Monday Article #79: Muscle fiber types: More than just slow and fast-twitch!

The Olympic Games: the place where the best of the best from each nation gathers. From the powerful weightlifters to the explosive sprinters, to the consistent marathon runners. Why is it that they all have such diverse athletic prowess?

It turns out that aside from talent and training, this assortment of capabilities may stem from the intricate composition of our skeletal muscles. Deep within these powerhouses lie molecular differences that define the typology of muscle fibers.

Muscle fiber anatomy and physiology

To understand how muscle fibers are classified into different types, as well as the different phenotypes of the muscle fibers, it is valuable that we understand the basic overview of muscle fiber anatomy and physiology.

Zooming out, a muscle is wrapped in a connective tissue, the epimysium, and consists of many fascicles wrapped in a layer of perimysium. Within the fascicles are muscle fibers (muscle cells), wrapped in a layer of endomysium. Within these muscle fibers, are myofibrils which themselves are composed of functional units known as a sarcomere (Dave, Varacallo and Shook, 2018). 

Figure 1: Structure of muscle. Image taken from eCampusOntario Pressbook.

Within each sarcomere are the thick filaments (myosin), and thin filaments (mainly actin). The heavy chains contain the myosin heads that interact with actin and allow the muscle to contract (Riddle et al., 2019). The head region of the myosin heavy chain (MHC) is also the site for the ATPase enzyme that is responsible for hydrolyzing ATP into ADP and Pi, which provides the energy for contraction (Dulyaninova and Bresnick, 2013).

Figure 2: Thin filament and thick filament of the sarcomere. Image taken from the Medical gallery of Mikael Häggström 2014.

Figure 3: Myosin structure. Image taken from “Myofilament dysfunction in diastolic heart failure” (Anahita Aboonabi and McCauley, 2023).

The exact mechanism of muscle contraction needs not be known, but the jist of it is the myosin head will bind to actin in the presence of ATP and calcium ions., forming a cross-bridge. The myosin heads will then pull the actin, release, and reattach. This process is known as cross-bridge cycling (Krans, 2010). 

Figure 4: The sliding filament theory describes the mechanism of muscle contraction. Image taken from Online Biology Notes.

How are muscle fibers classified?

In human skeletal muscles, different muscle fiber types have different isoforms (essentially types) of ATPases. These can be identified by histochemical staining, which separates the fibers based on the stain