Since before physical culture was even a thing, we have tacitly assumed a connection between lifting very heavy weights and having a muscular physique.
In fact, it was only very recently that scientists questioned whether heavy weights were necessary to produce muscle growth.
Yet, once these researchers started investigating, and pulled on that thread, they quickly discovered that training to failure with light weights could produce similar amounts of muscle growth as lifting heavy weights. Clearly, this means that lifting heavy weights is not necessary for achieving a muscular physique. After that discovery, our narrative of how strength training works unravelled.
So now we are living through a period in which many fitness professionals are *very* unclear about whether the size of the weight we lift affects muscle growth (spoiler: it does not) or strength gains (spoiler: it does).
Let me explain what is going on.
How does the size of the weight used affect muscle growth?
Muscle growth seems to occur *primarily* in response to mechanical loading on individual muscle fibers, which causes an increase in their diameter, thereby producing increases in whole muscle size.
This mechanical loading on the individual muscle fibers can be produced in *many* different ways.
The mechanical loading can be completely passive, such as by static stretching. Indeed, we have known for decades that applying a passive stretch to animals leads to muscle growth. And recently, this was confirmed in humans, by applying a progressively greater stretch in a leg press (still a completely passive stretch, with no muscle activation) several times a week, for a number of weeks.
More commonly, however, mechanical loading is achieved through active contractions of individual muscle fibers within the muscle, which are each controlled by motor units.
Motor units are grouped into low-threshold and high-threshold types. The high-threshold ones are difficult to recruit, which makes producing muscle growth of their attached muscle fibers challenging. Yet, high-threshold motor units *can* be recruited (and their fibers activated) by lifting very heavy weights, lifting light weights extremely quickly, and lifting light weights to muscular failure.
Although lifting light weights quickly involves a high level of motor unit recruitment (and fiber activation), it does not produce much muscle growth. This is because the mechanical loading on the muscle fiber is still small (despite the high level of activation), because few actin-myosin crossbridges form inside the muscle fiber on account of the high velocity, and these crossbridges are likely what determines the *amount* of active muscle fiber force (and hence the mechanical loading that produces muscle growth).
In contrast, lifting heavy weights and lifting light weights to failure both involve high levels of mechanical loading inside each muscle fiber, and therefore both muscle growth. Similarly high levels of muscle growth occur after both of these types of training because in both cases the contraction velocity is *compelled* to be slow, which allows the formation of a maximal number of crossbridges, and this produces a high level of active muscle fiber force (and hence the mechanical loading that produces muscle growth).
When lifting heavy weights, the velocity is slow even though all of the muscle fibers are working together, because they need a maximal number of crossbridges to form in order to produce sufficient force to lift the weight, and the faster they try to contract, the fewer crossbridges form. This is called the force-velocity relationship. When lifting light weights to failure, the velocity is also slow because fatigue interferes with the ability of the muscle fibers of the low-threshold motor units to produce force, so the high-threshold motor units have to pick up the slack, and the slow velocity occurs to allow a maximal number of crossbridges to form, so that they can keep doing this for as long as possible (this is why the final reps of a set are always a grind).
In short, the size of the weight you lift does *not* affect muscle growth, so long as a slow movement velocity is achieved in active muscle contractions, at the same time as the high-threshold motor units are active. This slow speed can be achieved either by a heavy weight, or by the presence of substantial fatigue.
How does the size of the weight used affect gains in maximum strength?
The active mechanical loading that a muscle fiber experiences in a strength training exercise is *probably* similar whenever it is activated and moving slowly, regardless of whether a heavy weight is lifted or a light weight is used under fatiguing conditions.
In contrast, (1) total muscle force, and (2) the passive mechanical loading on each muscle fiber will *probably* always be greater when using heavy weights, because of the greater external load, and the transmission of muscle forces between fibers inside the muscle.
These differences are very important, as they may underpin the key changes that occur after strength training with heavy weights, and which make it more effective for gaining maximum strength.
Although the greater force on the whole muscle (and its surrounding layer of collagen, and its tendon) does not seem to produce more hypertrophy of the individual muscle fibers, it does likely cause different adaptations in other parts of the body, which are designed to help us become better at lifting extremely heavy loads, such as:
- Increased tendon stiffness
- Increases in the amount of lateral force transmission inside the muscle
- Increases in the activation of the muscles
- Increases in coordination
#1. Tendon stiffness
Increasing the stiffness of a tendon (which typically only occurs after lifting heavy weights) is believed to be helpful for increasing maximum strength, and this effect is driven once again by the force-velocity relationship.
When we lift a very heavy weight, all of our muscle fibers are activated to produce force, and they try to shorten as quickly as they can. Yet, the weight resists being lifted. Therefore, the muscle fibers can only shorten as quickly as they are allowed to, and this speed is *initially* determined by the angular velocity at which our joint rotates as the weight moves.
Yet, our muscles are attached to the joint by tendons. So if the tendon is stiff, then it will not lengthen when the muscle produces force, and thus the shortening speed of the muscle fibers will be only *as fast* as the speed determined by joint rotation.
In contrast, if the tendon is compliant instead of stiff, then when the muscle fibers contract, the tendon will elongate, and this will permit the muscle fibers to shorten at a *faster* speed than the speed determined by the rotational velocity of the joint.
Since muscle fibers produce *more force* when they shorten slowly than when they shorten quickly (because slower contraction speeds allow more crossbridges to remain in place between the myosin and actin filaments inside the fibers), stiffer tendons should lead to us recording a greater level of maximum strength.
#2. Lateral force transmission
Very few people know that muscle fibers transmit the majority of their contractile force *laterally* (outwards to their surrounding collagen layer) and not longitudinally (towards the tendon).
Muscle fibers do in fact transmit 80% of their contractile force laterally by structures called “costameres” that link the fiber to their sarcolemma, and on to the collagen wrapping around them. This collagen layer, which is called the “endomysium,” transmits force down to the tendon, and also to neighboring muscle fibers grouped into a single fascicle (a bundle of fibers).
The other 20% of contractile force produced by a muscle fiber goes directly from one sarcomere to the next, along the length of the muscle fiber, and onwards to the tendon.
Researchers have shown that an increase in the amount of force transmitted laterally within a muscle can increase strength *relative to muscle size* and thereby enhance maximum strength. Although this is still a new area of research, the process of adaptation may involve an increasing number of costameres (links between the muscle fiber and its surrounding collagen layer), or perhaps an alteration in their structure.
Interestingly, when muscle fibers are badly damaged and can no longer transmit force along their whole length (such as when they are exposed to very heavy loads, while lengthening), they form new costameres on either side of damaged site. Force at this point is now transmitted entirely laterally, and this allows the fiber to continue contributing to muscle force, despite being damaged. So muscle damage subsequent to using heavy loads (especially in the eccentric or lowering phase) could be an important mechanism for increasing maximum strength.
#3. Activation of the prime movers
If we can increase muscle activation levels by training, this should increase maximum strength, even without increasing muscle size.
Since muscle fibers are activated when their motor unit is recruited, and full motor unit recruitment happens while lifting both heavy weights, and light loads to failure, we might expect both types of training to produce similar effects on muscle activation levels. Yet, this does not happen.
Why might this be?
Well, when lifting heavy weights, muscle activation is high for all repetitions of every set. In contrast, when we lift light weights, muscle activation starts low, and increases progressively over the set to a peak towards the end, because full motor unit recruitment is probably not reached until the final few repetitions.
The longer time that the high threshold motor units spend being activated while training with heavy weights may explain why *only* heavy weights tend to increase the maximum level of muscle activation (which is partly determined by motor unit recruitment, partly by rate coding, and somewhat less by other factors) that we usually record after long-term periods of strength training.
We know very little about how strength training exercises are coordinated, except that heavy loads require a different proportional involvement from each of the main muscle groups in multi-joint exercises (like squats and deadlifts) compared to lighter loads.
For example, as we increase the weight in squats, the hip extensors (the gluteus maximus, adductor magnus, and hamstrings) increase their contribution to the lift, while the quadriceps become *relatively* less important. Squats with a light load are very much a quadriceps exercise, while squats with a heavier weight start to share more of the overall load across the quadriceps and hip extensors.
There are likely other factors that differ between the same exercises performed with light weights or heavy weights, including the roles of the antagonist (opposing) and synergist (supporting) muscles. Antagonist activation usually increases as loads become heavier, and synergist muscles might need to be more active to control heavier weights.
Importantly, coordination typically improves only with practice, and the practice is really only applicable if it is very specific to the movement.
Since the size of the weight *does* change the way in which a strength training movement is performed, it would not be surprising if coordination was one day found to be a key mechanism by which lifting heavy weights improved maximum strength.
What is the takeaway?
We can use either heavy weights or light weights to produce similar amounts of muscle growth (so long as we train to failure with light weights). In contrast, heavy weights cause greater gains in maximum strength.
Using heavy weights most probably causes greater increases in maximum strength through a number of mechanisms that are related to the much higher mechanical loading on the *whole muscle and tendon*, including increased tendon stiffness, an increase in lateral force transmission, a greater increase in the activation of the muscles involved, and improved coordination.