How to Classify Skeletal Muscles?
Updated: Sep 6
The adult human body comprises more than 650 skeletal muscles, which make up about 40% of our body weight. All skeletal muscles are voluntary, meaning the somatic nervous system can control them. By definition, a Skeletal Muscle is a muscle connected to the skeleton to form part of the mechanical system that moves the limbs and other parts of the body. Let us look at how to classify skeletal muscles based on critical parameters.
Typically, muscles are classified as follows:
Skeletal Muscles are referred to as striated muscles because of their striated appearance.
Skeletal muscles can be classified using the following parameters:
#1 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 the sides, adjacent sides are unequal in length.
Pennate: Fibres arranged in 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.
#2 Based on Action:
Agonist muscle: Prime mover (may contain primary and secondary movers).
Antagonist muscle: Oppose the prime movers to allow the 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.
#3 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.
#4 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.
#5 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.
#6 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 critical parameters for classifying Skeletal Muscles are: Fibre Direction & Shape, Action, Number of Joints Crossed, Myoglobin Content, Muscle Contraction and Line of Pull.
We hope the article helped in understanding How to Classify Skeletal Muscles.