The stimulating reps model explains why hypertrophy is similar after the same number of sets to failure with a range of different loads (anything between 5RM and 30RM) without requiring us to hypothesize metabolic stress as a mechanism for muscle fiber growth.
Even so, once we have understood and accepted the stimulating reps model, one key question that remains. How many stimulating reps are there in each set that is performed to failure?
In this article, I revise the logic behind the stimulating reps model and add in some minor extra details, and then propose why there are usually five stimulating reps in each set (assuming that we are training without central nervous system fatigue). If you don’t want to revisit the model and see the minor extra details, simply navigate down to the section titled: “Why are only some of the reps in a set stimulating?”
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What determines muscle force during a strength training exercise?
During a strength training exercise, muscle force is required in order to lift and then lower the weight. This muscle force is produced by the binding of actin-myosin crossbridges along each of the many myofibrils that are located inside the thousands of single muscle fibers within the muscle.
Crossbridges form inside muscle fibers when they are activated by the central nervous system. The number of crossbridges that are formed at any one point in time determines the force that is exerted by each fiber, and the number of crossbridges that are formed at any one point in time inside each activated fiber is determined by the force-velocity relationship.
The sum total of all the crossbridges in all of the activated fibers in a muscle determines the force that is exerted by that muscle. The level of motor unit recruitment determines how many fibers are activated at any one point in time. This indirectly affects the total number of crossbridges that are formed, because the more muscle fibers that are active, the greater the potential number of crossbridges.
To understand how muscle force is produced, we must therefore understand (1) how the force-velocity relationship affects how many crossbridges are formed inside each activated muscle fiber, and (2) how the level of motor unit recruitment is increased, since this is what determines the number of muscle fibers that are activated at any one point in time.
What causes hypertrophy after strength training?
Understanding how muscle force is produced is essential for understanding how hypertrophy works. Muscle fibers grow after experiencing mechanical tension, and it is the force that a muscle fiber produces that is the mechanical tension that causes it to grow.
Even so, there is an enormous difference between the force (and mechanical tension) experienced by the whole muscle, and the force (and mechanical tension) experienced by each muscle fiber. Failing to understand this point leads even very knowledgable researchers and commentators astray.
Whole muscle force can be increased either (1) by increasing the number of activated muscle fibers, or (2) by increasing the force exerted by an individual muscle fiber. Yet, only the force exerted by each individual muscle fiber is relevant for hypertrophy. Muscle fibers do not grow because their neighboring fibers experience mechanical tension and grow. Only the force that is exerted by each individual muscle fiber is what triggers that muscle fiber to grow.
Ultimately, this means that the force-velocity relationship is what determines the mechanical tension that causes individual muscle fibers to grow, while the number of activated muscle fibers simply determines which muscle fibers are being trained at any one moment in time.
What does the force-velocity relationship mean?
Crossbridges attach and detach many times per second in response to the electrical signals from the central nervous system, which also arrive at the muscle fibers many times per second. The longer that the crossbridges can remain attached after being stimulated and before detaching, the greater the force that the muscle fiber produces.
When muscle fibers shorten slowly, crossbridges form and then detach slowly. Thus, each working muscle fiber exerts a high force, with many crossbridges being formed at any one time. Therefore, each working muscle fiber (and the whole muscle) exerts a high force.
When muscle fibers shorten quickly, crossbridges form and detach quickly. Thus, each working muscle fiber only exerts a low force, with only a few crossbridges being formed at any one time. Therefore, each working muscle fiber (and the whole muscle) exerts a low force.
This has two key implications.
Firstly, it means that whenever a muscle is contracting slowly, its muscle fibers must necessarily be exerting high forces. To us, it does not seem like there are high forces being exerted, because we do not have the ability to detect forces being produced by single muscle fibers. In fact, the only sensations that we can detect during a strength training exercise are (1) the level of effort, (2) fatigue, (3) stretch, (4) whole muscle force, and (5) awareness of joint position and movement (proprioception).
Secondly, it means that whenever a muscle is contracting very quickly, its muscle fibers must necessarily be exerting very small forces. This is why the forces that are exerted during jumping and throwing movements are so small in comparison with 1RM squats and bench presses, despite the fact that we devote similarly high levels of effort to them. It is also why hypertrophy is minimal after jumping and throwing programs.
What determines the degree of motor unit recruitment, and what does it mean?
Motor units are the control mechanism for groups of muscle fibers.
Motor units are recruited by the central nervous system in order of size. The smallest motor units (which control the fewest muscle fibers) are recruited first, while the largest ones (which control the most muscle fibers) are recruited last. Small motor units are called “low-threshold” motor units and large motor units are called “high-threshold” motor units. This terminology is a legacy of the early research that identified the recruitment of motor units at different thresholds of isometric force in unfatigued muscle. Yet, since we now understand that increasing contraction velocity and increasing fatigue can both reduce these thresholds, the terminology is not actually that helpful.
Motor units are actually recruited in response to certain levels of effort, and not force. As we exert greater efforts, additional motor units are recruited. Also, each motor unit can only be recruited after its preceding motor unit has been recruited. Therefore, if the last high-threshold motor units are recruited, all of the other motor units must also be recruited at the same time.
Since effort is the determinant of motor unit recruitment, when we exert a maximal effort to lift a weight, most (if not all) of the motor units that control the fibers of a muscle will be recruited. Consequently, most of its muscle fibers will be activated. In contrast, when we use a submaximal effort to perform a rep, only the smallest motor units are recruited, which means that only a small proportion of the muscle fibers will be activated.
This has two key implications.
Firstly, increasing levels of effort provide an opportunity to exert greater whole muscle forces and lift heavier weights, because the higher level of motor unit recruitment means that more muscle fibers are active, and each fiber can form a certain number of crossbridges. Yet, such greater levels of motor unit recruitment do not necessarily lead to greater whole muscle forces if the shortening speed of the muscle is fast, because of the force-velocity relationship.
Secondly, unless we perform some reps in a workout with maximal effort, we will not recruit all of the motor units in a muscle, and therefore not all of the muscle fibers will be trained. Hypertrophy occurs as a result of single fibers growing, and if we do not train a muscle fiber, it will not grow.
What happens during slow movements performed with low levels of effort?
When we perform slow movements with low levels of effort, such as lifting a light weight while unfatigued, walking, or carrying out other activities of daily life, we use small, low-threshold motor units. Therefore, only a small proportion of our muscle fibers are activated.
Even so, these muscle fibers must be exerting very high forces due to the high number of crossbridges that can be formed at any one time inside them. This means that the fibers controlled by low-threshold motor units experience high levels of mechanical tension during normal daily life. However, they do not grow, because they have already reached a plateau due to their constant exposure to this loading for many years.
This means that the only muscle fibers that grow after strength training are those that are not loaded in daily life, which are those that are controlled by high-threshold motor units. These high-threshold motor units can control both type I and type II muscle fibers, so we should not confuse discussions about fiber type-specific hypertrophy with motor unit-specific hypertrophy. Even so, this is a critical point for understanding muscle growth, since it explains why increasing motor unit recruitment is so important for causing hypertrophy during strength training, even though increasing motor unit recruitment does not affect the mechanical tension experienced by single muscle fibers.
This is also why cardiovascular exercise routinely causes only minor muscle growth, why slow tempos do not enhance hypertrophy, and why light load strength training to failure does not train different muscle fibers from more traditional, moderate load strength training.
What happens during fast movements performed with high levels of effort?
When we perform fast movements with high levels of effort, such as throwing a ball or jumping, we are using large, high-threshold motor units in addition to our small, low-threshold motor units. Therefore, a large proportion of our muscle fibers are activated.
However, the muscle fibers of all of these motor units are each exerting very low forces due to the tiny number of crossbridges that are formed at any one point in time inside the activated muscle fibers. This means that the muscle fibers controlled by these high-threshold motor units experience only very small amounts of mechanical tension and do not grow.
This is why throwing, jumping, and many forms of power training with light loads cause minimal muscle growth, despite their very high levels of motor unit recruitment.
How does fatigue change things?
Over the course of a set of strength training reps, we experience fatigue, which has two key physiological effects that are relevant for hypertrophy. Firstly, it causes the active muscle fibers to exert smaller forces (which can have an effect on motor unit recruitment, since additional motor units must be recruited so that other muscle fibers are activated in order to compensate). Secondly, it causes the shortening velocity of active muscle fibers to reduce (which obviously has an effect on the force-velocity relationship).
Although we might assume that fatigue increases steadily with each rep of a set of muscular contractions, this is not entirely the case. Since most of the muscle fibers controlled by low-threshold motor units are highly oxidative, they resist fatigue very effectively. Therefore, they can carry on producing a certain level of force for some time before they reach a point where they can no longer exert the force necessary to complete the task. In contrast, muscle fibers controlled by high-threshold motor units are less oxidative. They fatigue rapidly once they are activated. Consequently, most of the early reps performed during light or moderate load strength training with a normal, submaximal tempo will only involve the highly oxidative muscle fibers controlled by low-threshold motor units. Once fatigue does arise in these fibers, and the less oxidative muscle fibers of high-threshold motor units are recruited, the time to task failure is quite short.
Even so, the practical effects of fatigue are therefore determined by exactly how we perform a set. We can choose to lift with a maximal effort or with a submaximal effort. When lifting with a submaximal effort, we use a tempo. This tempo can be slow (defined as equal to or slower than the speed achieved when lifting a 1RM with maximum effort) or it can be moderate (defined as faster than the speed achieved when lifting a 1RM with maximum effort).
Each of these approaches has implications as follows:
- Maximum efforts — pushing (or pulling) the bar with maximum effort means that most motor units are recruited on every rep regardless of the level of fatigue. The only effect of fatigue is to cause a reduction in bar speed. Bar speed reduces gradually towards failure, and the speed of the final rep is equal to the speed of a 1RM performed with maximal effort. In fact, bar speed is identical at the same number of reps in reserve regardless of the weight on the bar.
- Submaximal efforts (slow tempo) — pushing (or pulling) the bar with a submaximal effort means that only some motor units are recruited on the early reps of the set, and therefore only some muscle fibers in the muscle are activated. If all reps of the set are performed at a speed equal to or slower than a 1RM bar speed, there is no reduction in bar speed over the set. Thus, mechanical tension on any active, working muscle fibers does not change from one rep to the next. Thus, the only effect of fatigue is to cause an increase in motor unit recruitment over the reps to failure.
- Submaximal efforts (moderate tempo) — pushing (or pulling) the bar with a submaximal effort means that only some motor units are recruited on the early reps of the set, and therefore only some muscle fibers in the muscle are activated. If the early reps of the set are performed at a speed faster than a 1RM bar speed, there is a reduction in bar speed over the set. Thus, fatigue increases the level of motor unit recruitment and also gradually increases the mechanical tension on working muscle fibers.
In practice, since tempo seems to have very little impact on the resulting hypertrophy after strength training to failure with the same number of sets, it seems likely that largely the same dosage of mechanical tension occurs in all of the above scenarios. This would require the contraction velocity (achieved during maximal efforts) that stimulates hypertrophy to occur at a similar proximity to muscular failure as the motor unit recruitment level (achieved during submaximal efforts) that stimulates hypertrophy. Given that the reps done after this point is reached are likely all carried out with maximal efforts and with bar speeds and motor unit recruitment levels determined by the external environment and not by the lifter, this is not unreasonable.
What about metabolites?
Some researchers and commentators have suggested that metabolites can be treated as a mechanism that triggers hypertrophy, because they cause motor unit recruitment to increase.
This is not correct.
The accumulation of metabolites is not necessary for motor unit recruitment to increase during a fatiguing contraction. It is perfectly possible to cause muscular fatigue, and thereby increase motor unit recruitment, without the presence of any metabolites.
Indeed, muscle damage often causes an increase in motor unit recruitment, because it decreases the level of force that each damaged muscle fiber can exert. The common factor in all cases is that some muscle fibers are no longer able to exert sufficient force for the muscle to be able to carry out the task with the current level of motor unit recruitment. This requires an increase in the number of recruited motor units, which requires an increase in the effort applied to the lift. Ultimately, effort levels determine motor unit recruitment, not metabolite accumulation.
Why are only some of the reps in a set stimulating?
During strength training with a heavy (5RM) load, motor unit recruitment is high (essentially maximal) during every rep of a set, and movement velocity is slow enough such that the forces exerted by each individual muscle fiber are high enough to trigger them to increase in size afterwards. In other words, all of the reps in a set stimulate hypertrophy.
In contrast, during strength training with moderate loads (6–15RM) and light loads (16–30RM), the situation is different. The exact situation depends on how the set is performed, as follows:
- Maximal effort — if each rep of a set is performed with maximal effort, then motor unit recruitment is still high (essentially maximal) during every rep of a set, but movement velocity is too fast on the early reps, so the forces exerted by each individual muscle fiber are too small to trigger them to increase in size afterwards. Only as fatigue is incurred towards the end of the set does movement velocity reduce to the point where forces exerted by each fiber are sufficiently high to trigger hypertrophy. Moreover, since the movement velocity on the final reps before failure is the same at the same number of reps in reserve regardless of the weight on the bar, the mechanical tension experienced by the working muscle fibers must also be the same.
- Submaximal efforts (slow tempo) — if each rep of a set is performed with a slow tempo (a speed equal to or slower than a 1RM bar speed), then movement velocity is slow enough that active muscle fibers experience high forces that could cause hypertrophy. Even so, the submaximal effort means that few motor units are recruited on the early reps of the set, and those that are recruited may not control muscle fibers that are capable of growing in response to mechanical stimulation. Only as fatigue is incurred towards the end of the set does effort increase to the point at which recruitment is high enough to allow the muscle fibers of high-threshold motor units to be stimulated.
- Submaximal efforts (moderate tempo) — if each rep of a set is performed with a moderate tempo (a speed faster than a 1RM bar speed), then movement velocity will reduce over the set (leading to increased levels of mechanical tension on working muscle fibers, and effort will increase over the set (causing increased levels of motor unit recruitment). Early reps may not be slow enough that active muscle fibers experience the high forces that cause hypertrophy. Motor units recruited in the early reps may not control muscle fibers that are capable of growing in response to mechanical stimulation. Only as fatigue is incurred will movement velocity slow and effort increase to the point at which mechanical tension is high on the working muscle fibers, and motor unit recruitment is sufficiently high to cause the fibers of high-threshold motor units to be stimulated.
Again, it is worth noting that since lifting tempo seems to have little impact on the resulting hypertrophy after strength training to failure with the same number of sets, it is likely that the same dosage of mechanical tension on the relevant muscle fibers occurs in all of the above scenarios.
How many reps in a set are stimulating? (part one)
Although we can guess the number of stimulating reps in a set by reference to the level of force at which motor unit recruitment is maximal (approximately 88% of 1RM or a 5RM), there are several other ways in which we can deduce the number of stimulating reps in a set performed to failure.
Firstly, we can perform studies that stop each set at a certain number of reps in reserve or when bar speed drops to a certain velocity. These studies allow us to determine how close we have to go to muscular failure in order to trigger muscle growth. For example, a recent study was able to trigger a small but still significant amount of hypertrophy by doing a very high number of sets per muscle group with 4.4 reps in reserve. This suggests that the number of stimulating reps in a set to failure is five or more. If it were only four, then training with 4.4 reps in reserve would not trigger hypertrophy at all. Even so, given that only a small amount of muscle growth was caused despite the very high number of sets, it is probably not much higher than five.
Secondly, we can perform studies that compare the hypertrophic effects of training with the same number of sets to failure with different loads. If the heavier of the two loads that are compared causes less hypertrophy than the lighter of the two loads, then we can assume that the number of reps done in those heavy sets is smaller than the number of stimulating reps in a set. In practice, most of these kind of studies have used 10RMs for the heavier load training groups, although one or two have used 8RMs or 6–10RM ranges. This suggests that the number of stimulating reps is likely to be less than eight reps per set. On the other hand, one study that used a 3RM found that a higher number of sets was necessary to achieve similar muscle growth to a 10RM. This finding also suggests that the number of stimulating reps is at least four reps per set.
Thirdly, we can compare the effects of fatigue when using heavy loads. If there are five stimulating reps in a set, then we should observe similar muscle growth when lifting five singles with a 5RM with complete rest between each rep and when lifting a 5RM to failure. In truth, the 5RM would be expected to cause a slightly greater amount of muscle growth due to the slower bar speeds used as fatigue develops, but this might well be negligible. In contrast, we should observe very different muscle growth when lifting five singles with a 6RM with complete rest between each rep and when lifting a 6RM to failure. To my knowledge, no such studies have been done, but there is a study that has assessed strength gains after lifting a 6RM either to failure or with 30 seconds of inter-rep rest (albeit there was some fatigue from one rep to the next even with this inter-rep rest). This study reported greater strength gains after the 6RM to failure, suggesting that there was a meaningful difference in hypertrophy. This hints that a 6RM load performed without fatigue may not stimulate hypertrophy, and therefore that the number of stimulating reps is less than six. However, this is only very weak evidence due to the lack of a measurement of muscle size.
In summary, there is strong evidence that the number of stimulating reps in a set taken to failure is five or greater. On the other hand, there is is moderate evidence that the number of stimulating reps in a set taken to failure is less than eight, and there is very weak evidence that the number is less than six. Since motor unit recruitment is maximal at approximately a 5RM, it makes most sense to use five reps as the number of stimulating reps in a set to failure, assuming no central nervous system fatigue from preceding sets.
How many reps in a set are stimulating? (part two)
Naturally, the answer to every question in strength and conditioning is always “it depends” and the question of how many reps in a set are stimulating is no exception. In this case, the answer depends on the training status of the lifters in question. This is easily understood by considering the atrophy that results from periods of inactivity (such as bed rest), and the subsequent hypertrophy that occurs when returning to normal levels of activity.
#1. Atrophy due to bed rest, and subsequent hypertrophy
During bed rest, atrophy occurs quickly, often within days. This happens because muscle fibers require exposure to a certain level of mechanical loading in order to be retained. If they are not exposed to this level of mechanical loading, they begin to decrease in size.
In order to achieve mechanical loading, muscle fibers must be (1) activated, and (2) shorten at slow speeds. Most activities of daily life involve fairly slow movement speeds and therefore the main factor that we need to consider is activation. Muscle activation occurs in response to motor unit recruitment levels, which is a function of effort. Most activities of daily life involve fairly low levels of effort, but bed rest involves even lower levels! Remaining in bed all day means that some low-threshold motor units that would normally be recruited just by walking around will cease to be recruited. Consequently, the fibers controlled by these motor units begin to reduce in size.
After stopping bed rest, we recommence our activities of daily life. As a result, we naturally regain our lost muscle size. This happens because we begin to recruit the low-threshold motor units that are recruited by walking around, but which are not recruited during bed rest. The activated fibers of these motor units are then exposed to high levels of mechanical loading by the slow movement speeds, and subsequently grow back to their previous levels of size. No strength training is necessary to reverse this effect, it just happens as a result of returning to our previous levels of activity.
#2. Hypertrophy in untrained lifters
When researchers recruit untrained lifters to take part in a strength training program, the subjects are sometimes classed as “recreationally active” and sometimes as “sedentary,” depending on the physical activities that they normally engage in during daily life. Either way, they are not athletes, who exert high levels of effort during muscular contractions, and so they will not have previously recruited high-threshold motor units to any great extent.
When these untrained individuals commence a strength training program, they experience very rapid muscle growth. There are likely two causes for this quick adaptation. Firstly, it is likely that some of their highest low-threshold motor units and some of their lowest high-threshold motor units contain muscle fibers that have not already reached a plateau in size. Consequently, untrained lifters have more muscle fibers that are capable of increasing in size than intermediate or well-trained lifters. Secondly, it is very likely that their high-threshold motor units have been largely unused prior to starting strength training, and their muscle fibers are therefore capable of very large increases in size.
When untrained lifters have been very sedentary prior to starting strength training, it is quite likely that some of their highest low-threshold motor units and some of their lowest high-threshold motor units contain muscle fibers that have not already reached a plateau in size. Therefore, each set they perform will necessarily include more stimulating reps per set than less sedentary individuals who have already achieved a plateau in the size of the muscle fibers of these motor units, because trainable motor units will be recruited earlier in a set as a result of fatigue.
What is the takeaway?
Whole muscle force is determined by both the number of activated muscle fibers (which is controlled by the level of motor unit recruitment) and the force-velocity relationship of single muscle fibers. This is because both the number of activated muscle fibers and the force-velocity relationship of single muscle fibers affect the number of actin-myosin crossbridges that are formed within the whole muscle, and it is the total number of crossbridges that are formed at any one point in time that determines the force exerted by the whole muscle.
Even so, the force produced by single muscle fibers (and the mechanical tension that stimulates them to grow after strength training) is determined only by the force-velocity relationship. The level of motor unit recruitment only affects which muscle fibers are trained. Ultimately, we therefore need a slow contraction velocity to cause hypertrophy of single muscle fibers, and a high level of motor unit recruitment is helpful for ensuring that we train muscle fibers that have not previously been trained by activities of daily life. This slow contraction velocity and high level of motor unit recruitment occur during the final reps of any set to failure, regardless of the weight on the bar. These are the stimulating reps.
In trained lifters, the number of stimulating reps in a set taken to failure is probably greater than five, but less than eight, and is more likely closer to five than eight. From a practical standpoint, it makes sense to never train below five reps in a set when training for muscle growth, and working consistently between 5 – 7 reps per set is likely to be optimal.
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