Overview
This lecture explains how skeletal muscles generate movement, the roles of different muscle groups, how muscle contractions occur, and the difference between isotonic and isometric contractions.
Structure and Function of Skeletal Muscles
- The human body contains about 640 skeletal muscles of various shapes and sizes.
- Skeletal muscles enable a range of power, duration, and subtle movements.
- Muscles never push; they always pull by contracting toward their origin bone.
- The origin is the immobile or less movable bone, while the insertion is the bone that moves.
Muscle Groups and Movement
- Prime movers (agonists) are the main muscles responsible for movement.
- Antagonists oppose the movement and help control or slow down the action.
- Synergists assist prime movers by adding force or stabilizing joints.
- Muscles are grouped functionally based on their role in a movement.
Motor Units and Muscle Contraction
- A motor unit consists of a single motor neuron and all the muscle fibers it stimulates.
- Large motor units control big, coarse movements; small units control fine movements.
- A muscle twitch has three phases: latent (stimulus received, no force yet), contraction (muscle shortens), relaxation (muscle returns to rest).
Graded Muscle Responses and Recruitment
- Muscle force is increased by raising the frequency of stimulation (temporal summation).
- Continuous rapid stimulation can lead to tetanus, a state of sustained contraction.
- Muscle strength also increases by recruiting more motor units (multiple motor unit summation).
- Recruitment follows the size principle: small units activate first, larger ones later as more force is needed.
Types of Muscle Contractions
- Isotonic contractions occur when muscles change length to move a load (e.g., lifting a mug).
- Isometric contractions occur when muscle tension increases without changing length (e.g., trying to lift an immovable object).
Key Terms & Definitions
- Skeletal muscles — muscles attached to bones, responsible for voluntary movement.
- Origin — fixed or less movable bone to which a muscle attaches.
- Insertion — the movable bone muscle attaches to.
- Prime mover (agonist) — main muscle causing a movement.
- Antagonist — muscle that opposes a prime mover.
- Synergist — muscle that assists a prime mover.
- Motor unit — a motor neuron and all its connected muscle fibers.
- Twitch — quick contraction/relaxation of a motor unit in response to a stimulus.
- Temporal summation — increasing muscle force by sending rapid, repeated stimuli.
- Tetanus — sustained maximal muscle contraction due to high-frequency stimuli.
- Recruitment — increasing muscle force by activating more motor units.
- Isotonic contraction — muscle changes length to move a load.
- Isometric contraction — muscle develops tension without changing length.
Action Items / Next Steps
- Review the sliding filament model of muscle contraction.
- Practice identifying prime movers, antagonists, and synergists in common body movements.