The muscles of the back can be divided into three groups – superficial, intermediate and deep: Superficial – associated with movements of the shoulder. Intermediate – associated with movements of the thoracic cage. Deep – associated with movements of the vertebral column. The deep muscles develop embryologically in the back, and are thus described as intrinsic muscles. The superficial and intermediate muscles do not develop in the back, and are classified as extrinsic muscles. This article is about the anatomy of the superficial back muscles – their attachments, innervations and functions. The superficial back muscles are situated underneath the skin and superficial fascia. They originate from the vertebral column and attach to the bones of the shoulder – the clavicle, scapula and humerus. All these muscles are therefore associated with movements of the upper limb. The muscles in this group are the trapezius, latissimus dorsi, levator scapulae and the rhomboids. The trapezius
Skeletal muscles are the voluntary and striated muscles. These skeletal muscles are responsible for the activities or movements around a particular joint. These skeletal muscles are not having the same structure and function.
There are three types of skeletal muscles (Table 2.1).
a. Type I fibers
b. Type IIa fibers
c. Type IIb fibers.
Type I Fibers
These fibers contain more number of the mitochondria, myoglobin and the blood capillaries. Because of the more number of myoglobin and blood vessels, the muscle fibers are red in color. It is having the capacity to generate the ATP from the aerobic system, so that it is called as oxidative fibers. It is innervated by the less number of nerve fibers. These muscle fibers are in small diameter and fatigue-resisted fibers.
Slow Twitch—Fatigue-resisted—Red Colored—Oxidative Muscle Fibers
Example: Postural muscles like neck and trunk muscles.
Type IIa Fibers
They contain more amounts of myoglobin, mitochondria and blood capillaries. But comparative with type I, these are less pink in color. These are fatigue-resisted fibers but not like Type I. They split the ATP much faster, as the result contraction velocity is also fast. These fibers also generate the ATP from aerobic system.
Oxidative—Fast Twitch—Pink Colored—Fatigue-resisted Muscle Fibers
Example: Leg muscles, which is responsible for walking and running.
Type IIb Fibers
They contain less numbers of mitochondria, myoglobin and blood capillaries. They generate the ATP from the anaerobic system, i.e. by the processes of glycolysis. They are large indiameter fibers and are fatigue earlier. The contraction of these muscle fibers will be more rapid and strong. These muscle fibers also contain the glycogen, they are in white color.
Glycolytic—Fast Twitch—Early Fatiguable—White Muscle Fiber
Example: The muscle responsible for rapid movement are—Shoulder flexors and abductors.
Normally, each and every muscle contains the mixture of all the muscle fibers. Depending on the activity the muscle fibers are working and also depends on the proportionate of muscle fibers in the muscle spindle determines the contraction of muscles. In postural muscles the proportion of the type I fibers are more and in rapid contraction muscles the type II b propor-tion is more. During the minimal contraction of the muscle, the Type I muscle fiber motor fibers are stimulated as a result type I muscle fiber goes for contraction. If the more amount of contraction is needed, the Type II motor units will be stimulated. Sometimes depend on the exercise the type of the muscle fibers also changing. If the strenuous activity is less, the type IIb fibers may be converted as type II a or type I. If the strenuous activity is more the type I fibers may become type IIa or type IIb.
For example, the gymnastic people, who are performing the rapid movements and exercise will be having the more proportionate of the white fibers (type IIb). The runners will be having more proportionate of the pink fibers (type IIa). The normal people like housewife, clerks, company MDs will be having the more of red fibers (Type I).
• Type I—Housewife, those who are performing the normal day-to-day activities.
• Type IIa—Runner, athletes.
• Type IIb—Gymnastic people, body-builders.
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