Force-Length Relationship

Examples of Force-Length Relationship in the following topics:

• Force of Muscle Contraction

  • The force a muscle generates is dependent on its length and shortening velocity.
  • The force a muscle generates is dependent on the length of the muscle and its shortening velocity.
  • If this attachment was removed, for example if the bicep was detached from the scapula or radius, the muscle would shorten in length.
  • In mammals, there is a strong overlap between the optimum and actual resting length of sarcomeres.
  • The force-velocity relationship in muscle relates the speed at which a muscle changes length with the force of this contraction and the resultant power output (force x velocity = power).

• Velocity and Duration of Muscle Contraction

  • The shortening velocity affects the amount of force generated by a muscle.
  • The force-velocity relationship in muscle relates the speed at which a muscle changes length to the force of this contraction and the resultant power output (force x velocity = power).
  • The force generated by a muscle depends on the number of actin and myosin cross-bridges formed; a larger number of cross-bridges results in a larger amount of force.
  • Though they have high velocity, they begin resting before reaching peak force.
  • As velocity increases force and power produced is reduced.

• Types of Muscle Contractions: Isotonic and Isometric

  • Muscle contractions are defined by changes in the length of the muscle during contraction.
  • Isotonic contractions maintain constant tension in the muscle as the muscle changes length.
  • A concentric contraction is a type of muscle contraction in which the muscles shorten while generating force, overcoming resistance.
  • An eccentric contraction results in the elongation of a muscle while the muscle is still generating force; in effect, resistance is greater than force generated.
  • In contrast to isotonic contractions, isometric contractions generate force without changing the length of the muscle, common in the muscles of the hand and forearm responsible for grip.

• Male Sexual Response

  • Physical or psychological stimulation, or both, leads to vasodilation in and the increased blood flow into the three spongy areas that run along the length of the penis (the two corpora cavernosa and the corpus spongiosum).
  • The muscles of the pelvic floor, the ductus deferens (between the testes and the prostate), the seminal vesicles, and the prostate gland may begin to contract in a way that forces sperm and semen into the urethra inside the penis.
  • The relationship between erection and arousal is not one-to-one.

• Principles of Electricity

  • When a neuron is stimulated, an electrical impulse is generated and conducted along the length of its axon, which is called action potential which underlies many functional activities in the nervous system
  • A voltage may represent either a source of energy (electromotive force), or it may represent lost or stored energy (potential drop).

• Stability and Range of Motion at Synovial Joints

  • Tendons traditionally have been considered a mechanism by which muscles connect to bone, functioning to transmit forces.
  • This allows tendons to passively modulate forces during locomotion, providing additional stability with no active work.
  • Furthermore, because the tendon stretches, the muscle is able to function with less or even no change in length, allowing the muscle to generate greater force.

• Interactions of Skeletal Muscles

  • Skeletal muscle contractions can be grouped based on the length and frequency of contraction.
  • The largest motor units have as much as 50 times the contractile strength as the smaller ones; thus, as more and larger motor units are activated, the force of muscle contraction becomes progressively stronger.
  • A concept known as the size principle allows for a gradation of muscle force during weak contraction to occur in small steps, which become progressively larger as greater amounts of force are required.
  • For skeletal muscles, the force exerted by the muscle can be controlled by varying the frequency at which action potentials are sent to muscle fibers.
  • In a typical circumstance, when a human is exerting as much muscular force as they are consciously able, roughly one-third of the fibers in that muscle will be contracting at once.

• Spinal Reflexes

  • It is a monosynaptic reflex that provides automatic regulation of skeletal muscle length.
  • The reflex functions to maintain the muscle at a constant length.
  • The tendon reflex operates as a feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn.
  • The tendon organ is a stretch receptor that signals the amount of force on the muscle and protects the muscle from excessively heavy loads by causing the muscle to relax and drop the load.
  • This maneuver is used often when testing the patellar reflex, as it forces the patient to concentrate on the interlocking of the fingers and prevents conscious inhibition or influence of the reflex.

• Bulk Flow: Filtration and Reabsorption

  • The movement of materials across the capillary wall is dependent on pressure and is bi-directional depending on the net filtration pressure derived from the four Starling forces.
  • Modern evidence shows that in most cases venular blood pressure exceeds the opposing pressure, thus maintaining a positive outward force.
  • This indicates that capillaries are normally in a state of filtration along their entire length.

• Knee Injuries

  • The knee is particularly vulnerable to injury due to enormous twisting forces that it is subjected to especially during sports.
  • In sports that place great pressure on the knees, especially with twisting forces, it is common to tear one or more ligaments or cartilages.
  • Before the advent of arthroscopy and arthroscopic surgery, patients having surgery for a torn ACL required at least nine months of rehabilitation, having initially spent several weeks in a full-length plaster cast.
  • The injuries to these tendons occur when there is forceful contraction of the knee.