Health Essentials: Muscle Functions and Structures Made Simple

What is Muscle Physiology?
Muscle physiology is the study of muscle functions. The muscular system is comprised of three muscle tissues being (i) skeletal, (ii) cardiac, and (iii) smooth. Skeletal muscle maintains posture, creates voluntary movement, and manages force. The cardiac muscle pushes blood through the circulatory system. Smooth muscles move fluids and solids, dilates arteries, and helps maintain balance.

Force, Muscle Tone, Motor Units, and Fatigue. So what does this mean?
Did you ever question why people have specific muscle tones? Typically, muscle tone occurs when many motor units are active even when the muscles are not contracting. Even though the motor units are not causing enough tension for movement, they do provide tone or firmness. Ultimately, muscle fibers are usually providing tension or none at all (known as the all or none). Muscle fibers are usually fast or slow twitch. Fast twitch muscles twitch about 10 times faster than slow twitch muscles. And typically, for muscles to produce force, there must be a period of complete tetanus, where the skeletal muscle contracts repeatedly to avoid relaxation.

Additionally, to produce more tension and force, more motor units have to stimulate and more fibers have to be gathered. Essentially, during high intense outputs, fast twitch muscle patterns are gathered and contracted, whereas slow twitch fiber patterns are seen more during endurance activities. Determining on the intensity of the exercise, muscle fibers and the rate of motor unit tension performance will decline proportionate to the level of fatigue. Recovery allows for working tissue to avoid fatigue; but high intensity workouts lead to higher activation of motor units, most likely an increase in muscle tone, and also rapid fatigue.

Muscle Contractions and Fibers and Distribution
There are different types of muscle contractions to deliver efficiency for the different types of work the body has to perform. Eccentric contraction is used to perform negative work and to do so the force applied is less than the resistive force, lengthening the muscle as it contracts. Concentric contraction accelerates joint movements and shortens the muscle so the applied force may overcome the resistive force. Isometric or static contraction is where muscle tension increases, but the angle of the joints remain the same as resistance cannot be overcome (i.e. the skeletal muscle contracts isometrically to not collapse from the pull of gravity).

Different types of muscle fibers have different shapes and uses. High force producing fibers are Type IIb, intermediate force producing fibers are Type IIa, and low power fibers are Type I. Moreover, fast twitch fibers are larger in diameter with more concentration of glycogen and myofibrils, while slow twitch fibers are smaller. Fast twitch fibers are better suited for anaerobic exercises, while the slow twitch fibers are better used during aerobic activities.  Due note that Type IIa (Intermediate) fibers are considered to be fast twitch muscles as they contain small quantities of myoglobin (oxygen transporting protein), even though these fibers have extensive blood supply similar to slow twitch muscles (which are more resistant to fatigue).

In terms of distribution, fiber types are, for the most part, genetically predetermined. Therefore, you may say that genes may help in performance advantages by the type of muscle fiber distributions. However, even if a person is not born with a specific distribution of the muscle fibers, they can change the recruitment patterns through training. Ultimately, people may improve performance, regardless of genetic predisposition. So the term you can change your genes still rings true.

How exactly do our muscles contract?
To produce force, our skeletal muscles contract. It starts with signals in our brains as the nervous system stimulates our muscles and produces tension. Nerves and blood vessels support muscle contraction and the production of force. These networks of vessels and nerves allow the vascular system to deliver energy and oxygen, from metabolic energy. This is done through action potentials (from the central nervous system), a short-lasting surge where electrical impulses along a muscular or nervous membrane of a cell, rises and falls.

Muscle fibers are constructed as a product line, where there are specialized functions. In the inner most part, you have myofibrils that contain the protein filaments of actin (thin) and myosin (thick) and where the contractile movements take place. Around the myofibrils are the calcium network of SRs (sarcoplasmic reticulum) which are T-tubules used to transfer nerve signals. Additionally, there is the mitochondrion, which is used to produce energy. The fibers are then encased by endomysium, where the capillaries are located; capillaries are tiny blood vessels that connect arteries and veins, help distribute oxygen and nutrients, and help remove waste.

If the body has to create tension in muscle fibers, the action potential current runs through the different levels of the muscle tissue. The small nerve fibers are motor neurons, and the entire motor unit is the motor neuron with remaining corresponding muscle fibers it supplies. Action potentials travel down the T-tubules and help stimulate the sarcoplasmic membrane. Before this, they connect the nerves and fibers. When relaxed, the fiber protein filaments inside the sarcomere (myofibril muscle thread) are blocked from interacting proteins known as troponin and tropomyosin. When calcium unlocks the bond from the SR, troponin molecules move and the tropomyosin rotates from the site. This allows myosin and actin protein to attach (this is known as the cross bridge attachment).