Muscle Tension Sim

Stimulus frequency1 Hz Stimulus strength100% Slow motion0.25×

Label the phases of a single twitch At 5 Hz and up, deliver shocks in bursts (shows the start and end of each contraction)

Tension trace Stimulus (shock) Recruited motor unit Ca²⁺ Thin filament Thick filament

Mini demo · Treppe (the staircase effect)

A muscle that has been resting gets the same stimulus over and over, slowly enough that it fully relaxes each time. The first few twitches get a little stronger each time, like a staircase. This is treppe (German for "staircase"). The usual explanation: Ca²⁺ builds up in the sarcoplasm a bit more with each twitch, so more is available to bind troponin. Treppe is different from summation: here the muscle relaxes completely between twitches.

Same stimulus strength every time. Only the twitch height changes.

Think of it like this

Sarcomere length2.10 µm

Thin filament (actin) Thick filament (myosin) Head forming a cross-bridge Head with nothing to grab Z disc

Length–tension relationship

Why there is a "just right" length

Length2.10 µm

% of resting100%

Tension100%

Tension comes from cross-bridges: myosin heads grabbing actin. A myosin head can only pull if a thin filament is lying right next to it. So the tension a sarcomere can make depends on how much the thick and thin filaments overlap before it starts to contract. This is the length–tension relationship.

Too short: thin filaments from both sides run into each other in the middle, and the thick filaments press against the Z discs. There is no room left to slide. Less tension.
Optimal: about 80–120% of resting length. The peak sits near 2.0–2.25 µm, where every myosin head has a thin filament to grab.
Overstretched: the filaments barely overlap. Few heads can reach actin, so few cross-bridges form. Low tension. With no overlap at all, no tension.

Your skeleton helps here: bones and joints keep most muscles near their best length.

Think of it like this

A tug-of-war team on a rope. If the team bunches up so close that they trip over each other, they pull badly. If the rope is so far away that only two people can grab it, they pull badly. The best pull is when every person has a good grip.

Length–tension relationship: Tension depends on how much thick and thin filaments overlap at the start of a contraction.
Optimal length: The sarcomere length that gives maximum tension (about 80–120% of resting).
Zone of overlap: The part of the sarcomere where thin filaments lie alongside thick filaments.

All three fibers contract over and over. Watch which ones keep going.

Capillary Mitochondrion Glycogen granule Deep red = lots of myoglobin

Three types of skeletal muscle fibers

Fibers differ in two ways: how fast they contract (set by how fast their myosin ATPase splits ATP) and how they make ATP (oxidative = aerobic, with oxygen, in mitochondria; glycolytic = anaerobic glycolysis). Most muscles contain all three types, in different proportions.

	SO	FO	FG
Full name	Slow oxidative	Fast oxidative	Fast glycolytic
Contraction speed	Slow■□□	Fast■■■	Fast■■■
Myosin ATPase	Slow	Fast	Fast
Main ATP source	Aerobic respiration	Mostly aerobic, some glycolysis	Anaerobic glycolysis
Mitochondria	Many■■■	Many■■□	Few■□□
Capillaries	Many■■■	Many■■□	Few■□□
Myoglobin / color	High: red	Lower than SO: lighter, pinkish	Little: white (pale)
Glycogen	Low	Medium	High
Fatigue	Very resistant	Fairly resistant	Tires quickly
Fiber diameter / tension	Small / low	Medium / medium	Large / high
Used for	Posture (neck, back), long endurance like a marathon	Leg muscles in walking; moderate activity	Short, powerful bursts: lifting heavy objects, sprinting (e.g., arm muscles)

Think of it like this

SO fibers are diesel trucks: slow, steady, go all day. FG fibers are drag racers: huge power for a few seconds, then out of fuel. FO fibers are in between, like a sporty commuter car.

Sorting game · Which fiber type fits best?

Card 1 of 12Right on first try 0Streak 0

Read the card. Which fiber type does it describe, or which type does that activity rely on most? Pick one.