What physically shortens during muscle contraction?
The sarcomere shortens. The actin and myosin filaments do NOT shorten — they slide past each other.
What defines the boundaries of a sarcomere?
Z line to Z line.
What is the M line?
The center of the sarcomere where myosin filaments are anchored
What is the role of calcium in muscle contraction?
Calcium binds to troponin → shifts tropomyosin → exposes myosin-binding sites on actin → allows cross-bridge formation.
According to sliding filament theory, what changes during contraction?
What defines concentric contraction at the joint level?
Muscle shortens while producing force greater than external load → joint motion occurs in direction of muscle action.
What defines eccentric contraction?
Muscle lengthens while under tension because external load > muscle force.
What defines isometric contraction?
Muscle produces force but no change in muscle length or joint angle.
What is a motor unit?
One motor neuron + all the muscle fibers it innervates.
How does muscle increase force?
Recruit more motor units (spatial summation)
Increase firing frequency (temporal summation)
What is the size principle?
Small motor units (slow-twitch) are recruited first; large fast-twitch units recruited as force demand increases.
What is the difference between force and strength?
Force = instantaneous push/pull produced by muscle.
Strength = maximum force a muscle can produce.
Define muscle power.
Force × velocity.
Trap: Power is not just strength.
Define muscle endurance.
Ability to sustain repeated contractions or maintain contraction over time.
Define muscle fatigue.
Decline in ability to generate force.
When is active force greatest?
When sarcomere is at optimal resting length (max actin-myosin overlap).
Too short → overlap interference
Too long → not enough cross-bridges
What causes passive tension?
Stretch of elastic components (titin, connective tissue).
How does velocity affect force in concentric contraction?
As velocity ↑, force ↓.
How does velocity affect force in eccentric contraction?
As velocity ↑, force ↑ (up to a point).
What is the overload principle?
Muscle must be challenged beyond normal load to adapt.
What must occur for strength to increase?
Muscle hypertrophy (increased cross-sectional area) and neural adaptations.
Key difference between strength and enduracne training?
Strength → High load, low reps
Endurance → Low load, high reps
Give the steps to the oxford clincial scale
0 – No contraction
1 – Flicker
2 – Full ROM gravity eliminated
3 – Full ROM against gravity
4 – Against some resistance
5 – Normal strength
