Skeletal Muscles

Skeletal Muscles: More Than Just Lifting

• Skeletal muscles are often associated with strenuous and difficult tasks.
• They perform essential brute-force actions.
• They are also capable of delicate and subtle movements.

Diversity of Skeletal Muscles

• There are approximately 640 skeletal muscles in the human body.
• They vary in shape and size:
  • Longest: Sartorius in the upper thigh.
  • Biggest: Gluteus maximus in the butt.
  • Tiniest: Stapedius in the middle ear.
• These muscles exhibit a range of power and duration.
• They can perform both powerful actions (crushing cans, punching holes) and delicate tasks (plucking an eyebrow, cuddling a kitten).

How Muscles Move: Pulling vs. Pushing

• Muscles never push; they always pull.
• Most skeletal muscles cross joints, connecting at least two bones.
• When a muscle contracts:
  • The bone that moves is the muscle's insertion point.
  • The muscle brings the insertion point closer to the origin (the bone that moves less or not at all).
• Movement is always a pull of the insertion bone toward the origin bone.
• Muscles cannot extend themselves to push a bone away.
• Example: In a push-up, the pectoralis major contracts, pulling its insertion (top of the humerus) toward its origin (sternum).
• This principle applies to all skeletal movements.

Functional Groups of Skeletal Muscles

• Muscles can be classified into four functional groups based on their roles:
  • Prime Movers (Agonists): Responsible for producing a specific movement.
    • Example: Pectorals and latissimus dorsi during adduction of arms in jumping jacks.
  • Antagonists: Work in opposition to the prime movers, providing resistance or control.
    • Example: Deltoids act as antagonists during adduction, preventing over-extension.
    • During abduction, deltoids become prime movers, and pecs/lats become antagonists.
  • Synergists: Assist prime movers by adding extra force or stabilizing joints.
    • Example: Rotator cuff muscles (teres minor, infraspinatus) act as synergists during arm movements.

Motor Units: The Key to Muscle Contraction

• A motor unit consists of a group of muscle fibers innervated by a single motor neuron.
• All fibers in a motor unit act together.
• Large Motor Units: Found in muscles for big movements (walking, squatting).
  • Each motor neuron may innervate thousands of muscle fibers.
  • Example: Rectus femoris (quadriceps).
• Small Motor Units: Found in muscles for fine motor control (eyes, fingers).
  • Each motor neuron innervates only a few muscle fibers.
• When a motor unit responds to an action potential, the fibers contract and release in a twitch.

The Three Phases of a Muscle Twitch

• A muscle twitch has three distinct phases:
  • Latent Period: Immediately after nerve stimulation.
    • Calcium ions flood sarcomeres, initiating the sliding filament model.
    • No force is produced yet; action is just starting.
  • Contraction Period: Myosin heads bind, pull, and release, causing muscle fiber contraction.
  • Relaxation Period: Calcium is pumped back into the sarcoplasmic reticulum.
    • Actin and myosin stop binding, and the muscle relaxes.

Graded Muscle Responses

• Muscles produce varying smooth forces called graded muscle responses.
• These responses are influenced by:
  • Frequency of Stimulation
  • Strength of Stimulation

Frequency of Stimulation

• Increasing force by increasing the firing rate of motor neurons.
• Faster nerve impulses lead to stronger twitches as the muscle doesn't fully relax between stimuli.
• Temporal Summation: Twitches add up when they occur close together in time.
• Tetanus: Twitches blend into one continuous contraction, reaching maximum tension.

Muscle Fatigue

• Prolonged contraction leads to muscle fatigue due to ATP depletion.
• Tension drops to zero when muscles can no longer sustain activity.

Strength of Stimulation

• Increasing the signal strength involves sending action potentials to more motor units.
• Each motor unit twitches at slightly different times, smoothing out the overall contraction.
• Recruitment (Multiple Motor Unit Summation): More motor neurons stimulate more muscle fibers, intensifying contractions.

Size Principle

• The order of recruitment follows the size principle:
  1. Smallest motor units with the smallest fibers are activated first.
  2. Larger motor units with larger fibers are then enlisted.
  3. Largest motor units with the biggest muscle fibers are recruited last.
• Larger motor units generate significantly more force (up to fifty times) than smaller ones.
• More motor units recruited = greater force generated.

Isotonic vs. Isometric Contractions

• Isotonic Movement: Muscle tension overcomes the load's weight, changing muscle length during contraction.
• Isometric Contraction: Muscles contract and develop tension without changing length.

Summary

• Skeletal muscles work together to create and reverse movements.
• Motor unit size influences muscle function.
• Muscle twitches occur in three phases.
• Impulse strength and frequency affect contraction strength and duration.
• Twitch summation and tetanus are distinct processes.
• Isotonic and isometric movements differ in muscle length changes.