muscle forces

Examples of muscle forces in the following topics:

• Force of Muscle Contraction

  • The force a muscle generates is dependent on its length and shortening velocity.
  • The force a muscle generates is dependent on the length of the muscle and its shortening velocity.
  • The force-velocity relationship in muscle relates the speed at which a muscle changes length with the force of this contraction and the resultant power output (force x velocity = power).
  • The force generated by a muscle depends on the number of actin and myosin cross-bridges formed; a larger number of cross-bridges results in a larger amount of force.
  • Although the force of the muscle is increased, there is no velocity of contraction and zero power is generated (left edge of graph).

• Velocity and Duration of Muscle Contraction

  • The shortening velocity affects the amount of force generated by a muscle.
  • In a twitch contraction, a short burst of stimulation causes the muscle to contract, but the duration is so brief that the muscle begins relaxing before reaching peak force.
  • The force-velocity relationship in muscle relates the speed at which a muscle changes length to the force of this contraction and the resultant power output (force x velocity = power).
  • The force generated by a muscle depends on the number of actin and myosin cross-bridges formed; a larger number of cross-bridges results in a larger amount of force.
  • The reverse is true for stretching of muscle; although the force of the muscle is increased, there is no velocity of contraction and zero power is generated (left edge of graph).

• Arrangement of Fascicles

  • Skeletal muscles are grouped into fascicles, which are bunches of muscle fibers surrounded by a perimysium.
  • Skeletal muscle tissue is composed of numerous muscle fibers which are separated from adjacent muscles and other tissues by a layer of dense, elastic connective tissue termed the fascia.
  • This fascia can project beyond the end of the muscle and attach to bones, other muscles, and other tissues.
  • Muscle fascia is predominately composed of cross-linked collagen and elastin fibers oriented parallel to the direction of muscle force, making them able to resist high-tension forces while remaining somewhat elastic.
  • The organization of connective tissue throughout and around a muscle provides strength and flexibility while distributing the force evenly.

• Muscle Fatigue

  • Muscle fatigue refers to the decline in muscle force generated over sustained periods of activity or due to pathological issues.
  • Long-term muscle use requires the delivery of oxygen and glucose to the muscle fiber to allow aerobic respiration to occur, producing the ATP required for muscle contraction.
  • Nerves are responsible for controlling the contraction of muscles, determining the number, sequence, and force of muscular contractions.
  • Most movements require a force far below what a muscle could potentially generate, and barring disease nervous fatigue is seldom an issue.
  • With sufficient training, the metabolic capacity of a muscle can change, delaying the onset of muscle fatigue.

• Structure and Function of the Muscular System

  • Muscle is a highly-specialized soft tissue that produces tension which results in the generation of force.
  • Numerous myocytes make up muscle tissue and the controlled production of tension in these cells can generate significant force.
  • Some skeletal muscle can attach directly to other muscles or to the skin, as seen in the face where numerous muscles control facial expression.
  • Smooth muscle is non-striated and involuntary.
  • The body contains three types of muscle tissue: skeletal muscle, smooth muscle, and cardiac muscle, visualized here using light microscopy.

• Types of Muscle Contractions: Isotonic and Isometric

  • Muscle contractions are defined by changes in the length of the muscle during contraction.
  • A concentric contraction is a type of muscle contraction in which the muscles shorten while generating force, overcoming resistance.
  • Cross-bridge cycling occurs, shortening the sarcomere, muscle fiber, and muscle.
  • An eccentric contraction results in the elongation of a muscle while the muscle is still generating force; in effect, resistance is greater than force generated.
  • In contrast to isotonic contractions, isometric contractions generate force without changing the length of the muscle, common in the muscles of the hand and forearm responsible for grip.

• How Skeletal Muscles Produce Movements

  • Muscles exist in groupings that work to produce movements by muscle contraction.
  • For muscle pairings referred to as antagonistic pairs, one muscle is designated as the extensor muscle, which contracts to open the joint, and the flexor muscle, which acts opposite to the extensor muscle.
  • They often act to reduce excessive force generated by the agonist muscle and are referred to as neutralizers.
  • The muscles of the rotator cuff are also synergists in that they fix the shoulder joint allowing the bicepps brachii to exert a greater force.
  • The brachioradialis and brachialis are synergist muscles, and the rotator cuff (not shown) fixes the shoulder joint allowing the biceps brachii to exert greater force.

• Types of Muscle Tissue

  • The function of muscles is movement, but the types of movement elicited differ between skeletal, cardiac, and smooth muscle.
  • Skeletal muscles are voluntary and striated in nature that allow movement of an organism by the deliberate generation of force.
  • Skeletal muscle fibers are the longest muscle fibers and have stripes on their surface.
  • Although cardiac muscle is involuntary in nature, it is structurally different from smooth muscle.
  • Cardiac muscle is striated, similar to skeletal muscle, but beats involuntarily.

• How Skeletal Muscles Are Named

  • Parallel muscles are characterized by fascicles that run parallel to one another, and contraction of these muscle groups acts as an extension of the contraction of a single muscle fiber.
  • These muscles do not tend to exert as much force on their tendons.
  • In Pennate muscles, the tendon runs through the length of the muscle.
  • However, these muscles tend to have relatively more muscle fibers than similarly sized parallel muscles, and thus carry more tension.
  • If the central tendon branches within a pennate muscle, the muscle is called multipennate.

• Abnormal Contractions of Skeletal Muscle

  • Involuntary muscle contractions are referred to as spasms, and can be due to abnormal activity of the nerve or the muscle.
  • Examples of spasms include muscle contractions due to abnormal nerve stimulation, or abnormal activity of the muscle itself.
  • A spasm may lead to muscle strains or tears of tendons and ligaments, if the force of the spasm exceeds the tensile strength of the underlying connective tissues, such as with a particularly forceful spasm, or in the case of weakened connective tissues.
  • In this case, the hypertonic muscle tone is excessive and the muscles are unable to relax.
  • Hypertonic muscle spasms is the state of chronic, excessive muscle tone, or tension in a resting muscle – the amount of contraction that remains when a muscle is not actively working.