Sports Science HL B.1.3 Muscular Function

Three Types of Muscles

Skeletal Muscle, Cardiac Muscle, Smooth Muscle

Muscle Functions

Move bones, transportation within the body, maintain body position, generate body heat.

Contractibility

Contract (shorten in length) to generate force, can stretch to about 50%.

Extensibility

Stretch beyond normal length, can stretch to about 150%.

Elasticity

Ability to return to normal size after being stretched.

Motor Neurons

Carry messages from CNS to muscles. About 200,000 make up the efferent system.

Motor Neuron Structure

Structure includes a cell body (soma) which is in the spinal cord. Axon extends out through the body to the muscle. End plate / synapse at the muscle. Dendrites attach to the neurons. Myelin sheath covers axon to insulate electrical impulse down the axon.

Motor Neuron Structure (cont.)

End of motor neuron reaches a muscle, branches at the end plate reach out to various muscle cells of the muscle, space between end of neuron and muscle cell called the synapse. Neurotransmitters released at synapse recognized by muscle cell.

Motor Unit

Combination of a motor neuron and all the muscle fibers it innervates. More muscle fibers per motor unit means a large force, fewer muscle fibers mean more precision.

All-or-none principle

States that a motor neuron will activate all the muscle fibers it innervates, cannot be selective.

Types of Motor Units - Type I

Slow-twitch, slow to active, small force. High endurance / fatigue resistance. Slow oxidative. Ex. walking. Greater amount if mitochondria and capillarization.

Types of Motor Units - Type IIa

Fast-twitch. Fast to activate, stronger, some fatigue resistant. Fast oxidative. Ex. swimming, when more force needed for longer. Fast Fatigue (FF). Utilizes greater amount of glycolysis for faster and stronger contractions.

Types of Motor Units - Type IIx

Fast-twitch. Fastest, strongest, fatigue quickly. Fast Fatigue (FF). Fast glycolytic. Useful when large force is needed, ex. sprinting, jumping, weights. Utilizes greater amount of glycolysis for faster and stronger contractions.

Motor Units in Muscle

Generally 50% Type I, 25% Type IIa, 25% Type Iix

Orderly Recruitment

Type I → Type IIa → Type IIx. Small motor neurons are quick to recruit, large ones are slower.

Hypertrophy

Increase in size of body tissue / organ. Increase in size of muscle cells. Transient hypertrophy is due to increase in fluid in muscles directly following a workout. Chronic hypertrophy is due to increase in fluid in muscles directly following a workout.

Chronic Hypertrophy

Due to long term training. Can be muscle cells increasing in size from additional myofibrils OR additional muscle cells (hyperplasia).

Atrophy

Decrease in muscle size. Due to disease, disuse, or poor diet. Disuse can be simply not exercising and using a muscle, or immobilization due to an injury.

Types of Muscle Contractions - Isometric

Muscles contract but remain same length (iso means equal, metric means measurement)

Types of Muscle Contractions - Isotonic

Muscles contracting with the same tension (tonic means tension)

Isotonic - Isotonic Concentric

Muscles contracting with the same tension, but length of muscle decreases (often using muscles to lift something)

Isotonic - Isotonic Eccentric

Muscles contracting with the same tension, length of muscle increases (often using muscles to put something down, the lowering phase of a lift)

Types of Muscle Contractions - Isokinetic

Equal movement. Often there is acceleration. Most often is use is rehabilitation and is rare without regulatory equipment.

Relaxing

No contractile forces, but can still have some resistance as fibers stretch.

Muscle Roles - Agonist

The mover. The muscle that is contracting to generate force / movement. Can be prime (ex. biceps brachii) or assistant (ex. brachialis and brachioradialis)

Antagonist

The muscle that works in contrast of the agonist. Does the opposite joint movement. Has to relax during contraction of agonist through reciprocal inhibition (triceps vs biceps). OR it is a muscle that contracts eccentrically to control a drop (biceps).

Reciprocal Inhibition

Relaxation of the antagonistic muscle during a muscle contraction.

Coactivation

Conscious overriding of a reciprocal inhibition to stabilize a joint.

Fixator

Stabilizer. Prevents a movement at the other end of a muscle contraction. (Ex. back of shoulder during bicep curl, or core during almost anything.)

Synergist

Neutralizer. Prevents a secondary movement at a joint, ex. muscles preventing supination of a radioulnar during a bicep contraction.)