GLOSSARY

The Frank–Starling law states that the more the ventricle fills with blood before contraction (greater fibre stretch), the greater the force of contraction and therefore the stroke volume; put simply, “the heart pumps whatever it receives”.
The law is named after Otto Frank (German) and Ernest Starling (British).

Mechanism

• Optimal overlap: stretching the fibres brings actin–myosin overlap closer to the optimal length.
• Increased calcium sensitivity: stretch increases the sensitivity of the myofilaments to calcium; this mechanism is considered more important.
• Intrinsic nature: this response occurs without nerves or hormones.

Importance

• Matching the two ventricles: if the right ventricle pumps slightly more, left ventricular filling rises and its output rises too, preventing blood pooling in the lungs.
• Limitation: with excessive stretch (as in heart failure), contractile force no longer rises and may even fall.

Relevance to Sport

• During exercise: increased venous return from the muscle and respiratory pumps raises preload and, through this law, increases stroke volume.
• Endurance athletes: a larger, more compliant ventricle and greater blood volume allow greater use of this mechanism.
• Practical example: swimming in a horizontal position, with better venous return, increases stroke volume compared with running.