Notes on Muscle Physiology and Activity

Complete Tetanus

• Muscle contraction that occurs when stimulation frequency is high enough.
• Continuous contraction:
• Muscle fibers do not relax.
• High levels of Ca²+ are maintained.
• Relaxation phase is eliminated, leading to plateau of tension at a maximum level.

Tension Produced by Whole Skeletal Muscles

• Depends on:
• Tension produced by stimulated muscle fibers.
• Total number of muscle fibers stimulated.
• Frequency of stimulation.

Motor Units in a Skeletal Muscle

• Motor Unit: All muscle fibers controlled by 1 motor neuron.
• Comprises few to hundreds of muscle fibers that contract simultaneously.
• Fibers from one motor unit intermingle with fibers from other motor units.

Recruitment (Multiple Motor Unit Summation)

• In whole muscles, smooth motion & increasing tension is produced by:
• Gradually increasing size or number of motor units stimulated.
• Maximum tension is reached when all motor units achieve tetanus; sustained only for a short time.
• Sustained tension allows motor units to rest in rotation.
• This recovery allows motor units to be stimulated again after rest.

Muscle Tone

• Refers to the normal tension and firmness of muscle at rest.
• Muscle units actively maintain body position without motion.
• Increased muscle tone raises metabolic energy usage, even at rest.

Muscles and Body Movements

• Muscles are attached at least at two points:
• Origin: The fixed attachment (often proximal).
• Insertion: The movable attachment (often distal).

Resistance (Load) and Speed of Contraction

• Heavier resistance leads to:
• Longer time before shortening of muscle begins.
• Slower contraction progress (e.g., riding a bike uphill).

Muscle Relaxation

• After contraction, a muscle returns to resting length through:
• Elastic forces (pull of tendons & ligaments).
• Opposing muscle contractions (reverse original motion).
• Gravity (may assist or replace opposing muscle movements).

Muscle Actions

• Prime Mover: Main muscle responsible for movement.
• Synergist: Helps prime mover in action.
• Antagonist: Opposes the movement of prime mover.

Muscle Energy Use and Activity

Resting Muscle:

• Low ATP demand, ample O₂ available; fatty acids are broken down in mitochondria; excess ATP stored as CP and glycogen.

Moderate Activity:

• ATP demand increases, met by mitochondria; aerobic catabolism of pyruvic acid occurs; no surplus ATP.

Peak Exertion:

• Lack of oxygen shifts reliance to glycolysis for ATP; lactic acid builds up due to pyruvic acid conversion.

Muscle Fatigue

• Occurs when muscles can no longer perform a required activity, resulting in:
• Depletion of metabolic reserves.
• Damage to sarcolemma and sarcoplasmic reticulum.
• Decreased pH due to lactic acid deposition.
• Muscle exhaustion and pain.

Recovery Period

• Time needed post-exertion for muscles to return to normal, involving:
• Availability of oxygen.
• Resumption of mitochondrial activity.
• Recycling of lactic acid.

The Cori Cycle

• Involves removal and recycling of lactic acid by the liver.
• Lactic acid is converted to pyruvic acid, and glucose is released to replenish muscle glycogen reserves.

Types of Skeletal Muscle Fibers

1. Fast Fibers: Quick contraction, large diameter, high glycogen reserves, few mitochondria, fatigue rapidly.

• AKA: White muscle fibers, fast-twitch fibers, Type II-B.

1. Slow Fibers: Slow to contract, low fatigue, small diameter, more mitochondria, extensive capillary network, high oxygen supply.

• AKA: Red muscle fibers, slow-twitch fibers, Type I.

1. Intermediate Fibers: Mid-sized, low myoglobin content, more capillaries than fast fibers, slower to fatigue.

• AKA: Fast-twitch oxidative fibers, Type II-A.

Hypertrophy vs. Atrophy

• Hypertrophy: Increases fiber diameter, myofibrils, mitochondria, and glycogen reserves.
• Atrophy: Decrease in muscle size and strength due to disuse or lack of stimulation.

Physical Conditioning

• Improves muscle power and endurance through both aerobic and anaerobic activities.
• Importance of warm-up activities in enhancing flexibility.

Anaerobic Endurance

• Involves fast fiber use, fatigue quickly, improved through frequent, brief, intensive workouts leading to hypertrophy.

Aerobic Endurance

• Involves prolonged activities supported by mitochondria, requires oxygen and nutrients, improved through repetitive training and cardiovascular conditioning.