There are two different types of muscular hypertrophy: sarcoplasmic and myofibrillar. During sarcoplasmic hypertrophy, the volume of sarcoplasmic fluid in the muscle cell increases with no accompanying increase in muscular strength. During myofibrillar hypertrophy, actin and myosin contractile proteins increase in number and add to muscular strength as well as a small increase in the size of the muscle. Sarcoplasmic hypertrophy is characteristic of the muscles of bodybuilders while myofibrillar hypertrophy is characteristic of weightlifters.[1]

[1] Kraemer, William J.; Zatsiorsky, Vladimir M. (2006). Science and practice of strength training. Champaign, IL: Human Kinetics. pp. 50. ISBN 0-7360-5628-9.

The sarcoplasmic reticulum (SR), from the Greek sarx, ("flesh"), is a special type of smooth ER found in smooth and striated muscle. The only structural difference between this organelle and the SER is the medley of proteins they have, both bound to their membranes and drifting within the confines of their lumens. This fundamental difference is indicative of their functions: the SER synthesizes molecules while the SR stores and pumps calcium ions. The SR contains large stores of calcium, which it sequesters and then releases when the muscle cell is stimulated.[8] The SR's release of calcium upon electrical stimulation of the cell plays a major role in excitation-contraction coupling.

Excitation-contraction (EC) coupling is a term coined in 1952 to describe the physiological process of converting an electrical stimulus to a mechanical response [1]. This process is fundamental to muscle physiology, whereby the electrical stimulus is usually an action potential and the mechanical response is contraction. EC coupling can be dysregulated in many disease conditions.

Though EC coupling has been known for over half a century, it is still an active area of biomedical research. The general scheme is that an action potential arrives to depolarize the cell membrane. By mechanisms specific to the muscle type, this depolarization results in an increase in cytosolic calcium that is called a calcium transient. This increase in calcium activates calcium-sensitive contractile proteins that then use ATP to cause cell shortening.


sarcoplasmic hypertrophy

sarcoplasmic fluid:

As the muscle continues to receive increased demands, the synthetic machinery is upregulated. Although all the steps are not yet clear, this upregulation appears to begin with the ubiquitous second messenger system (including phospholipases, protein kinase C, tyrosine kinase, and others).[citation needed] These, in turn, activate the family of immediate-early genes, including c-fos, c-jun and myc. These genes appear to dictate the contractile protein gene response.[citation needed]


  • c-fos
  • c-jun
  • myc

  • phospholipases

  • protein kinase C
  • tyrosine kinase

Ultimately the message filters down to alter the pattern of protein expression. The additional contractile proteins appear to be incorporated into existing myofibrils (the chains of sarcomeres within a muscle cell). There appears to be some limit to how large a myofibril can become: at some point, they split. These events appear to occur within each muscle fiber. That is, hypertrophy results primarily from the growth of each muscle cell, rather than an increase in the number of cells. Skeletal muscle cells are however unique in the body in that they can contain multiple nuclei, and the number of nuclei can increase.

Cortisol decreases amino acid uptake by muscle tissue, and inhibits protein synthesis.[2]

A small study performed on young and elderly found that ingestion of 340 grams of lean beef (90g protein) did not increase muscle protein synthesis any more than ingestion of 113 grams of lean beef (30g protein). In both groups, muscle protein synthesis in both groups increased by 50%. The study concluded that more than 30g protein in a single meal did not further enhance the stimulation of muscle protein synthesis in young and elderly.[3]

For persons training heavy, a high protein intake is common, usually in smaller meals every 2-3 hours. Some aim to consume at least 2 grams of protein per pound (0.45 kg) of bodyweight.[citation needed] Others[who?] advise consuming closer to 1.5 grams of protein per kilogram (2.2 lbs) of bodyweight.[4]

Mechanism of work-induced hypertrophy of skeletal muscle

The increase in muscle weight reflects an increase in protein, especially sarcoplasmic protein, and results from greater protein synthesis and reduced protein breakdown. Within several hours after operation, the hypertrophying soleus shows more rapid uptake of certain amino acids and synthesis of phosphatidyl-inositol. By 8 hours, protein synthesis is enhanced. RNA synthesis also increases, and hypertrophy can be prevented with actinomycin D. Nuclear DNA synthesis also increases on the second day after operation and leads to a greater DNA content. The significance of the increased RNA and DNA synthesis is not clear, since most of it occurs in interstitial and satellite cells. The proliferation of the non-muscle cells seems linked to the growth of the muscle fibers; in addition, factors causing muscle atrophy (e.g. denervation) decrease DNA synthesis by such cells.

meh Acute effects of resistance exercise on muscle protein synthesis rate in young and elderly men and women


The relationships among IGF-1, DNA content, and protein accumulation during skeletal muscle hypertrophy

Calcium-dependent regulation of protein synthesis and degradation in muscle

MUSCLE proteins undergo continuous intracellular turnover as do proteins in other cells1,2. Furthermore, hormones, nutrients and work load can alter rates of protein synthesis and degradation in muscle, resulting in growth or atrophy of these tissues1,2. In hereditary muscular dystrophies, where there is prominent wasting of the affected tissues, rates of both total protein synthesis and degradation are elevated3−10. The immediate cause of this muscle atrophy is the imbalance resulting from an increase in protein degradation which exceeds a smaller enhancement in average protein synthesis. No simple explanation based on a defect in the response of dystrophic cells to known hormonal or nutritional factors has satisfactorily explained the elevation in the rates of both protein synthesis and degradation. We have now investigated the possible role of increased cellular Ca2+ as a mediator of such changes in protein metabolism based on other known structural and biochemical alterations in dystrophic muscles11,12. Lesion(s) involving membranes in muscle as well as other cells occur in hereditary dystrophies, including the main human form, Duchenne dystrophy11,12. One characteristic of the dystrophic plasma membrane seems to be an increased permeability to the high concentrations of Ca2+ normally present in extracellular fluid11,12. In addition, studies have suggested a decreased ability of sarcoplasmic reticulum to sequester Ca2+ in dystrophic muscles13,14. Thus, it is possible that increased Ca2+ might be responsible for the stimulation of both protein synthesis and degradation which occurs in these muscles. To test this idea, we have experimentally increased the uptake of external Ca2+ into rat muscles by using the divalent cation ionophore, A23187. The ability of this ionophore to increase the transport of Ca2+ across membranes has resulted in its application as a widely used tool for the study of many Ca2+-dependent cellular processes15. The experiments reported here demonstrate that increased movement of Ca2+ into muscle can produce effects which closely resemble dystrophic muscle and that the increased net catabolism can be reversed by certain factors.

Muscle hypertrophy in bodybuilders

Muscle biopsy samples were obtained from m. vastus lateralis and m. deltoideus of three high caliber bodybuilders. Tissue specimens were analysed with respect to relative distribution of fast twitch (FT) and slow twitch (ST) fiber types and different indices of fiber area. In comparison to a reference group of competitive power/weight-lifters the following tendencies were observed: the percentage of FT fibers was less, mean fiber area was smaller and selective FT fiber hypertrophy was not evident. Values for fiber type composition and fiber size were more similar to values reported for physical education students and non-strength trained individuals. The results suggest that weight training induced muscle hypertrophy may be regulated by different mechanisms depending upon the volume and intensity of exercise.

  • Contrasts in muscle and myofibers of elite male and female bodybuilders

  • Exercise induced increases in muscle fiber number

  • A weight-lifting exercise model for inducing hypertrophy in the hindlimb muscles of rats

"inducing hypertrophy"

Myosin heavy chain composition of single fibres from m. biceps brachii of male body builders Skeletal muscle hypertrophy and structure and function of skeletal muscle fibres in male body builders Morphological and biochemical evidence of muscle hyperplasia following weight-lifting exercise in rats Morphological observations supporting muscle fiber hyperplasia following weight-lifting exercise in cats The effect of resistive exercise rest interval on hormonal response, strength, and hypertrophy with training

Skeletal muscle hypertrophy and structure and function of skeletal muscle fibres in male body builders

Needle biopsy samples were taken from vastus lateralis muscle (VL) of five male body builders (BB, age 27.4 ± 0.93 years; mean ±s.e.m.), who had being performing hypertrophic heavy resistance exercise (HHRE) for at least 2 years, and from five male active, but untrained control subjects (CTRL, age 29.9 ± 2.01 years). The following determinations were performed: anatomical cross-sectional area and volume of the quadriceps and VL muscles in vivo by magnetic resonance imaging (MRI); myosin heavy chain isoform (MHC) distribution of the whole biopsy samples by SDS-PAGE; cross-sectional area (CSA), force (Po), specific force (Po/CSA) and maximum shortening velocity (Vo) of a large population (n= 524) of single skinned muscle fibres classified on the basis of MHC isoform composition by SDS-PAGE; actin sliding velocity (Vf) on pure myosin isoforms by in vitro motility assays. In BB a preferential hypertrophy of fast and especially type 2X fibres was observed. The very large hypertrophy of VL in vivo could not be fully accounted for by single muscle fibre hypertrophy. CSA of VL in vivo was, in fact, 54% larger in BB than in CTRL, whereas mean fibre area was only 14% larger in BB than in CTRL. MHC isoform distribution was shifted towards 2X fibres in BB. Po/CSA was significantly lower in type 1 fibres from BB than in type 1 fibres from CTRL whereas both type 2A and type 2X fibres were significantly stronger in BB than in CTRL. Vo of type 1 fibres and Vf of myosin 1 were significantly lower in BB than in CTRL, whereas no difference was observed among fast fibres and myosin 2A. The findings indicate that skeletal muscle of BB was markedly adapted to HHRE through extreme hypertrophy, a shift towards the stronger and more powerful fibre types and an increase in specific force of muscle fibres. Such adaptations could not be fully accounted for by well known mechanisms of muscle plasticity, i.e. by the hypertrophy of single muscle fibre (quantitative mechanism) and by a regulation of contractile properties of muscle fibres based on MHC isoform content (qualitative mechanism). Two BB subjects took anabolic steroids and three BB subjects did not. The former BB differed from the latter BB mostly for the size of their muscles and muscle fibres.

single muscle fiber hypertrophy

Effect of swimming on myostatin expression in white and red gastrocnemius muscle and in cardiac muscle of rats

Inducible activation of Akt increases skeletal muscle mass and force without satellite cell activation

A better understanding of the signaling pathways that control muscle growth is required to identify appropriate countermeasures to prevent or reverse the loss of muscle mass and force induced by aging, disuse, or neuromuscular diseases. However, two major issues in this field have not yet been fully addressed. The first concerns the pathways involved in leading to physiological changes in muscle size. Muscle hypertrophy based on perturbations of specific signaling pathways is either characterized by impaired force generation, e.g., myostatin knockout, or incompletely studied from the physiological point of view, e.g., IGF-1 overexpression. A second issue is whether satellite cell proliferation and incorporation into growing muscle fibers is required for a functional hypertrophy. To address these issues, we used an inducible transgenic model of muscle hypertrophy by short-term Akt activation in adult skeletal muscle. In this model, Akt activation for 3 wk was followed by marked hypertrophy (~50% of muscle mass) and by increased force generation, as determined in vivo by ankle plantar flexor stimulation, ex vivo in intact isolated diaphragm strips, and in single-skinned muscle fibers. No changes in fiber-type distribution and resistance to fatigue were detectable. Bromodeoxyuridine incorporation experiments showed that Akt-dependent muscle hypertrophy was accompanied by proliferation of interstitial cells but not by satellite cell activation and new myonuclei incorporation, pointing to an increase in myonuclear domain size. We can conclude that during a fast hypertrophic growth myonuclear domain can increase without compromising muscle performance.—Blaauw, B., Canato, M., Agatea, L., Toniolo, L., Mammucari, C., Masiero, E., Abraham, R., Sandri, M., Schiaffino, S., Reggiani, C. Inducible activation of Akt increases skeletal muscle mass and force without satellite cell activation.

increase in myonuclear domain size

'nuclear domain'

"We can conclude that during a fast hypertrophic growth myonuclear domain can increase without compromising muscle performance."

"cytoplasmic volume"

Compensatory hypertrophy of the plantaris muscle in relation to age

RJ Tomanek, YK Woo - The Journal of Gerontology, 1970 -

... In con- trast to these results an increase in actomyosin concentration without a change in sarcoplasm was reported by Helander (1961). It has been concluded that muscle fiber hypertrophy may result from either an increase in myofibrilar or sarcoplasmic protein, depending ... Cited by 22 - Related articles - All 2 versions

Enzymatic changes in hypertrophied fast-twitch skeletal muscle

Changes in the protein composition of muscles of the rat in hypertrophy and atrophy

Hume (1952) repeated the work and confirmed the increase in total protein but found no change in the relative amounts of sarcoplasmic, contractile and connective tissue proteins. (in this study however, "Changes in the protein composition.." they did find a change in sarcoplasmic protein levels)

Sarcoplasmic Hypertrophy Muscle fibers adapt to high volume training by increasing the number of mitochondria (organelles in the cell that are involved in ATP production) in the cell. This type of training also leads to the elevation of enzymes that are involved in glycolytic and oxidative pathways. The volume of sarcoplasmic fluid inside the cell and between the cells is increased with high volume training. This type of training contributes little to maximal strength while it does increase strength endurance due to mitochondria hypertrophy. Growth of connective tissue is also present with sarcoplasmic hypertrophy.

"sarcoplasmic hypertrophy is referred to as non-functional muscle"

"Sarcoplasmic hypertrophy is an increase in the volume of the non-contractile muscle cell fluid, sarcoplasm. This fluid accounts for 25-30% of the muscle?s size. Although the cross sectional area of the muscle increases, the density of muscle fibers per unit area decreases, and there is no increase in muscular strength . This type of hypertrophy is mainly a result of high rep, ?bodybuilder-type? training" - Joe De Franco

"The sarcoplasm is soft tissue that surrounds the muscle fiber, ..."

"non-contractile proteins"

"satellite cells" <--- but the sarcoplasmic reticulum is not a cell..

"Research from Russia even suggests that there are two different types of muscle hypertrophy: sarcomere hypertrophy (of the actual contractile components) and sarcoplasmic hypertrophy (of non-contractile proteins and semifluid plasma between the muscle fibres), with the latter type of hypertrophy being more in evidence in bodybuilding (Siff M C "Supertraining", 2000, Ch 1.13)."

""" This might suggest that all muscle fibre hypertrophy lowers work capacity. Hypertrophy is an adaptive response to physical stress and does offer the benefit of increased mitochondrial surface area, which provides for more efficient energy processes than would an increased number of mitochondria. With a rapid increase in loading, the size of the mitochondria continues to increase markedly, but their number decreases and the concentration of ATP drops, thereby diminishing the partial volume of the contractile myofibrils.

The resulting energy deficit soon inhibits the formation of new structures and the decreased amount of ATP stimulates various destructive processes associated with decrease in the number of myofibrils. This process is referred to as irrational adaptation.

Growth of any living structure is related to the balance between its volume and its surface area. When muscle hypertrophy occurs, the surface of the fibres grows more slowly than their volume and, this imbalance causes the fibres to disintegrate and restructure in a way which preserves their original metabolic state (Nikituk & Samoilov, 1990). """

structure of a skeletal muscle

"Every organelle and macromolecule of a muscle fiber are arranged to ensure form meets function. The plasma membrane is called the sarcolemma with the cytoplasm known as the sarcoplasm. In the sarcoplasm are the myofibrils. The myofibrils are long protein bundles about 1 micrometer in diameter each containing myofilaments. Pressed against the inside of the sarcolemma are the unusual flattened nuclei. Between the myofibrils are the mitochondria. While the muscle fiber does not have a smooth endoplasmic reticulum it contains a sarcoplasmic reticulum. The sarcoplasmic reticulum surrounds the myofibrils and holds a reserve of the calcium ions needed to cause a muscle contraction. Periodically it has dilated end sacs known as terminal cisternae. These cross the muscle fiber from one side to the other. In between two terminal cisternae is a tubular infoldings called a transverse tubule (T tubule). The T tubule are the pathway for the action potential to signal the sarcoplasmic reticulum to release calcium causing a muscle contraction. Together two terminal cisternae and a transverse tubule form a triad. [1]" "Once a cell is sufficiently stimulated, the cell's sarcoplasmic reticulum releases ionic calcium (Ca2+), which then interacts with the regulatory protein troponin. Calcium-bound troponin undergoes a conformational change that leads to the movement of tropomyosin, subsequently exposing the myosin-binding sites on actin. This allows for myosin and actin ATP-dependent cross-bridge cycling and shortening of the muscle."

Lack of myostatin results in excessive muscle growth but impaired force generation

""" What is the Sarcoplasm?

Within the sarcoplasm there are soluble (or aqueous) components (making up 80 percent of it); composed of ions and soluble macromolecules like enzymes, carbohydrates, different salts and proteins, as well as a great proportion of RNA. This watery component can be more or less gel-like or liquid depending on the condition and the activity phases of the cell. In general, margin regions of the cell are gel-like and the cell's interior is liquid.

The insoluble constituents of the sarcoplasm are organelles (such as the mitochondria, the chloroplast, lysosomes, peroxysomes, ribosomes), several vacuoles, cytoskeletons as well as complex membrane structures (e.g. sarcoplasmic reticulum).

The muscle protein fraction that makes up the cytoplasm (sarcoplasm in a muscle cell) is made up of mostly enzymes participating in cell metabolism, such as the anaerobic energy conversion from glycogen to ATP, intracellular transport, and several other enzyme functions. This fraction adds up to about 25 or 30% of the total muscle protein versus the larger and more talked about structural protein (myofibril protein) makes up about 40%. """

decrease in count of mitochondria in muscles undergoing 6mo weight training program

""" MACDOUGALL, J. D., D. G. SALE, J. R. MOROZ, G. C. B. ELDER, J. R. SUTTON and H. HOWALD. Mitochondrial volume density in human skeletal muscle following heavy resistance training. Med Sci. Sports. Vol. 11, No. 2, pp. 164-166, 1979. Needle biopsies were taken from triceps brachii of 6 healthy males before and after a 6 month intensive weight training programme. The tissue was sectioned, photographed under a Philips EM200 and subjected to stereological analysis. Cross sectional fibre areas were also calculated from cryostat sections stained for ATPase activity. Morphometric analysis indicated that training resulted in a significant 26% reduction in mitochondrial volume density and a 25% reduction in the mitochondrial volume to myofibrillar volume ratio. These changes were accompanied by significant increases in fibre area for both FT (33%) and ST (27%) fibres as determined from the light microscope. There was a significant correlation between the reduction in mitochondrial volume density and the increase in FT fibre area following training (r = 0.845). It was concluded that heavy resistance training leads to a dilution of the mitochondrial volume density through an increase in myofibrillar size with hypertrophy. """

"Viral mediated expression of insulin-like growth factor I blocks the aging-related loss of skeletal muscle function"

Contractile and cytoskeletal proteins in smooth muscle during hypertrophy and its reversal

Hypertrophy of rat urinary bladder smooth muscle was induced by partial urethral obstruction. Bladder weight increased from 70 to 240 mg after 10 days and to 700 mg after 7 wk. Removal of the obstruction after 10 days caused a regression of bladder weight to 130 mg. The relative volume of smooth muscle in the bladder wall increased during hypertrophy. The concentration of myosin in the smooth muscle cells decreased in 10-day hypertrophied bladders, whereas the concentration of actin was unchanged. The actin-myosin ratio was 2.3 in controls, 3.3 in 10-day obstructed bladders, and 2.9 in 7-wk obstructed bladders. After removal of obstruction, the ratio was normalized. Two isoforms of myosin heavy chains were identified (SM1 and SM2). The relative amount of SM2 decreased during hypertrophy. The relative proportion of actin isoforms (alpha, beta, and gamma) was altered toward more gamma and less alpha. These changes were reversible upon removal of the obstruction. Desmin was the dominating intermediate filament protein. The concentration of desmin and filamin increased in the hypertrophic bladders. The increased desmin-actin and filamin-actin ratios in obstructed bladders were normalized after removal of the obstruction. The results suggest that the turnover of contractile and cytoskeletal proteins is fast and can be regulated in response to changes in the functional demands in smooth muscle.