MSK – Tension

Master answer

Muscle length and tension

• during isometric stimulation the total tension can vary with muscle length (total tension)
• also when unstimulated, tension can vary with muscle length (passive tension)
• this relationship can be studied in a muscle preparation study
• muscle length can be varied by changing the distance between its two attachments -> passive tension measured.
• muscle then electrically stimulated -> total tension measured.
• the difference between the two values at any length is the amount of tension generated by the contractile process -> the active tension.

Passive tension curve = tension exerted by this skeletal muscle at each length when not stimulated.
Total tension curve = tension developed when the muscle contracts isometrically in response to a maximal stimulus.
Active tension = difference between the two.

• the length at which the active tension is maximal is usually = resting length
• when muscle fibers contract isometrically -> tension developed is proportionate to the number of cross-linkages between the actin & myosin molecules.
• when muscle stretched -> overlap reduced -> potential tension reduced
• when muscle shorter -> distance filaments have to move is reduced -> potential tension reduced.
• Optimal length = resting length

Frank-Starling Law

“energy of contraction is proportional to the initial length of the cardiac muscle fiber.”

• For the heart, the length of the muscle fibers (ie, the extent of the preload) is proportionate to the end- diastolic volume.
• Increased venous return increases the end-diastolic volume and therefore preload, which is the initial stretching of the cardiac myocytes prior to contraction.
• Myocyte stretching increases the sarcomere length, which causes an increase in force generation. Increasing the sarcomere length increases troponin C calcium sensitivity, which increases the rate of cross-bridge attachment and detachment, and the amount of tension developed by the muscle fiber. The effect of increased sarcomere length on the contractile proteins is termed length-dependent activation.
• This ability of the heart to change its force of contraction and therefore stroke volume in response to changes in venous return is called the Frank-Starling mechanism.
• The mechanism is very important for:
  • Rapidly responding to acute changes in venous return
  • Keeping the right and left ventricle outputs exactly equal

Frank-Starling curve: Relationship between ventricular stroke volume and end-diastolic volume

JC 2019

Answer by JC