The skeletal muscle has 2 main functions

  1. To allow “movement”: walking, jumping, moving the fingers to grab a bag, raising the arms to carry the bag, pushing pulling, smiling, chewing, dodging, … skeletal muscle can also allow us to maintain static postures.
  2. To produce heat.

The unique structure of the muscle cell:

Muscle cells, called myofibers, do not divide. The growth of a muscle is the result of cellular enlargement, not cell division. The size of a myofiber increases with strength and conditioning.

Muscle cells look very different than most other cells but have the same fundamental structure. One important difference is that a myofiber contained not one (like all other cells in our body) but many nuclei. This is because a myofiber is created by the fusion of many progenitors of myofibers, each progenitor having one nucleus. Myofibrils are rod-like structures inside the myofibers and represent the functional units of the muscles.

A muscle fiber with multiple nuclei and surrounded by satellite cells

When muscle injury occurs, satellite cells differentiate into progenitors of myofibers and fuse either to repair existing muscle fibers or to form new fibers.

Energy usage and production of force:

Muscles are formed by several types of fibers, called type I and type II. Duration and speed of contraction is dependent on the fiber type.

  • Type I fibers (slow twitch) provide muscle support during endurance exercise but have a low strength of contraction.
  • Type II fibers (fast twitch) provide muscle support for burst of energy and high strength exercise.

A predominance of type I fibers has been observed in LGMDs and in some LGMD2I patients. Type I fibers have a lower force generating capacity than type II fibers.

The 2 types of fibers use 3 systems to generate energy

1 the adenosine triphosphate-creatinine phosphate system, which is used for intense exercise lasting a very short time such as, in athletes, the 100 meter sprint (anaerobic system which means that it does not need oxygen brought via the lungs); 2 the lactic anaerobic system, which is used for intense muscle activity lasting about 2 minutes such as, in athletes, the 400 meter sprint (does not need oxygen and produces lactic acid from glucose); 3 the aerobic system, which is used for lower intensity exercises that lasts more than several minutes (requires oxygen and produced lots of energy from glucose and fatty acids).

The type I muscle fibers only use the aerobic system (#3) for low intensity endurance activities, while the type II muscle fibers use the 2 anaerobic systems for bursts of high intensity energy.

In LGMD2I, the muscle suffers extensive injury:

A muscle fiber is enveloped by a “highly delicate, transparent, and probably elastic” sheath called the basement membrane. The integrity of the basement membrane and its interaction with the myofiber is essential for muscle strength, integrity, and maintenance.

The basement membrane is formed by several layers, each of which layer is composed of molecules highly interconnected into solid networks. The strengthened layers cross-link and, at the same time, bind to the muscle fiber they encircle, rendering the fiber strong, less breakable, and seemingly elastic.

In LGMD2I, extensive muscle injury and degeneration is due to the weak (or lack of) binding between the muscle fiber and the basement membrane (see section “LGMD2I”).

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