Describe the length–tension relationship of skeletal muscle and give its significance.

Definition

Types of Tension

The Relationship

• Active tension is maximum at the optimal length (L0), about 2.0–2.2 µm sarcomere length
• At L0 there is maximum overlap between actin and myosin → the greatest number of cross-bridges can form

• Passive tension, in contrast, rises steeply with stretch beyond L0 — due to titin and connective tissue

Significance

• In the body, muscles are held near their optimal length by their bony attachments
• Provides the cellular basis of Starling's law of the heart — ↑ end-diastolic volume stretches the fibres toward L0 → ↑ force of contraction
• In cardiac muscle the resting length is below optimal, so stretch improves performance — unlike skeletal muscle, which normally sits at L0

Force–velocity Relationship

• Velocity of shortening is inversely proportional to the load
• Zero load → velocity is maximum (Vmax)
• As load increases, velocity falls
• At maximum load the muscle cannot shorten at all → velocity = 0 → the contraction becomes isometric
• Vmax depends on myosin ATPase activity, not on load

Applied Aspects

• Over-distension of the heart (as in dilated cardiomyopathy or severe volume overload) stretches sarcomeres beyond L0 → force falls → descending limb of the Starling curve → decompensation
• Over-distension of the bladder or uterus similarly weakens contraction — a cause of postpartum haemorrhage in an overdistended uterus
• Surgical shortening or lengthening of a tendon alters the resting length and therefore the power of the muscle