Related Experiment Video
Updated: Jun 4, 2025

Ex Vivo Assessment of Contractility, Fatigability and Alternans in Isolated Skeletal Muscles
Published on: November 1, 2012
Muscle Contractility in Hypokalemic Periodic Paralysis
Sonja Holm-Yildiz1, Thomas Krag1, Tina Dysgaard1
1Copenhagen Neuromuscular Center, Department of Neurology, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.
Introduction/Aims:
Primary hypokalemic periodic paralysis (HypoPP) can present with periodic paralysis and/or permanent muscle weakness. Permanent weakness is accompanied by fat replacement of the muscle. It is unknown whether the permanent muscle weakness is solely due to fat replacement or if other factors affect the ability of the remaining muscle fibers to contract. We aimed to investigate muscle fat replacement and contractility in persons with HypoPP-causing variants in CACNA1S and to compare the results to healthy controls.
Methods:
In this cross-sectional study, we used T1-weighted and 2-point Dixon magnetic resonance imaging (MRI) to assess fat replacement of the muscle and stationary dynamometry to assess muscle strength. Contractility was determined by maximal muscle contraction divided by the contractile cross-sectional muscle area.
Results:
We included 45 persons with HypoPP-causing variants in CACNA1S and data from 37 healthy controls. We found that fat fraction was increased in ankle dorsiflexors and knee extensors and flexors, and further found that muscle strength was decreased in knee extensors and flexors in persons with HypoPP-causing variants in CACNA1S compared to healthy controls. Additionally, we found decreased contractility of thigh muscles in persons with HypoPP-causing variants in CACNA1S compared to healthy controls.
Discussion:
The decreased contractility could relate to skeletal muscle voltage-gated calcium channel dysfunction, subclinical attacks of paralysis, and/or changed muscle architecture, but this needs further investigation.
Related Concept Videos
Relaxation of Skeletal Muscles
When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated sodium channels. Sodium ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated calcium channels to open....
Motor Unit Stimulation
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...
Muscle Contraction
Cross-bridge Cycle
Muscle Stimulation Frequency
Wave summation
At low firing rates, motor neurons induce individual twitch contractions in muscle fibers. These twitches...
Specialized Characteristics of Cardiac Muscles
Cardiac muscle cells are smaller than skeletal muscles, averaging 10–20 mm in diameter and 50–100 mm in length. However, they have large energy demands for continuous contraction and relaxation. This energy is almost exclusively derived from aerobic metabolism of energy...

