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Related Concept Videos

Motor Unit Stimulation01:20

Motor Unit Stimulation

When the neuron of a motor unit fires an action potential, it triggers a series of events, leading to a twitch contraction in the muscle fibers. The process of excitation-contraction coupling is crucial in relaying the action potential to the muscle fibers.
The latent period of contraction marks the onset of excitation-contraction coupling, when the action potential propagates across the sarcolemma, preparing the muscle fibers for contraction. As the fibers enter the contraction phase, the...
Excitation-Contraction Coupling in Skeletal Muscles01:20

Excitation-Contraction Coupling in Skeletal Muscles

Excitation-contraction coupling is a series of events that occur between generating an action potential and initiating a muscle contraction. It occurs at the triad, a structure found in skeletal muscle fibers that comprise a T-tubule and terminal cisternae of the sarcoplasmic reticulum on each side. These triads are visible in longitudinally sectioned muscle fibers. They are typically located at the A-I junction — the junction between the A and I bands of the sarcomere.
When an action potential...
Energy Supply for Muscle Contraction01:25

Energy Supply for Muscle Contraction

Skeletal muscle fibers have the unique ability to switch between rest and contraction states, using different sources of ATP for energy. The contraction cycle and Ca2+ transport back into the sarcoplasmic reticulum for relaxation require significant ATP. However, the ATP reserves in muscle fibers are limited and can only sustain contractions for a few seconds. Additional ATP production becomes necessary for prolonged contractions. As a result, muscle fibers generate ATP through various sources,...
ATP Driven Pumps III: V-type Pumps01:30

ATP Driven Pumps III: V-type Pumps

V-type pumps are ATP-driven pumps found in the vacuolar membranes of plants, yeast, endosomal and lysosomal membranes of animal cells, plasma membranes of a few specialized eukaryotic cells, and some prokaryotes. They are also known as the V1Vo-ATPase, that couple ATP hydrolysis to transport protons against a concentration gradient.
The peripheral or cytosolic V1 domain with eight subunits is involved in ATP hydrolysis. The integral or transmembrane V0 domain containing at least five subunits...
Actin and Myosin in Muscle Contraction01:16

Actin and Myosin in Muscle Contraction

Actin and myosin are contractile proteins that form the sarcomere found in skeletal muscle tissues for regulating muscle contraction. Actin, a globular contractile protein, interacts with myosin for muscle contraction. The skeletal tissue appears striped or striated under a microscope due to the repeated arrangement of contractile proteins actin and myosin along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes...
Fascicle Arrangement in Skeletal Muscles01:25

Fascicle Arrangement in Skeletal Muscles

Fascicles are bundles of muscle fibers in a skeletal muscle. Muscle fascicle arrangement is directly associated with the power and range of motion of various muscles. The configuration of these fascicles can vary, leading to different functional outcomes.
The four primary types of muscle based on fascicle arrangement are:

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Related Experiment Video

Updated: Jul 7, 2026

Construction of Constant-Load (Isotonic) and Constant-Velocity (Isokinetic) Torque-Velocity-Power Profiles In vivo for the Rat Plantar Flexors
07:44

Construction of Constant-Load (Isotonic) and Constant-Velocity (Isokinetic) Torque-Velocity-Power Profiles In vivo for the Rat Plantar Flexors

Published on: October 3, 2025

Variable gearing in pennate muscles.

Emanuel Azizi1, Elizabeth L Brainerd, Thomas J Roberts

  • 1Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, USA. manny_azizi@brown.edu

Proceedings of the National Academy of Sciences of the United States of America
|January 31, 2008
PubMed
Summary
This summary is machine-generated.

Pennate muscles automatically adjust their gear ratio based on contraction force. Dynamic shape changes allow muscles to favor velocity at low loads and force at high loads.

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Area of Science:

  • Biomechanics
  • Muscle Physiology

Background:

  • Muscle fiber architecture significantly influences mechanical function.
  • In pennate muscles, fiber orientation and rotation affect force and velocity output.
  • Fiber rotation alters the gear ratio, impacting muscle performance.

Purpose of the Study:

  • To investigate if muscle gear ratio is fixed or varies with contraction force.
  • To determine the role of dynamic muscle shape changes in modulating fiber rotation and gear ratio.
  • To understand how pennate muscles adapt their performance for diverse mechanical functions.

Main Methods:

  • Analysis of muscle fiber architecture and its relationship to mechanical output.
  • Quantification of fiber rotation and gear ratio during contractions of varying force.
  • Modeling of dynamic muscle shape changes in orthogonal dimensions.

Main Results:

  • Muscle gear ratio is not fixed and significantly decreases with increasing contraction force.
  • Dynamic muscle shape changes promote fiber rotation at low forces and resist it at high forces.
  • Gearing automatically adjusts with load, optimizing for velocity at low loads and force at high loads.

Conclusions:

  • Pennate muscle shape changes function as an automatic transmission system.
  • Muscles shift from high gear (velocity) at low loads to low gear (force) at high loads.
  • Variable gearing provides a mechanism for modulating muscle performance across different mechanical demands.