Skeletal muscle or striated muscle cells are some of the largest cells in the body and it is the job of these muscles to create movement. Muscle cells create movement by contracting (shortening), which pulls the opposite ends of the muscle together. There are over 600 muscles in the human body, and more than 400 are skeletal muscles. Muscle does not connect to muscle throughout the body. Fascia, tendons, and ligaments all play a role in aiding joint movement with the skeletal muscle.
Fascia is a sheet or band of fibrous connective tissue which separates and contains muscles within compartments. A tendon is a fibrous cord (consisting primarily of collagen, due to collagen’s ability to withstand tension stress and its elasticity) which attaches muscle to bone, although fascia can also act as a site for muscle attachment. There are three layers of fascia which surround, protect, and enable separate innervation of muscle fibers (muscle cells) within a muscle.
The outermost layer of fascia around muscle fibers is the epimysium, which completely surrounds muscle. The next layer groups muscle fibers in bundles called fasciculi and is referred to as perimysium. The innermost layer, the endomysium, surrounds individual muscle fibers. The epimysium is continuous (becomes one) with the endomysium and perimysium. Continuations of the epimysium form tendons that become continuous with the periosteum (outer layer of bone). This interweaving of connective tissue makes the muscle-bone connection extremely strong. Along with the neuroendocrine system, this intramuscular fascia enables some muscle fibers within a muscle to be relaxed and others to be active.
Skeletal muscle is composed of bundles of long, parallel fibers (cells). A muscle is not just one big fiber. Embedded in the muscle fibers are filaments called myofibrils which are further divided into sarcomeres. Sarcomeres are the smallest contractile unit of skeletal muscle. Muscle fibers are called striated because they have striations of light and dark bands created by the repeating actin and myosin filaments in the sarcomeres. Muscles are divided by layers of connective tis-sue to enable fibers within each muscle to be innervated (in order) from smaller to larger numbers of fibers and sizes (smaller fibers will always be recruited first, to conserve energy).
A motor neuron and all the muscle fibers it activates is known as a motor unit. The size principle of recruitment describes how smaller numbers of fibers (and smaller fibers) will al-ways be selected before larger numbers of fibers (and larger fibers) to preserve muscular energy. The larger the neuron, the larger the number and size of fibers it innervates. Once a muscle fiber is innervated (an action potential is sent by the nervous system from the neuron(s) to the muscle fibers), it is either on or off, there can be no partial contraction; this is the all or none theory of muscle fiber activation. If all muscle fibers were on all the time, it would not be a pretty sight (un-controllable joint movements), and the constant high intensity contractions would be downright dangerous!
You will learn about muscle fiber types, muscle actions and how to train people based on these scientific facts in the NESTA Personal Fitness Trainer Certification, Wexford University Personal Training Certification, the MMA Conditioning Coach Certification and the ITCA Triathlon Coach Certification programs.
