Traditionally, it was assumed that lifting heavy weights was necessary to achieve muscle growth.
Indeed, a simple comparison between strength training, which has historically involved doing repeated muscular contractions with heavy weights, and aerobic exercise, which involves a far greater number of repeated muscular contractions with lower forces, suggests that lifting heavy is required for hypertrophy to occur.
Nevertheless, recent research has shown that muscle growth is similar after strength training with either heavy or light weights, at least when training to the point of muscular failure.
Some researchers believe that this similar overall muscle growth may occur in conjunction with a greater increases in the size of the muscle fibers that are controlled by low-threshold motor units after light load strength training, and a greater increase in the sizes of the muscle fibers that are controlled by high-threshold motor units after heavy load strength training. The foundation of this proposal is that low-threshold motor units are very likely subjected to a larger volume of mechanical loading (or time under tension) during light load strength training, compared to in heavy load strength training.
But does this actually make a difference?
What are motor units, and how are they recruited?
Motor units comprise motor neurons and the muscle fibers that they innervate. There are usually a few hundred motor units in any muscle, but the exact number varies between muscles.
When the central nervous system sends an action potential (an electrical signal) along a motor neuron, this “recruits” the motor unit, and causes all of the muscle fibers governed by that motor unit to be activated. Once the fibers have been activated, they exert force, and shorten as quickly as they can.
Motor units are recruited in order of motor neuron (not muscle fiber) size, and the size of the motor neuron can be identified by the amplitude of the action potential. Larger motor neurons display large action potentials. This is called “Henneman’s size principle.”
In unfatigued conditions, motor units are recruited at particular force levels, which are called recruitment thresholds.
Once a certain level of force is required in a given contraction type, the motor unit will be recruited. Those motor units that are recruited at relatively low levels of force are called “low-threshold motor units,” while those motor units that are recruited at higher levels of force are called “high-threshold motor units”
Motor units that have larger motor neurons also control far more muscle fibers, and the difference in the number of muscle fibers controlled by each motor unit can be very dramatic. The lowest-threshold motor units typically control only a dozen muscle fibers, while the highest-threshold motor units can control thousands. The number of muscle fibers controlled by a motor unit increases exponentially with increasing recruitment threshold.
In practice, this means that the capacity for low-threshold motor units to contribute to muscle growth is very small, because they only control a very small number of muscle fibers, while the capacity of high-threshold motor units to contribute to muscle growth is huge, because they control the majority of the muscle fibers inside a muscle.
Additionally, it means that the muscle fibers of low-threshold motor units are routinely called upon to contribute to force production during activities of daily life, and therefore they experience huge volumes of mechanical loading on a regular basis. Therefore, they have likely reached their plateau for how large they can grow in most recreationally active people. In contrast, the muscle fibers of high-threshold motor units only experience mechanical loading on those unusual occasions that we require the muscle to produce high levels of force (or expose the muscle to very high levels of local fatigue). Therefore, they probably only reach their plateau for how large they can grow in advanced bodybuilders.
What are fiber types?
Muscle fibers can be categorized into different types.
The categorization of muscle fibers can be done based upon several different factors. Today, the most common way of categorizing muscle fibers is by reference to the isoforms of certain of their proteins, but muscle fibers were originally classified by reference to their maximum shortening velocity (which is why they are often called slow twitch and fast twitch), and later by their oxidative capacity, which coincides with their color (more oxidative fibers have greater greater myoglobin and capillary content, which causes them to appear red rather than white).
When typing muscle fibers by shortening velocity, it is possible to test muscle fiber shortening speeds directly. However, it is now more common to assess the rate at which the ATPase of the myosin head can hydrolyze ATP by performing histochemical staining for myosin ATPase. This rate is associated with the speed that the muscle fiber can shorten. Using this method usually yields three basic fiber types (I, IIA, and IIX), although some researchers have identified types that have intermediate myosin ATPase staining characteristics between these three major types, and therefore record additional types.
When typing muscle fibers by reference to their isoforms, the most common approach is to refer to the myosin heavy chain (MHC) isoform, although the myosin light chain (MLC) can also vary between fibers, and contribute to its functional properties. When using this method of typing fibers, researchers refer to MHC I and MHC II to differentiate between slow and fast fibers, and these types can be further subdivided into other categories, such as MHC IIA and MHC IIX, and hybrids designated in the format MHC IIAX.
In general, muscle fibers tend to decrease in oxidative capacity, increase in contraction velocity, increase in diameter, and increase in responsiveness to the anabolic stimulus provided by mechanical loading during strength training, with increasing motor unit recruitment threshold.
Fibers that are governed by low-threshold motor units therefore tend to be mainly type I (MHC I) fibers, with high oxidative capacity and a red appearance, and have a slow maximum shortening velocity. Fibers that are governed by high-threshold motor units tend to be mainly (but not exclusively) type II (MHC II) fibers, with a low oxidative capacity and a white appearance, and have a fast maximum shortening velocity.
Across various animals, the oxidative capacity of muscle fibers is inversely related to their diameter. The relationship between oxidative capacity and fiber diameter seems to be a universal one, which is caused by the prevailing level of interstitial oxygen tension. Consequently, for highly oxidative muscle fibers to increase in diameter, they need to increase mitochondrial density (and not just the number of mitochondria). It has been estimated that large increases in mitochondrial density of highly oxidative muscle fibers will require very large increases in capillarization (measured as the number of capillaries per fiber), while only modest increases in capillarization seem to be necessary for less oxidative muscle fibers to increase in diameter. This unique requirement of highly oxidative, type I muscle fibers may be part of the reason that they tend to increase less in size after strength training, compared to less oxidative, type II muscle fibers.
In practice, this means that the capacity for the individual (mainly type I) muscle fibers controlled by low-threshold motor units to contribute to muscle growth is small, because they grow little after strength training, while the capacity of individual (mainly type II) muscle fibers controlled by high-threshold motor units to contribute to muscle growth is large, because they grow substantially after strength training.
How are motor units and muscle fiber types related?
It is often believed that low-threshold motor units control only slow-twitch (type I) muscle fibers, and high-threshold motor units control fast-twitch (type II) muscle fibers.
However, the reality is more complex.
In muscles that contain approximately similar numbers of type I and type II muscle fibers, the large majority of the motor units control only type I muscle fibers. This happens because of the exponential relationship between muscle fiber number and motor unit threshold.
For example, when a muscle contains 50% type I and 50% type II muscle fibers, 84% of the motor units will control type I muscle fibers, and the remaining 16% will control type II muscle fibers.
Clearly, when a muscle contains a slightly greater proportion of type I muscle fibers, as is common for the soleus, biceps femoris, and deltoids, then even more of its motor units will control type I fibers, and only a very small number of the highest-threshold motor units will control any type II fibers.
In practice, this means that large numbers of type II muscle fibers are activated together in large groups at similar force levels, while type I muscle fibers are recruited in far smaller groups, and at a range of different force levels. In addition, it means that type II muscle fibers are probably first activated only after voluntary activation reaches a high level, while type I muscle fibers are first activated at a range of force levels, from the smallest possible level of force right through to high levels of force.
Additionally, it is important to note that muscles are formed of multiple regions, and these regions tend to comprise different fiber type proportions from one another. In conjunction with this, some research suggests that the fibers of each motor unit may be clustered into certain specific regions, which may in turn explain why muscle growth in certain specific regions of a muscle is linked to changes in maximum strength.
What causes individual muscle fibers to grow?
Individual muscle fibers grow once they have experienced a sufficiently high level of mechanical loading, and in sufficient volume.
This mechanical loading can be applied when the muscle fiber is active, or it can be applied when the fiber is not active, by passively stretching it. Indeed, both heavy strength training and static stretching programs lead to increases in muscle size in both animals and humans, although the changes after stretching are smaller than after strength training.
The mechanical loading that is experienced by single muscle fibers during strength training depends largely upon its shortening velocity in the lifting (concentric) phase, because of the force-velocity relationship.
The force-velocity relationship is the observation that greater forces (and therefore higher levels of mechanical loading) are produced by individual muscle fibers when the shortening velocity is slow, compared to when it is high. This happens because slower shortening velocities allow a greater number of actin-myosin crossbridges to form, which are the driving engines of force production.
This means that when muscle fibers are activated, but shorten quickly, they are not stimulated to increase in size. This is why training programs involving high-velocity movements, like jumping, do not increase muscle size, even though they can involve fairly high volumes of muscular contractions in which motor unit recruitment is very high.
In practice, this means that when muscular contractions are performed involving high muscle fiber shortening velocities, muscle growth will be minimal, regardless of whether motor unit recruitment is high or low.
Can aerobic exercise cause individual muscle fibers to grow?
Aerobic exercise comprises repeated muscular contractions of low force levels, often at slow velocities.
If the muscle fibers controlled by low-threshold motor units can grow meaningfully in size after experiencing mechanical loading, then they should grow most after aerobic exercise, since this involves the greatest volume of relevant muscular contractions.
However, how long-term aerobic exercise affects the size of individual muscle fibers is unclear.
When testing sedentary and older individuals, aerobic exercise does often have a beneficial effect on muscle size, and can increase the size of both type I and type II fibers. Yet, adding aerobic exercise into the training programs of athletes who are already doing strength training often reduces the gains in muscle size that are achieved, perhaps because of competing signaling that causes endurance-related adaptations (this is called the interference effect).
It therefore seems likely that training status may affect the results that occur after aerobic exercise, with aerobic exercise being beneficial for muscle size in sedentary and untrained people, but detrimental in trained lifters. This may happen because sedentary individuals have not reached a plateau in the size of the muscle fibers belonging to low-threshold motor units, because of their inactive lifestyles. Findings in these populations are therefore not particularly relevant for understanding the effects of long-term strength training.
Moreover, the type of aerobic exercise may be important.
Marathon running training seems to reduce both type I and type II muscle fiber size in recreationally-active subjects. On the other hand, some studies have found that endurance cycling training in untrained individuals increases type I but not type II muscle fiber size.
These findings are hard to interpret.
On the one hand, it seems that aerobic exercise has the capacity to reduce muscle fiber size. On the other hand, aerobic exercise can increase the size of either both type I and type II fibers, or just type I fibers.
It seems likely that training status and the type of exercise are responsible for these differing results. The following framework may help explain the current picture in the literature.
Sedentary individuals will find even aerobic exercise very challenging, and this will lead to a high level of motor unit recruitment, and therefore hypertrophy of both fiber types. Individuals with a high strength training status will experience competing adaptations, which will lead to aerobic exercise having a negative impact on hypertrophy. And exercise that involves a high aerobic demand (like running) will produce a greater competing signaling effect compared to exercise that involves a more local muscular endurance requirement (like cycling), which will lead to it causing reductions in muscle fiber size.
In cases where exercise is performed by recreationally active individuals and involves a local muscular endurance requirement, there is the possibility for type I muscle fiber growth to occur without type II muscle fiber growth. Yet, such exercise involves a very high workload (approximately 30 minutes at 70 rpm, for a total of 2,100 revolutions or reps per workout). Comparable studies that have tested strength training with light loads while avoiding failure (10 sets of 36 reps, for a total of 360 reps per workout) have not found any type I muscle fiber growth, perhaps because the volume of work done is insufficient.
Does fiber type-specific hypertrophy happen after strength training?
If the muscle fibers of low-threshold motor units were to grow more after light load strength training to failure, and the fibers of high-threshold motor units were to grow more after heavy load strength training, even though overall muscle growth is the same after both heavy and light load strength training, we would predict the following:
Fiber type-specific hypertrophy — we would expect research to record greater increases in type II muscle fiber cross-sectional area after heavy load strength training, and greater increases in type I muscle fiber cross-sectional area after light load strength training. Some research has reported such changes, but recent research has not.
Different regional hypertrophy — we would expect changes in muscle size to occur in different regions of a muscle after training with heavy or light loads, because of differences in fiber type between regions of a muscle. There has been little research done in this area, but there seems to be no effect of load on the regional nature of muscle growth.
Greater hypertrophy after training with slow lifting tempos — we would expect hypertrophy after strength training to be greater after using a slow tempo in the lifting (concentric) phase, than after using a fast tempo, because fast tempos would stop the fibers of low-threshold motor units from experiencing mechanical loading in the early reps of a set. However, lifting (concentric) tempo definitely does not affect muscle growth, since both fast and slow tempos cause similar hypertrophy.
Greater hypertrophy after programs involving both heavy and light loads — we would expect hypertrophy to be greater after using a mixture of both heavy and light loads, compared to after using heavy, moderate, or light loads, when training to failure. This is most easily tested by reference to the load periodization literature, because these studies usually compare a single, unchanging rep range with a number of rep ranges carried out in sequence over a period of time. However, there seems to be little beneficial effect of load periodization on muscle growth.
Greater increases in capillarization after training with light loads, especially around type I muscle fibers — we would expect large increases in capillarization after light load strength training, particularly around type I muscle fibers, as well as increases in mitochondrial density in these fibers, but much smaller increases in capillarization after heavy strength training. While recent research suggests that capillarization of both type I and type II muscle fibers does increase after strength training, light load and heavy load strength training cause similar increases in capillarization and mitochondrial content.
In summary, there is some evidence that fiber type-specific hypertrophy might occur, but the magnitude of the effect seems to be very small, because (1) it does not lead to any differences in regional muscle growth, (2) reducing lifting (concentric) tempo does not enhance hypertrophy, (3) training with a mixture of heavy and light loads does not cause greater muscle growth compared to training with a single load, and (4) increases in capillarization are similar after strength training with light and heavy loads.
What is the most likely explanation for the similar overall muscle growth after training with heavy and light loads?
The most likely explanation for the equal muscle growth after strength training with heavy and light loads when training to failure is the way in which fatigue affects the behavior of motor units and muscle fibers.
When fatigue is present (whether this happens in combination with metabolic stress or not), the recruitment threshold of motor units decreases. This means that high-threshold motor units are recruited at lower levels of force. During light load strength training to failure, the last reps of each set involve the high-threshold motor units.
Light load strength training requires a large number of reps to be performed before fatigue leads to increased motor unit recruitment, while heavy load strength training only involves small number of reps, albeit these reps are all performed with a high level of motor unit recruitment. In practice, the number of reps performed with a high level of motor unit recruitment are probably the same in both types of training.
Similarly, when fatigue is present (whether this happens in combination with metabolic stress or not), the maximum shortening velocity of the working muscle fibers decreases. We can observe this happening as a reduction in bar speed over the set. This means that the working shortening velocity of any newly-activated muscle fiber, as well as the already-activated muscle fibers, is reduced. Since slower shortening velocities allow the muscle fibers to produce greater forces, this increases the mechanical loading on any of the muscle fibers that are active.
Light load strength training requires a large number of reps to be performed before fatigue leads to reduced bar speed (and therefore reduced muscle fiber shortening velocity), while heavy load strength training only involves small number of reps, although these reps are all done with a slow fiber shortening velocity. In practice, the number of reps performed with a slow fiber shortening velocity are probably the same in both types of training.
(Technically, light load strength training could be performed with a slow tempo on the early reps of each set, which would increase the mechanical loading on the muscle fibers of low-threshold motor units, while reducing motor unit recruitment slightly on those reps. As explained above, if the muscle fibers of low-threshold motor units contributed substantially to overall muscle growth, this practice should enhance hypertrophy, but it doesn’t).
In summary, fatigue has two effects that influence the hypertrophic stimulus of a set of strength training exercise. It increases the number of motor units (and therefore muscle fibers) that are stimulated, and it increases the size of the stimulus on each muscle fiber, by increasing the mechanical loading that it experiences.
What is the takeaway?
When strength training to failure, the rep range does not affect the amount of muscle growth that occurs. Training with either heavy loads and training with light loads produce similar hypertrophy.
This effect can probably be attributed to the effects of fatigue during light load strength training, which leads to an increase in the level of motor unit recruitment (to the same level as when training with heavy loads) and a reduction in muscle fiber shortening velocity (to the same level as when training with heavy loads). The increase in the level of motor unit recruitment increases the number of muscle fibers that are stimulated, and the reduction in muscle fiber shortening velocity increases the size of the mechanical loading stimulus on each muscle fiber.
Muscle fibers of low-threshold motor units are recruited in activities of daily life, and have probably reached their maximum possible size in most recreationally active people. They are few in number, and respond little to the strength training stimulus. Muscle fibers of high-threshold motor units are only recruited by strength training, and probably do not reach their maximum size except in advanced bodybuilders. There are thousands of muscle fibers for each high-threshold motor unit, and they respond markedly when exposed to the strength training stimulus. Consequently, while there is some evidence that fiber type-specific hypertrophy can occur, it is likely fairly meaningless in the context of strength training.