Skeletal Muscle Mechanics

• The lecture is about Skeletal Muscle Mechanics

Learning Objectives

• Define muscle tension
• List the factors that affect the muscle tension
• Define the muscle twitch
• List Phases of Twitch
• Define Twitch Summation and Tetanus
• Identify Motor Unit Recruitment
• Identify Lever Systems
• List the types of Contraction
• Identify Muscle fatigue

Muscle Twitch

• Defined as a brief contraction-relaxation response to a single action potential.
• It has three phases: Latent period, Contraction phase, and Relaxation phase.

Phases of Twitch

• Latent Period: Occurs before contraction where the action potential moves through the sarcolemma, causing Ca2+ release.
• Contraction Phase: Calcium ions bind, and tension builds to peak.
• Relaxation Phase: Ca2+ levels fall, active sites are covered, and tension falls to resting levels.

Muscle Tension

• Defined as the pulling force on muscle cells generated during contraction.
• It happens as a result of cross-bridge cycling.
• Muscle tension is directly related to the number of crossbridges.
  • Increased crossbridges equals increased muscle tension.
  • Decreased crossbridges equals decreased muscle tension.

Factors Influencing Muscle Tension Extent

• Length of fiber at the onset of contraction
• Frequency of stimulation
• Extent of fatigue
• Thickness of fiber

Optimal Length

• Resting length is the length of a muscle fiber before the start of contraction.
• The optimal length is where the muscle develops maximum tension in subsequent contractions.
• A sarcomere's optimal length to produce max tension is 80%-120% of its resting length
• At this length, overlap between thin and thick filaments is greatest, allowing the maximum number of crossbridges to interact and bind to actin molecules.
• The muscle tension is greatest at the optimal length.
• If the muscle fiber length is shorter or longer than optimal, tension decreases.

Sarcomere Length vs. Optimal Length

• Sarcomeres longer than optimal length: Thick and thin filaments do not fully overlap, resulting in less tension produced.
• Sarcomeres shorter than optimal length: The zone of overlap is reduced, eventually diminishing tension as thin filaments have nowhere else to go.

Frequency of Stimulation: Twitch Summation and Tetanus

• A single action potential in muscle fiber produces a very weak and brief twitch.
• Repeated stimulation of fiber can produce contraction of longer duration and greater tension.
• Two twitches from two action potentials add together to produce greater tension, resulting in twitch summation.
• Twitch summation is possible because the duration of action potential is much shorter than the duration of twitch.
• Summation occurs because the second stimulus triggers the release of more Ca++ ions, which become available to activate more cross-bridging while the muscle is still contracting from the first stimulus.
• Summation results in greater contraction of the motor unit.

Tetanus

• Tetanus occurs if muscle fiber is stimulated so rapidly that it does not have a chance to relax between stimuli.
• It is defined as a smooth sustained contraction of maximum strength.

Tetanus of Skeletal Muscle

• All contractions of our skeletal muscles are smooth tetani in normal life (plural of tetanus).
• Walking, writing, and turning your head are examples.
• Blinking of an eyelid is possibly the only example of a twitch (lasts about 0.1 second).

Pathological Tetanus

• Rough tetanus can become visible when coordination between nerves and muscles is problematic.
• Shaking, tremors, and spasms are examples of abnormal contractions.

Motor Unit Recruitment

• A motor unit is one motor neuron and the muscle fibers it innervates.
• It is defined as the successive activation of the same and additional motor units with increasing strength of voluntary muscle contraction.
• Greater the number of fibers contracting, greater the total muscle tension.
• Motor unit recruitment increases muscle tension.
• Increased frequency of stimulation will increase muscle tension by twitch summation, tetanus, and motor unit recruitment.

Muscle Fatigue

• Fatigue occurs when repeated, intense use of muscles leads to a decline in performance.
• It is of two main types: Peripheral fatigue and Central fatigue.
  • Peripheral fatigue occurs in the muscle or neuromuscular transmission and is a defense mechanism that protects the muscle by preventing it from reaching a point where no ATP will be available.
  • Central fatigue occurs when the nervous system no longer activates the motor neurons supplying the muscles, causing the person to stop exercising even though the muscles can still perform.

Muscle Fatigue Causes

• Depletion of Glycogen energy stores
• Accumulation of Hydrogen ions from lactic acid interferes with cross- bridge functions
• Intracellular acidosis from lactic acid inhibits glycolysis enzymes and slows ATP production
• Accumulation of extracellular K+
• Neurotransmitter depletion at the NM
• Muscle fatigue decreases the muscle tension

Thickness of Fiber

• Muscle fiber hypertrophy from regular exercise increases tension and force of contraction by increasing the number of myosin and actin molecules in muscles.

Types of Contraction

• Two primary types: Isotonic and Isometric.
  • Isotonic: Muscle tension remains constant as muscle changes length, but length shortens.
  • Isometric: Muscle remains the same length, and tension changes.
• The same internal events occur in both isotonic and isometric contractions.

Summary Questions

• How does exercise increase muscle tension?
• List causes of fatigue.

Classroom Question

• Which of the followings increases skeletal muscle tension? Answer: Increased frequency of stimulation.