Strength gain and muscle size are two key motivating factors for those of us who engage in resistance training. You can find wildly differing opinions out there on how frequently a muscle group should be trained to optimize size and strength, ranging from daily workouts at one extreme to one workout every month at the other. In this post we will look at some of the various physiologic factors we need to consider when planning frequency of training for strength and size, including:
- Neuromuscular adaptation
- Glycogen Storage
- The “pump”
What I intend to do is tease out what the research may suggest about a minimal optimal frequency for strength gain, muscle gain and appearance concerns. A frequency that can account for these key physiologic factors mentioned above.
Our first port of call is muscle hypertrophy, which refers to an increase in skeletal muscle size “accompanied by an increase in mineral, protein, or substrate abundance”1. Hypertrophy is the factor most individuals consider first when thinking about the physical results of lifting weights. Let’s take a deeper look at what it is. Researchers have proposed that skeletal muscle hypertrophy can be split into three categories: connective tissue hypertrophy, myofibrillar hypertrophy and sarcoplasmic hypertrophy.
Connective Tissue Hypertrophy
Connective tissue hypertrophy refers to an increase in volume of the extracellular matrix that surrounds muscle cells and acts as a support structure/signalling environment. This extracellular matrix usually accounts for between 1-10% of the mass of a muscle 2. Much of it consists of collagen and plays an important role in both strength and elasticity. After the stimulus of strength training there is a remodelling of the extracellular matrix including synthesis and accumulation of collagen 3.
The sarcoplasm is the cell plasma that surrounds the myofibrils (strands of muscle cells) within a muscle fiber, it contains a reticulum, mitochondria, t-tubules, enzymes and substrates. The majority of muscle glycogen, a critical fuel source for muscle, is stored within this sarcoplasm. Sarcoplasmic hypertrophy refers to a chronic increase in the size of sarcoplasm, the volume of its content and its enclosing sheath, the sarcolemma. Resistance training stimulates, for example, an increase the amount of glycogen stored in the sarcoplasm 4, we will revisit glycogen later. Some have suggested that sarcoplasmic hypertrophy may be the “bodybuilder’s” hypertrophy associated with moderate loads and reps in the 8-12+ range 5.
Myofibrillar hypertrophy concerns an increase in the size or number of the strands of muscle cells (myofibrils), including a greater number of sarcomeres, or an increased amount of sarcomeric protein within the myofibrils. Conversely to sarcoplasmic hypertrophy, some have suggested myofibrillar hypertrophy may be the “strength athlete’s” hypertrophy associated with heavier loads and reps in the 3-5 range 5.
When performing exercises to momentary failure with a set length of between 30-90 seconds you will, in all likelihood stimulate all three types of hypertrophy simultaneously. It is however, as mentioned, possible that higher-load/lower-rep training has the potential to favor myofibrillar hypertrophy whilst moderate load/higher-rep training may favor sarcoplasmic hypertrophy 5. If this proves to be the case, then spending some of your time training with heavier weights and a lower time under load (~30 seconds) and some of your time with moderate weights and slightly longer TULs (~60-90 seconds) may ultimately work best to optimize both. This is easily programmable utilizing progressive overload and appropriate load selection for your exercises.
Stimulating hypertrophy results in chronic adaptations that are not acutely transient 6. In other words, this is hypertrophy you will get to keep for a significant period even if you take some time off training. We will return to how long you can retain hypertrophy later.
Initial Muscle Size Gains: Hypertrophy or Edema?
It is a common perception to think of hypertrophy as a near immediate stimulus-result outcome once an individual takes up resistance training, however there is research that suggests this may not be the case. In the first 3 or so weeks of engaging in regular resistance training, muscle size increases may be due to transient swelling or edema in response to muscle damage 7. Then at some point in time between 3-10 weeks after commencement of training, hypertrophy as defined previously, begins.
The initial edema of the first few weeks occurs as a response to an unusual stimulus (resistance training) for the skeletal muscle tissue, it then appears that muscle damage needs to be repaired before growth can take place 8. As the muscles become acclimated to resistance training, further muscle damage is attenuated 9, 10, unless there is a significant change in protocol/approach- another unusual stimulus. An example of this would be introducing eccentric only training where previously only normal concentric/eccentric reps had been performed- the new stimulus would cause renewed muscle damage and edema.
If we stop training how long do hypertrophic and strength gains last?
The answer to this partly depends on why you stopped training. If it is just because you have fallen out of the habit or gone away on vacation and the rest of your active life goes on as usual, there is good news. Some research suggests that you will hold on to your lean mass for at least 2 weeks and possibly as long as 3 weeks before noticeable atrophy begins 11, 12. Other research however has reported shorter timeframes, one paper noting that sarcoplasmic hypertrophy can last for up to 8 days following cessation of training 5, and another paper finding that mean cross-sectional of muscle fibers peak 3 days after the last workout and had significantly reduced again by day 10 of no training 13. It should be noted with this last paper, that no measurements were taken between days 3 and 10, so an actual peak may have occurred at a point later than day 3.
It appears that strength is somewhat better preserved than hypertrophy and strength loss may not occur until some point after 3 weeks without training. In fact, strength tends to be retained quite well even after 6 weeks without training 14, 15, 16.
What happens if you have paused training because of an immobilized limb, or an illness that means you are confined to bed 24-7? In these scenario’s atrophy and strength loss will occur much more rapidly than above, beginning as quickly as a day or two into your incapacitation. Within two weeks without change to your condition it is possible to lose in the region of 5-15% of a muscle’s volume and a 23% strength loss 17.
Whilst hypertrophy is the reason that we gain muscle size over the long term, it is not the only reason that we gain strength. Neuromuscular adaptations play a key role in strength gain. Neuromuscular adaptations made in response to strength training include improved intermuscular coordination of agonists, antagonists and synergists and enhanced motor unit firing rates. On initial commencement of resistance training as an outright beginner, a frequency for performing a specific exercise of 3 times per week is probably a practical ideal to reasonably rapidly acquire the neuromuscular adaptations needed to become skilled in performing that exercise. However, once we have acquired these adaptations, they are preserved for at least 2 weeks, and possibly, as long as 4 weeks, without any training stimulus before any detrimental decrease begins to occur 18.
Transient Glycogen Storage
We have already mentioned that part of sarcoplasmic hypertrophy is due to an increase in the amount of glycogen stored in muscle. For those that care about how their muscles look between training bouts we need consider transient glycogen storage in more detail.
The normal range for glycogen stored in muscle cells in the human body is considered between 350-700g 19. Glycogen is also stored in the liver- about 100g, and smaller amounts are found in other tissues such as the heart and the brain. Let’s focus on the glycogen in muscle tissue: where it is stored to be available for use in energy production of ATP, the required fuel for muscle contractions.
Each gram of glycogen stored in muscle tissue is bound to ~3-4 grams of water. When the sarcoplasmic stores are full, glycogen and its bound water can account for up to about 16% of a muscle’s total mass 20. Having a “full-tank” in terms of glycogen stores can make a muscle appear somewhat fuller and more visually impressive: primarily due to the amount of glycogen bonded water now present in the muscle tissue.
During resistance training glycogen is used to resynthesize the phosphate pool and therefore the muscle’s stores of glycogen are depleted. How much the stores are depleted depends on the effort level of the sets and the total time under tension a muscle group is exposed to. Typically, glycogen depletion from a resistance training workout can be in the range of ~24-40% 19.
Note that glycogen stored in muscle tissue cannot be transported for use elsewhere. For example, if you perform a challenging set for the biceps that depletes glycogen in that muscle group, glycogen stored in the triceps (or any other muscle) cannot be shuttled to the biceps for top-up. Therefore, if you use a split routine, muscle glycogen will only be depleted from the muscles you have exercised in that workout.
Continuing with the biceps as an example, let’s say that on completion of your workout you have depleted its glycogen stores by 40%. What happens next? It will now take in the region of 20-24 hours 21 for the body to restock the biceps muscle cells with glycogen to a similar level they were prior to the workout. Once restocked and at peak, glycogen levels are then maintained for at least 3 more days (assuming the biceps does not endure another depleting workout during this timeframe).
This means that your muscles can potentially be looking at their best with workouts for a muscle group spaced about 4 days apart. After this timeframe, without further stimulus, glycogen stores in muscle cells will start to decrease somewhat, along with their bonded water content. In other words, muscles will start to look a little flatter.
This time course can vary somewhat, for example, eccentric exercise appears to delay post-workout glycogen replenishment, likely due to the increased microdamage that it causes 22. If your training includes eccentric-only exercise or eccentric-overload sets, then workouts could be spaced further apart, primarily for the repair of microdamage, but also from the perspective of glycogen replenishment too.
Via nutritional manipulation glycogen stores in muscle can even be made to transiently supercompensate above their resting baseline peak. This is a strategy used by both competitive bodybuilders: for the purpose of looking good on stage, and endurance athletes: to force their muscles to store more fuel for an important race. Glycogen supercompensation requires a) a workout that significantly depletes glycogen followed by b) three days of a high carbohydrate diet. When this strategy is followed, glycogen stores are replenished as normal within the first 24 hours, but then go on to supercompensate over the next few days. After a supercompensation peak has been reached glycogen stores gradually revert to baseline but can remain elevated above baseline for 3-5 days after the peak was reached 23, 24, 25.
Finally, the famous “pump”
Water plays a part in physical appearance during another physical phenomenon associated with exercise: “the pump”. The pump, made famous by Arnold Schwarzenegger in the appropriately titled Pumping Iron, occurs during a workout and lasts for at most a couple of hours post workout. Blood plasma, which is ~92% water, increases in the muscle fibers and interstitial space when a muscle is worked 26. This gives worked muscles a “pumped-up” appearance, as fleeting as it is, until the plasma subsides. Useful, perhaps, only for those who are about to step on a bodybuilding stage, have a photo-shoot or want an Instagram worthy post-workout selfie 😉
Combining all the factors
Let’s now draw all this information together into a practical overview.
- When starting resistance training for the first time or coming back to it after a very long layoff, train fairly frequently (~3x per week) to acquire desirable neuromuscular adaptations to the exercises you are going to be using. It may also be advisable to employ a moderate effort level (perhaps a 7/10) to minimize initial microdamage and resultant edema at the outset. Three weeks of doing this should prepare your body well for higher effort training. If you are a personal trainer working with clients who are unable to commit to this higher frequency, then you will need to work with the frequency the client can commit to. Once past this initial stage you can up the effort level, working towards muscle failure with the skill you have acquired, and reduce frequency.
- Performing sets to muscle failure that last 30-90 seconds are likely to stimulate both sarcoplasmic and myofibrillar hypertrophy. However, to cover your bases, it may be useful to spend time both at the lower end of that spectrum as well as the higher end of that spectrum over the course of your training. This can happen “organically”, if you work with a load that you can initially only get ~30 seconds with, then sticking with that load until your sets for that exercise double or even triple in their time to muscle failure over the following weeks. Then restart with a load that brings you back to that low end of the spectrum. Rinse and repeat.
- To look at their absolute best muscle groups may need to be trained every around every 3rd-4th day. This is to take advantage of the glycogen depletion, repletion and peak-maintenance time course. However, this doesn’t necessarily mean that each muscle group needs to be used to lift weights 2x per week. For example, as an alternative you could train each muscle group 1x per week in the gym and on the weekend be involved in a sport or physical activity that somewhat depletes glycogen from some or most of your muscle groups. What I’m saying is there is a degree of flexibility here and your lifestyle and interests can be factored in when choosing how you deplete glycogen.
- Even training a muscle group effectively just 1x per week will be enough to stimulate strength and myofibrillar gains and will still likely have your sarcoplasmic volume up near peak for most of the week. Experiment for yourself to find what frequency works better for you in this regard.
- If you alter your workout significantly or are using eccentric-overload then it is highly likely you will cause significantly more muscle damage than usual. It is going to take longer to repair that damage and longer to replenish glycogen stores, especially for the first handful of workouts training in this new manner. Adjust your frequency accordingly: training a muscle group 1x per week is likely better than more frequently under these circumstances especially at the outset, before the repeat bout effect has kicked in.
- Unless you are confined to bedrest or a muscle group is immobilized you needn’t be overly concerned about atrophy or strength loss from taking a week or three away from the gym every now and again. Unless you are being occasionally vigorously active in your time off yes sarcoplasmic volume will likely decrease but strength and myofibrillar gains will probably be maintained. And sarcoplasmic volume will rebound quickly when you do get back to the gym.
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