Muscle Fiber Types

Fiber Types

• Limb muscles contain a relatively equal distribution of each fiber type.

Type I Fibers

• Type I are slow twitch, oxidative, and consists of small fibers.
• They are red due to high myoglobin content.
• Type I fibers are used by endurance athletes and resist fatigue.

Type IIa Fibers

• Type IIa are intermediate fast-twitch and oxidative-glycolytic.

Type IIb/X Fibers

• Type IIb/X are fast twitch, glycolytic, and consists of large fibers.
• They are white.
• Sprinters and jumpers primarily utilize Type IIb/X fibers and are most fatigable.

Motor Unit Types

• Fast Fatigable (FF) or Type IIb/X fibers are fast glycolytic with large motor units and high innervation ratio.
• Fast Fatigue-Resistant (FR) or Type IIa fibers are fast oxidative-glycolytic with a smaller motor unit and innervation ratio than FF.
• Slow (S) or Type I fibers are slow oxidative with smaller motor units and a low innervation ratio.

Fatigue Resistance

• Type IIb/X fibers are easily fatigued.
• Type IIa fibers are less prone to fatigue.
• Type I fibers are fatigue resistant.

Muscle Attachments

• Contraction produces equal force on the two attachments of a muscle.
• Muscles attach to bone in three ways: directly, via a tendon, or via an aponeurosis.
• A tendon is an inelastic bundle of collagen fibers.
• An aponeurosis is a sheath of fibrous tissue.
• The origin is the more proximal attachment.
• The insertion is the more distal attachment.

Coordination of the Muscular System

• Muscles act together to accomplish a task.
• Coordination involves the "co-ordering" of muscles to accomplish the task efficiently and effectively.
• Coordination is regulated by the nervous system.

Role of Muscles

• Prime mover muscles are primarily responsible for a given movement or action.
• Assistant mover muscles contribute to movement.
• Agonist muscles create the same joint movement.
• Antagonist muscles oppose agonist action; they must first relax to permit movement then act as a brake near the end of movement and check ballistic movements.
• Synergist muscles or forces work with the muscle that is contracting.
• Stabilizers (fixators) stop unwanted action at the fixed attachment (body part that does not move) of the muscle that is producing motion.
• Neutralizers can stop unwanted action at the mobile attachment.
• Co-contraction is the simultaneous contraction of agonists and antagonists.

Isometric Muscle Action

• Tension is produced without visible change in joint angle.
• An example of isometric muscle action is holding arms out to the sides.

Concentric Muscle Action

• The muscle visibly shortens while producing tension.
• The up phase of a sit-up is an example of concentric muscle action.

Eccentric Muscle Action

• The muscle visibly lengthens while producing tension.
• The lowering phase of a squat is an example of eccentric muscle action.

Stretch-Shortening Cycle (SSC)

• SSC occurs when a muscle or muscle group is eccentrically loaded (stretched) and is immediately followed by a concentric contraction.
• Pre-stretch involves quick lengthening of a muscle before contraction.
• Pre-stretch generates greater force than contraction alone and utilizes the elastic component of muscle.
• Type I fibers benefit most from slower pre-stretch due to slow cross-bridging.
• Type II fibers benefit most from faster pre-stretch due to fast cross-bridging.

Plyometrics

• Plyometrics is a conditioning protocol that utilizes pre-stretching.
• Examples of plyometrics include single-leg bounds, depth jumps, and stair hopping.

One- and Two-Jointed Muscles

• Muscles can cross one or two joints.
• Brachialis and pectoralis major are examples of one-jointed muscles.
• Two-jointed muscles (biarticulate) save energy.
• Gastronemius, hamstrings, and biceps brachii are examples of two-jointed muscles.