Learning Objective

By the end of this lesson, you should be able to interpret length–tension curves and predict how changes in muscle preload affect passive tension, active tension, total tension, and identify the optimal sarcomere length for maximal active force.

Length–Tension Curves

Length–tension curves are fundamental for understanding skeletal and cardiac muscle function. While the graphs shown are generated from skeletal muscle in vitro, the principles apply to both skeletal and cardiac muscle in vivo.

Passive Tension Curve

• The green line represents passive tension.
• Muscle behaves like a rubber band: stretching generates resistance due to elastic properties.
• Passive tension rises non-linearly with increased stretch.

Key points on the curve:

• Point A: No preload → no stretch → no passive tension.
• Point B: Preload of 1 g → slight stretch → ~1 g passive tension.
• Point C: Preload of 5 g → greater stretch → higher passive tension.

Active Tension

• The purple line shows tension generated by maximal isometric contraction at different preloads.
• Active tension comes from cross-bridge cycling between actin and myosin.
• The curve is bell-shaped, reflecting optimal overlap of actin and myosin filaments.

Key points on the curve:

• Preload A: Minimal stretch → actin filaments overlap → fewer cross-bridges → lower active tension (~2 g).
• Preload B (Optimal length, Lo): Ideal sarcomere length → maximal actin-myosin overlap → greatest active tension (~4 g).
• Preload C: Excessive stretch → reduced filament overlap → fewer cross-bridges → diminished active tension.
• Extreme stretch: Actin pulled past myosin → no cross-bridges → active tension = 0 (experimental scenario).

Total Tension

• Total tension = passive tension + active tension.
• As preload increases, total tension initially rises (due to increasing active tension), then plateaus or slightly falls if sarcomeres are overstretched.

Activity