How to classify SKELETAL MUSCLES?
Updated: Nov 12
What are skeletal muscles? How do we classify skeletal muscles?
The article will teach us to classify Skeletal Muscles based on critical parameters.
Typically muscles are classified as:
Google dictionary defines a Skeletal Muscle as a muscle which is connected to the skeleton to form part of the mechanical system which moves the limbs and other parts of the body.
Skeletal Muscles are referred to as striated muscles because of their striated appearance.
There can be numerous classifications based on various parameters for Skeletal Muscles. The most common types are as follows:
Based on Fibre Direction and Shape
Parallel: Fibres extend the whole length of the muscle and are parallel to each other
Rectangular: Fibres parallel to each other having equal length abdomen
Rhomboidal: Fibres parallel in sides, adjacent sides are unequal in length
Pennate: Fibres arranged in the form of a feather-like pattern, are short and parallel, lying obliquely to a central axial tendon
Unipennate: Fibres are attached to one side of the tendon resembling half of a feather
Bipennate: Fibres are attached to both sides of the tendon resembling a complete feather
Multipennate: Collection of multiple bipennate structures
Circumpennate: Fibres attached to all sides of the tendon
Fusiform: Arranged in the form of a spindle as a belly and two tendons
Triangular: One side triangle or two side triangle-shaped
Fan-shaped: Triangle shaped on one side
Trapezoidal: Triangle shaped on two sides
Sphincter: Arranged in a circular form
Based on Action
Agonist muscle: Prime mover (may contain primary and secondary movers)
Antagonist muscle: Oppose the prime movers to allow agonist to move
Synergist muscle: Assistor to the agonist muscle
Stabiliser muscle: Keeps joints stable
For example, when you flex your elbow:
Biceps brachii is the agonist muscle
Triceps brachii is the antagonist muscle
Brachioradialis and brachialis (forearms) are the synergist muscles
Shoulder muscles like anterior deltoid, middle deltoid, posterior deltoid etc. are the stabiliser muscles
Based on the Number of Joints Crossed
Uniarticulate muscle: Muscle crossing only one joint
Example – brachialis, soleus etc.
Biarticulate muscle: Muscle crossing two joints
Example – gastrocnemius, hamstrings, rectus femoris
Multiarticulate muscle: Muscle crossing more than two joints
Example – erector spinae, multifidus
Based on Myoglobin Content
Slow-twitch muscle: Contain more red fibre, responsible for movements requiring slow action for an extended time
Fast-twitch muscle: Contain less red fibre, responsible for movements requiring fast action for a short time
Based on the Type of Muscle Contraction
Isometric contraction: Muscle contracts but does not shorten
Concentric contraction: Helps in moving weight against the line of gravity; muscles origin and insertion come closer while contracting
Eccentric contraction: Helps in moving weight in the line of gravity; muscles origin and insertion move farther while contracting
Based on the Orientation of the Line of Pull in Relation to the Joint
Flexor: Line of pull passes anterior to the joint axis
Extensor: Line of pull passes posterior to the joint axis
Adductor: Line of pull passes medial to the joint axis
Abductor: Line of pull passes lateral to the joint axis
Typically muscles in the human body are classified as: Skeletal Muscles, Smooth Muscles and Cardiac Muscles.
Skeletal Muscles can be classified using various parameters.
The key parameters for classification of Skeletal Muscles are based on: Fibre Direction & Shape, Action, Number of Joints Crossed, Myoglobin Content, Muscle Contraction and Line of Pull.
Now we understand how to classify Skeletal Muscles using critical parameters.