Does muscle loss happen within a training week?

Currently, the usual fitness influencers are in uproar about the idea that muscle atrophy occurs quite quickly. They are arguing that we do not lose muscle mass until two or three weeks without strength training. Let’s look at the data to see what it says.

How do we know that muscle loss occurs in less than one week?

Immobilization (and space travel)

Bed rest, immobilization, space travel, water immersion, and dry immersion studies show that meaningful muscle atrophy can happen within just a few days and even within 48 hours. So how does this happen? Well, mechanistic investigations have revealed that there is a large reduction in the myofibrillar protein synthesis rates without any large increase (or even a decrease) in the myofibrillar protein breakdown rates. Moreover, the consumption of dietary protein has no meaningful effect on the reduction in the rate of myofibrillar protein synthesis. Together, this data tells us that the reason for the reduction in muscle size is the absence of a mechanical tension stimulus on all of the muscle fibers in the muscle owing to the lack of any muscular contractions against resistance. There is no increase in the rate of breakdown of the muscle proteins, nor any problem with the supply of raw material to make new myofibrils, only a decrease in the customary stimulus that is being applied.

This is important because some commentators have argued that the rapid rate of atrophy during such immobilization conditions only occurs due to inflammatory conditions (that thereby increase the rate of muscle protein breakdown). Clearly, this is not true because higher rates of muscle protein breakdown are not responsible for the changes that are being observed.

We know that muscle growth only happens because individual muscle fibers produce a mechanical tension stimulus that causes them to add myofibrils. Hence, muscle hypertrophy occurs independently in each muscle fiber as a result of its own stimulus. Since the immobilization data shows that atrophy is caused by the ABSENCE of a stimulus, we must now accept that muscle loss occurs independently every muscle fiber due to its own LACK of stimulus. Since every muscle fiber is independent of the other muscle fibers in a muscle, we can therefore use the immobilization literature to give us insights into the rate of muscle loss while performing the normal activities of daily life between strength training workouts.

Assuming that activities of daily life maybe take us up to 50% of maximum levels of motor unit recruitment for any given muscle, this suggests that the time between workouts will be able to resist muscle atrophy for the bottom half of the available motor unit pool but not the top half. When comparing with the immobilization data, we should consequently expect meaningful levels of whole muscle atrophy to occur within 4–10 days (because bed rest and other immobilization models cause meaningful levels of whole muscle atrophy to occur within 2–5 days (technically, muscle atrophy is happening gradually for all the inactive muscle fibers throughout the whole detraining period in both cases but it’s helpful to provide a numerical comparison). Clearly, this suggests that we should expect meaningful muscle atrophy at the whole muscle level to occur within a single training week.

N.B. When strength training workouts are performed just prior to a period of immobilization, this reduces the amount of atrophy that happens. Yet, this reduction only applies to the first two days after the workout, even though the volumes are relatively high (6–10 sets per muscle group). When measurements are taken at 5–7 days after the workout, there is still atrophy from the immobilization period. This tells us that the stimulus period that is generated by the strength training workout only applies for a couple of days at most before muscle loss starts to happen. This corresponds with the existing data regarding the post-workout elevation in myofibrillar protein synthesis rates that occurs for approximately 48 hours (and the elevation is correlated with long-term hypertrophy over this period). This tells us that the stimulus period after a strength training workout is less than five days (and clearly less than four days if we leave a day for atrophy to occur). In reality, it will be two days if it lines up with the post-workout elevation in myofibrillar protein synthesis rates, which it probably does.

Strength training maintenance studies

The maintenance literature involves finding a volume of strength training that neither allows muscle loss nor muscle gain. Interestingly, this cannot be achieved when doing multiple workouts per week, because this almost always causes hypertrophy even when only a single set is performed in two workouts per week. However, equilibrium can be achieved by doing three (or maybe four) sets once per week.

The most obvious explanation for this maintenance of muscle size is that the 3-set workout causes some hypertrophy and this added muscle size is then lost over the next few days prior to the following workout. This is logical because we know that post-workout elevations in myofibrillar protein synthesis rates last for 48 hours, which suggests that the post-workout stimulus period also lasts for approximately the same duration of time. Once it is outside of this stimulus period, the muscle fiber begins to atrophy and all the hypertrophy is then lost by the following workout.

If you want to argue that atrophy does not happen within a training week, you are going to have a hard time explaining this data within the confines of muscle physiology. To argue that atrophy does not happen, you will need to argue that a single 3-set workout cannot cause hypertrophy, which is just absurd because there are dozens of studies that show that 3-set workouts (and even 1-set and 2-set workouts) can cause hypertrophy when they are done multiple times per week. To get around that limitation, you might go completely off the rails and argue that weekly volume (rather than workout volume) needs to hit a certain threshold to stimulate hypertrophy, which is the realm of fantasy. We have known for a long time that muscle growth is stimulated after single workouts. There is literally no evidence that muscle fibers hold short-term information about previous workouts and take that information into account when deciding whether to add myofibrils after a given bout of strength training exercise. And if you try to argue that there is something magical about doing a 3-set workout once per week that makes the muscle fiber decide to neither grow nor shrink for a week because it is only detecting a “maintenance” stimulus then I don’t know what to tell you other than to say that’s not remotely how muscle physiology works.

Detraining studies

Detraining studies are often wheeled out as evidence in favor of the idea that muscle loss does not happen until two weeks of no strength training, although there are studies showing large numerical losses of muscle fiber cross-sectional area within ten days of detraining, studies that have shown significant losses of muscle thickness within one week of detraining, and also studies showing significant losses of muscle fascicle length within one week of detraining. Therefore, it is clear that detraining after a strength training program can easily lead to atrophy within one week.

It seems quite likely that stretch-mediated hypertrophy (as indicated by increases in muscle fascicle length) is lost somewhat more rapidly than contraction-mediated hypertrophy (as indicated by increases in pennation angle). Indeed, in detraining periods that follow eccentric-only training programs, literally everything is lost within two weeks. Moreover, when the losses in fascicle length (during detraining after eccentric-only training) and pennation angle (during detraining after concentric-only training) are compared, the rate of loss of muscle fascicle length is much faster than the rate of loss of pennation angle.

Additionally, it seems likely that fast twitch muscle fiber size is lost more rapidly than slow twitch muscle fiber size during detraining following a strength training program. The probable reason for this is the reduced likelihood that they will be used during activities of daily life. Indeed, if there is any large difference in the rates of atrophy between single slow twitch and fast twitch muscle fibers, it is likely that the slow twitch muscle fibers atrophy more quickly than the fast twitch ones, when exposed to the exact same lack of mechanical tension stimulus.

SUMMARY

• Immobilization studies tell us that muscle loss occurs within two days when a mechanical tension stimulus is removed from a muscle fiber (which is not due to the inflammation caused by the immobilization, since there is no increase in the rate of muscle protein breakdown). Therefore, we should similarly expect muscle loss to start happening within a couple of days of training cessation, once we leave the period of elevated myofibrillar protein synthesis (which is usually 48 hours).
• Immobilization studies with strength training workouts beforehand tell us that muscle loss does not occur within 48 hours of a workout but absolutely does occur within 5–7 days of a workout. This tells us that the stimulus period after a strength training workout is less than five days (and clearly less than four days if we leave a day for atrophy to occur).
• Maintenance studies tell us that muscle loss happens within a training week when only training a muscle once per week (because otherwise even a single, low-volume workout a week would cause hypertrophy).
• Detraining studies shows us that losses in muscle mass can happen within 7 days of training cessation that still involves normal physical activity. The losses in muscle fascicle length (stretch-mediated muscle growth) are faster than the losses in muscle fiber cross-sectional area (contraction-mediated muscle growth). Also, in terms of muscle fiber cross-sectional area, the greatest reductions in size do occur in the fast twitch muscle fibers.

What are the conclusions?

The strength training literature uniformly tells us that muscle loss will happen within a single week of not training a muscle group and that the fastest losses will occur in the [1] stretch-mediated muscle growth, and in the [2] the fast twitch muscle fibers at the top end of the motor unit pool. Consequently, performing a single workout per week for a muscle group will necessarily incur a great deal of muscle atrophy and reduce the net hypertrophy stimulated by this training frequency. In contrast, training each muscle more frequently will likely cause greater net hypertrophy across the week owing to less muscle loss between workouts.