L14 Skeletal muscle mechanics

What is the length–tension relationship?

The relationship between muscle length and the force (tension) it produces during contraction.

What is passive tension?

What is active tension?

Force needed to stretch the muscle

Force generated by muscle contraction

At what length does maximal active tension occur?

At optimum length (L₀), where all myosin heads can bind to actin.

What happens when muscle is longer than L₀?

What happens when muscle is shorter than L₀?

Fewer cross-bridges can form as actin myosin overlap is reduced → less force

Actin filaments overlap and myosin hits z discs causing interfere (steric hindrance) → less force.

What is the typical range for active force generation?

70–130% of L₀.

Series vs parallel elastic

Series: tendons, connective tissues in series with contractile elements

Parallel: sarcomere proteins especially titin and extracellular sheaths around fibres

What is an isometric contraction?

What is an isotonic contraction?

What is an auxotonic contraction?

What are concentric and eccentric contractions?

Muscle develops tension but does not change length (e.g. holding a load).

Muscle shortens under constant load (force remains constant)

• Concentric: Muscle shortens while contracting (lifting a weight).

• Eccentric: Muscle lengthens while contracting (lowering a weight).

Contraction against a variable load (e.g. stretching a spring)

What is preload?

What is afterload?

When does muscle shortening begin?

The initial load (stretch) on the muscle before contraction, setting its starting length.

The load the muscle must lift during contraction.

Shortening begins when the muscle’s force exceeds the afterload(isotonic)

What is the relationship between force and velocity?

Inversely related — as load increases, shortening velocity decreases.

How does the load determine myosin attachment and detachment for light and heavy loads

If the load is heavy and the filament isn’t moving (isometric):

• Myosin head attach and stay bound longer (detachment slower under strain)

• More heads accumulate in the attached site so more force generated but little shortening

If the load is light the filament moves quickly (concentric):

• heads detach faster (detachment faster under low strain)

• Fewer head attached so less force but higher velocity 

high force= lower velocity (many cross bridges attached) 

low force= high velocity (few cross bridges attached) 

What happens in eccentric contractions?

External force exceeds muscle tension → muscle is stretched while contracting → very high force generated.

What is a muscle twitch?

A single contraction from one AP in a motor neuron.

What is summation?

Why can twitches summate but APs cannot?

The additive effect of multiple twitches due to repeated stimulation before full relaxation.

Twitches are mechanical and last longer than APs; APs are refractory and brief.

What is tetanus?

What is clonus?

Sustained maximal contraction from high-frequency stimulation — many fused twitches.

Partial, rhythmic contractions — incomplete tetanus.

Why does more frequent stimulation increase force?

• AP triggers calcium release from SR so calcium levels rise fast but falls quickly as calcium is pumped back into the SR

• Force lags behind calcium as calcium bind troponin causing it to move and allowing cross bridges formation. This then cycle and stretch elastic elements so when force peaks calcium is already falling

• When there is more frequent APs, calcium levels kept high so mor cross bridges meaning greater force 

How long is a muscle AP compared to a twitch?

AP ≈ 5–10 ms; Twitch ≈ 100+ ms.

What is a motor unit?

small vs large motor units: how many fibres and use

One motor neuron and all the muscle fibers it innervates.

• Small motor units: few fibers, fine control (e.g. eye muscles)

• Large motor units: many fibers, powerful movement (e.g. leg muscles)

What are the three main energy sources for ATP in muscle?

What is their speed, how many ATP they make per glucose and when are they used

PGO

• Phosphocreatine (creatine phosphate): very very fast, direct transfer (no glucose used) rapid ATP regeneration, immediatly 

• Glycolysis: fast, 2 ATP per glucose, short high power

• Oxidative phosphorylation: slow, ~36 ATP per glucose or fat oxidation, long term

What is the order of motor unit recruitment?

Slow (small) → fast oxidative → fast glycolytic.

What are the three main muscle fiber types?

Mention speed, force, fatigue resistance, metabolism method it uses

• Type I (slow oxidative): slow, low force, fatigue-resistant, use oxidative phosphorylation

• Type IIa (fast oxidative):moderate speed, moderate force, moderate fatigue resistance, glycolysis and oxidation

• Type IIb (fast glycolytic): very fast, high force, fast fatigue, glycolysis

Name the type of muscle fibers they are 

1, 2a, 2b