Muscle Fiber Types, Contraction Mechanics, and Motor Unit Recruitment

What are the three major muscle fiber types?

Type I (slow oxidative), Type IIa (fast oxidative-glycolytic), Type IIb/IIx (fast glycolytic)

Which fiber type is fatigue resistant and used for posture?

Type I slow-twitch oxidative

Which fiber type is the most trainable?

Type IIa fast-twitch oxidative-glycolytic

Which fiber type is the fastest and generates the most force?

Type IIb/IIx fast-twitch glycolytic

Which fiber type has the highest mitochondria and capillary density?

Type I slow-twitch

Which fiber type fatigues the easiest?

Type IIb/IIx fast-twitch glycolytic

What determines optimum tension in a muscle fiber?

Sarcomere length (optimum length → maximum crossbridges)

What is summation?

Increased contraction force when the muscle does not fully relax between action potentials

What is tetanus?

Maximal sustained contraction with no relaxation

What is a motor unit?

One motor neuron and all the muscle fibers it innervates

What does recruitment mean?

Activating more motor units to increase contraction force

What is the order of motor unit recruitment?

Type I → Type IIa → Type IIb

Why is asynchronous recruitment important?

Helps delay fatigue by having different motor units take turns working

What is an isotonic contraction?

Muscle changes length and moves a load

What is a concentric contraction?

Muscle shortens while producing force

What is an eccentric contraction?

Muscle lengthens under tension

What is an isometric contraction?

Muscle produces force without changing length

Why doesn't the muscle shorten during an isometric contraction?

Elastic elements stretch (tendons) while sarcomeres shorten internally

In an isotonic contraction, what happens once tension equals the load?

The muscle shortens and the load moves

What happens to tension when the muscle relaxes after isotonic contraction?

Tension drops back to baseline