equally limited contraction. The muscles that contain them are defined as oblique or pinnate bundles (the fibers are arranged like the beards of a feather compared to calamus). Even if the contraction is limited, the pennation allows to compact a large number of fibers in a smaller transverse area; consequently, the greater number of fibers guarantees the development of a considerable force, greater than that generated by the parallel bundle muscles. .
The arrangement of the fibers is associated with the function of the muscle: the fast muscles are usually in parallel bundles, the strong ones pinnate.
Both types of muscle are present in the human body, but those with oblique bundles prevail. The latter can be further divided into different categories (unipennati or semipennati, bipennati and multi-pinnates) according to the method of attack on the tendons.
EFFECT OF THE PENING ANGLE ON FORCE DEVELOPMENT:
Parallel fibers (parallel bundle muscles) transmit all their contractile capacity to the tendon; the pinnate ones, on the other hand, transmit only a part of it; an angle of pennation equal to 30 °, for example, transmits to the tendon about 87% of the tension exerted by the fibers (cos (30 °) = 0.866).
The pennation angle is the angle between the axis of the muscle and the axis of its fibers.
Although it involves a loss of contractile power, pennation allows a large number of fibers to be compacted into a smaller transverse area and thus to produce greater force.
Beyond a certain limit of muscle volume, the pinnate muscles undergo a change in the pinnation angle, which increases and becomes more and more unfavorable, limiting the possibilities for the muscle to express force. Also for this reason, beyond a certain threshold, the linearity between increase in muscle volume (hypertrophy) and increase in strength loses linearity. Once this threshold is exceeded, hypertrophy can still increase, but is accompanied only by a modest increase in strength.
The parallel bundle muscles can be divided, on the basis of their macroscopic shape, into ribbon-like, fusiform, flat and fan-shaped.
In the ribbons, the flattened and long bundles remain organized parallel from one "end to the other" (eg sartorius). In fusiforms, on the other hand, the bands, long and more voluminous, converge on a tendon at one or both ends (eg brachial biceps); this particular arrangement of the fibers, as the word itself says, gives the muscle a tapered shape, voluminous in the central part and flattened at the ends.
The laminar or broad muscles are flattened and large, provided with aponeurosis, like the diaphragm and the muscles of the abdominal wall.
Finally, in the fan-shaped muscles, the fibers develop to form a triangular structure, as in the temporal muscle (they diverge at one end and are inserted on the same insertion tendon at the other end).
Based on the type of union between muscle bundles and tendons, they are classified into:
Muscles with parallel bundles
- Ribbon-like muscles: those that have muscle bundles organized parallel to each other from one end to the other
- Fusiform muscles: those that have muscle bundles almost parallel to each other and that converge on a tendon at one or both ends
- Wide muscles: are those that have flat muscle bundles that merge with the aponeuroses at the extremities
- Fan-shaped muscles: are those in which the muscle bundles diverge at one end and converge on an insertion tendon at the other end
Muscles with oblique or pinnate bundles
- Pinnate muscles: they are those that have a central tendon on which the muscle fibers converge and stretch
- Semi-finned muscles: these are those that have two tendon laminae between which the muscle fibers are stretched
- Multipinnati muscles: these are those that have many tendons of origin on which the muscle fibers converge and stretch
Other articles on "Pinnate Muscles"
- Muscles classification
- muscles of the human body
- Skeletal muscle
- Muscle anatomy and muscle fibers
- myofibrils and sarcomeres
- actin myosin
- muscle contraction
- muscle innervation
- neuromuscular plaque
