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. 2021 Aug 10;120(17):3649–3663. doi: 10.1016/j.bpj.2021.07.023

Figure 1.

Figure 1

The “textbook” hierarchy in the anatomy of skeletal muscle. The overall muscle is characterized by its cross-sectional area (CSA), which contains a certain number (Nc) of muscle fibers (the muscle cells with multiple nuclei or multinucleate myocytes). A given muscle has a nearly fixed number of myocytes: between Nc ≈ 1000 for the tensor tympani and Nc > 1,000,000 for large muscles (gastrocnemius, temporalis, etc. (1)). Muscle cells contain a variable number (Nm) of parallel myofibrils (organelles), each of which can be divided into repeated mechanical elements called sarcomeres. The typical length of a sarcomere is ∼2 μm, so there are ∼105 of these elements in series along a fiber in a typical large muscle (2). Each sarcomere contains a number of parallel thick filaments (helical bundles of myosin, red) whose constituent myosins pull on the actin polymers in the thin filaments (F-actin, blue) to generate force. Within the myofibrils, the spacing between neighboring myosin filaments is ∼0.046 μm at rest (3,4). The typical cross-sectional area of a single muscle fiber substantially varies between individuals and muscle types but is of the order of 4000 μm2 (5). Accounting for some 15% of the cell volume being outside of the myofibrils (6), this means that a typical muscle fiber has ∼2,000,000 parallel filaments, between which the macroscopic force F must be divided. Rather than using this awkward number, we will express our results in terms of the total myofibrillar CSA within a single muscle fiber. A chemically activated muscle fiber with a CSA of 4000 μm2 shows a force in the vicinity of 300–1000 μN for untrained individuals (with a very large individual variation) (7), which translates to an average filament force of 150–500 pN (see Supporting materials and methods, Section A.6). Training can increase the neural activation level (8) as well as the number of active myosin heads and the maximal voluntary contraction force per filament (e.g., by stretch activation (9)). Because of this, we would expect resistance training to lead the filament forces to tend toward the upper end of the range (∼500 pN).