Resistance exercise research indicates acute alterations of intra- and extracellular water balance, the extent of which is dependent on the type of exercise and intensity of training(1).
These changes in fluid balance occur during
intense muscular contractions when the veins taking blood out of working muscles are compressed, while arteries continue to deliver blood into the working muscles, thus creating an increased concentration of intramuscular blood plasma. There is an enhanced reperfusion of blood that occurs and culminates in a phenomenon commonly referred to by sports scientists as “cellular swelling” and by bodybuilders as “the pump,” whereby muscles become engorged with blood(2).
The pump is amplified by resistance exercise that relies heavily on anaerobic glycolysis, essentially “bodybuilding-style training” that involves moderate to higher repetitions with limited rest intervals(3).
This style of training results in a substantial accumulation of metabolic byproducts including lactate and inorganic phosphate, which in turn function as osmolytes and thereby draw additional fluid into the cell(4).
The pump is generally thought to be a transient phenomenon. Bodybuilders “pump up” by performing high repetition sets immediately before a competition in an effort to make their muscles appear full and dense while on stage. Although these short-term effects of the pump are well documented, recent research suggests that the pump may, in fact, mediate long-term adaptive responses.
Heavy Loads and Muscle Growth
Muscle hypertrophy represents the dynamic balance between protein synthesis and breakdown. Three primary factors have been postulated to mediate hypertrophic adaptations pursuant to resistance training: mechanical tension, metabolic stress, and muscle damage(5).
There is compelling evidence that
mechanical tension is the primary impetus for this adaptive response.
Tension on muscles initiates a phenomenon called mechanotransduction whereby sarcolemmal-bound mechanosensors, such as integrins and focal adhesions, convert mechanical energy into chemical signals that mediate various intracellular anabolic and catabolic pathways in a manner that shifts muscle protein balance to favor synthesis over degradation(6).
Heavy loads are an effective means for increasing muscle growth as noted by the importance of mechanical tension in promoting anabolism. The use of higher intensities places greater tension on muscles, thus stimulating greater mechanotransduction.
However, other factors are purported to play a role in post-exercise muscle protein accretion. There is compelling evidence that exercise-induced metabolic stress can mediate a hypertrophic response, and cell swelling is believed to be an important component to this process(3).
Probable Hypertrophic Mechanisms of Cell Swelling
The pump represents an increase in intracellular hydration that causes the muscle fiber to swell. Evidence indicates that cell swelling acts as a physiological regulator of cell function(7), stimulating protein accretion by both increasing protein synthesis and decreasing protein breakdown(8).
In muscle, fast-twitch fibers have been found to be particularly sensitive to osmotic changes, presumably related to their high concentration of water transport channels called aquaporin-4. Aquaporin-4 is strongly expressed in the sarcolemma of mammalian fast-twitch glycolytic and fast-twitch oxidative-glycolytic fibers, facilitating the entry of plasma into the cell(4).
Numerous studies indicate that fast-twitch fibers display a superior potential for growth as compared with slow-twitch fibers(9), suggesting that cell swelling may promote hypertrophy by favorably impacting
net protein balance in these fibers.
Although the underlying mechanisms remain to be fully elucidated, it has been hypothesized that cell swelling-induced anabolism is a means of cell survival. It is theorized that an increased pressure against the cytoskeleton and/or cell membrane is perceived as a threat to cellular integrity, thereby initiating an intracellular signaling response that promotes reinforcement of its ultrastructure(5).
The signaling response is believed to be facilitated by integrin-associated volume osmosensors within muscle fibers(10).
When the membrane is subjected to swelling-induced stretch, these sensors initiate activation of anabolic protein-kinase transduction pathways.
Hyperhydration also may have a direct effect on amino acid transport systems.
It is also thought that cellular swelling may enhance hypertrophic adaptations through increased satellite cell activity(11).
Satellite cells are muscle stem cells that reside between the basal lamina and sarcolemma. While resting, these precursor cells remain inactive. During skeletal muscle overload, satellite cells enter the cell cycle and initiate muscular repair by first undergoing proliferation and then differentiating into myoblast-like cells(12).
Once differentiated, myoblasts are then able to fuse to traumatized myofibers and donate their nuclei to enhance the cell’s ability to synthesize new contractile proteins. Research investigating the myogenic properties of creatine monohydrate, an osmolyte, show an explicit impact on satellite cell accretion and differentiation, as well as myogenic regulatory factor expression(13). Research indicates that the osmolytic properties of creatine monohydrate may instigate proliferation of satellite cells and facilitate their fusion to hypertrophying myofibers(11).
Practical Applications
When looking to achieve a pump, there are essentially two different sets, repetitions, and timing schemes that can conceivably be used interchangeably to maximize the pump.
- – Utilize several high repetition sets combined with short rest periods (e.g., 2–3 sets of ∼20 repetitions with 60 seconds of rest in between sets)
- – Utilize repeated medium repetition sets combined with short rest periods (e.g., 5–10 sets of 8–12 repetitions with 30 seconds of rest in between sets).
Another alternative for enhancing the pump is to perform a drop set (e.g., a high intensity set is immediately followed by a lower intensity bout with the load decreased by ∼25–50%).
This type of training results in significant metabolite accumulation and enhances cellular hydration.
Choose exercises that maintain constant tension (e.g., cable curl) to maximize the pump. In addition, perform exercises in a continuous manner so that the target muscles are not allowed to relax. It is vital to maintain continuous tension on the working muscles if the goal is to maximize cellular swelling.
Summary
I recommend a combination of low-to-medium repetition ranges with “pump-type” training. Heavy loads maximize muscle activation, and progressive overload ensures that muscles receive increased mechanical tension over time. Therefore, increasing strength on heavy multi-joint movements should be
the foundation of long-term hypertrophy training.
Also, incorporate exercises and training styles that achieve a “pump” as mentioned above. This training stimulus provides a potent hypertrophic stimulus that is synergistic to heavy compound lifting. I would recommend dedicating a component of your training sessions toward “pump” training, ideally after heavier strength work, to take advantage of the multiple pathways involved in muscle hypertrophy.
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References:
1. Sjøgaard G: Water and electrolyte fluxes during exercise and their relation to muscle fatigue. Acta Physiol Scand Suppl 556:129-36, 1986
2. Schoenfeld BJ, Contreras B: The Muscle Pump: Potential Mechanisms and Applications for Enhancing Hypertrophic Adaptations. Strength & Conditioning Journal 36:21-25, 2014
3. Schoenfeld BJ: Potential mechanisms for a role of metabolic stress in hypertrophic adaptations to resistance training. Sports Med 43:179-94, 2013
4. Frigeri A, Nicchia GP, Verbavatz JM, et al: Expression of aquaporin-4 in fast-twitch fibers of mammalian skeletal muscle. J Clin Invest 102:695-703, 1998
5. Schoenfeld BJ: The mechanisms of muscle hypertrophy and their application to resistance training. J Strength Cond Res 24:2857-72, 2010
6. Zou K, Meador BM, Johnson B, et al: The α₇β₁-integrin increases muscle hypertrophy following multiple bouts of eccentric exercise. J Appl Physiol (1985) 111:1134-41, 2011
7. Häussinger D: The role of cellular hydration in the regulation of cell function. Biochem J 313 ( Pt 3):697-710, 1996
8. Grant AC, Gow IF, Zammit VA, et al: Regulation of protein synthesis in lactating rat mammary tissue by cell volume. Biochim Biophys Acta 1475:39-46, 2000
9. Adams GR, Bamman MM: Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy. Compr Physiol 2:2829-70, 2012
10. Low SY, Rennie MJ, Taylor PM: Signaling elements involved in amino acid transport responses to altered muscle cell volume. Faseb j 11:1111-7, 1997
11. Dangott B, Schultz E, Mozdziak PE: Dietary creatine monohydrate supplementation increases satellite cell mitotic activity during compensatory hypertrophy. Int J Sports Med 21:13-6, 2000
12. Philippou A, Halapas A, Maridaki M, et al: Type I insulin-like growth factor receptor signaling in skeletal muscle regeneration and hypertrophy. J Musculoskelet Neuronal Interact 7:208-18, 2007
13. Willoughby DS, Rosene JM: Effects of oral creatine and resistance training on myogenic regulatory factor expression. Med Sci Sports Exerc 35:923-9, 2003
