A skeletal muscle can be visualised as a series of nested tubes. At the outermost level, you have the whole muscle; as you move inwards, you reach bundles of muscle fibres (fascicles), then individual muscle fibres (muscle cells), and finally the myofibrils and sarcomeres inside each fibre.
Epimysium is the tough outer layer of connective tissue that surrounds the entire muscle.
It helps protect muscles from friction with other tissues and maintains their structural integrity.
Perimysium Beneath the epimysium, bundles (or fascicles) of muscle fibres are each wrapped by the perimysium.
Fascicles are groups of 10–100+ individual muscle fibres.
Endomysium
Each muscle fibre within a fascicle is then individually surrounded by the endomysium, a fine connective tissue layer that helps supply the muscle fibre with capillaries and nerves.
Muscle Fibres (Muscle Cells)
Skeletal muscle fibres are large, cylindrical cells.
Each fibre contains many nuclei (because they are formed from the fusion of multiple myoblasts) and is packed with contractile elements called myofibrils.
Myofibrils
Myofibrils are long strands of protein filaments.
Each myofibril is subdivided into repeating units called sarcomeres (the functional contractile units).
Sarcomeres
Sarcomeres are made of thick (myosin) and thin (actin) filaments.
The arrangement of these filaments gives muscle its striated (striped) appearance.
This is where the actual contractile “power stroke” takes place, fueled by ATP.
Important “Sarco-” Terms
Sarcolemma
The sarcolemma is the plasma membrane of a muscle fibre.
It is analogous to a cell membrane in other cell types, but it has special features like T-tubules (transverse tubules) that help propagate electrical signals into the cell’s interior.
Sarcoplasm
The sarcoplasm is the cytoplasm of a muscle fibre.
Unlike generic cell cytoplasm, it is rich in glycogen (stored form of glucose) and myoglobin (an oxygen-binding protein), both of which support energy production.
Sarcoplasmic Reticulum
The sarcoplasmic reticulum (SR) is a specialised form of endoplasmic reticulum that surrounds each myofibril.
The SR stores and releases calcium ions (Ca²⁺), which are essential for triggering muscle contraction.
Sarcomere
As noted above, the sarcomere is the fundamental contractile unit of muscle.
The boundaries of each sarcomere are marked by Z-lines. Within each sarcomere, the overlapping arrangement of actin (thin) and myosin (thick) filaments produces force when they slide across one another.
Sarcopenia vs. Sarcoplasm(ic) Hypertrophy
Sarcopenia
Definition: Age-related, involuntary loss of muscle mass, strength, and function.
Cause: Often due to a combination of reduced physical activity, hormonal changes, poor nutrition, and chronic inflammation.
Result: Decrease in muscle fibre size, particularly fast-twitch Type II fibres, and overall decline in muscle strength and function.
Sarcoplasmic Hypertrophy
Definition: An increase in the muscle glycogen storage, fluid, and non-contractile components within the sarcoplasm of the muscle fibre.
Cause: Often associated with higher-volume, moderate-to-high rep bodybuilding-style training.
Result: Muscle fibres can appear larger because of increased sarcoplasm volume, but this doesn’t necessarily equate to a proportional increase in “contractile” strength (that’s more related to myofibrillar hypertrophy).
Myofibrillar Hypertrophy
Contrast: Involves an increase in the number or size of the actin-myosin filaments within the sarcomeres, thus creating stronger contractions.
Cause: Typically linked to heavy, lower-rep, high-intensity resistance training.
Result: Greater density of contractile proteins leads to improved strength and power, sometimes with slightly less visible size gain compared to sarcoplasmic hypertrophy.
Cross-Sectional Changes with Different Stimuli
Strength/Power Training (e.g., heavy lifting)
Emphasises myofibrillar hypertrophy (growth of contractile proteins).
Muscle cross section sees a greater increase in density rather than primarily in sarcoplasm volume.
Some increase in sarcoplasm does occur but is secondary to contractile protein synthesis.
High-Rep Bodybuilding Training
Emphasises sarcoplasmic hypertrophy.
Muscle cross section often shows enlarged sarcoplasmic volume, more glycogen storage, and fluid retention around muscle fibres.
Gains in raw strength are present but not always proportional to the size gains.
Endurance Training
Less focus on muscle fibre diameter.
More emphasis on mitochondrial density, capillary networks, and overall oxidative capacity of the muscle (especially Type I fibres).
Muscle cross section doesn’t typically grow substantially; instead, it adapts for long-duration energy production.
Detraining and Sarcopenia
When training load is reduced significantly (or with ageing, if unaddressed), muscle fibre cross section can shrink (atrophy).
Sarcopenia involves a chronic decline in muscle cross-sectional area and functional capacity over time.
The Role of Mitochondria in the Cross Section
Mitochondria, though not “sarcoplasmic” structures in the strict sense, are found within the sarcoplasm and contribute heavily to muscle endurance and recovery.
In endurance-trained athletes, muscle cross sections often reveal increased mitochondrial density, improving oxidative (aerobic) metabolism.
In power or sprint-trained athletes, mitochondrial adaptations are present but to a lesser extent. The spotlight remains on increasing the contractile apparatus (actin and myosin filaments).
Practical Takeaways
Hypertrophy Goals
If you want larger muscles with a “fuller” look, incorporate higher-rep, moderate-intensity, and high-volume workouts (sarcoplasmic hypertrophy).
If you want stronger, denser muscles with maximal contractile force, focus on heavier weights with fewer repetitions (myofibrillar hypertrophy). In reality, many lifters use a blend of both.
Combating Sarcopenia
Resistance training is one of the best defences against sarcopenia.
Ensuring adequate protein intake, engaging in strength work, and maintaining an active lifestyle are crucial to preserving muscle cross-sectional area and functionality as you age.
Balance of Energy Systems
Including cardio or high-intensity interval training (HIIT) can support mitochondrial health, cardiovascular function, and overall metabolism without necessarily diminishing strength or size gains—provided volume is managed well.
Muscle cross section and its internal architecture revolve around a symphony of connective tissues, muscle fibres, myofibrils, and sarcomeres—all orchestrated by “sarco-” components like the sarcolemma, sarcoplasm, and sarcoplasmic reticulum. Understanding these structures clarifies why we can grow muscle in different ways (sarcoplasmic vs. myofibrillar hypertrophy), why sarcopenia occurs with age, and how targeted training approaches can preserve or enhance muscle mass. By tailoring your exercise routine—strength training, hypertrophy work, endurance training—you can optimise both the size and functional capacity of your skeletal muscles, no matter your age.
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