Many strength coaches will tell you that heavy loads produce the high levels of mechanical tension that trigger hypertrophy, while light loads do not.
They explain the similar hypertrophy achieved after strength training with heavy and light loads to failure by claiming that metabolic stress can also stimulate muscle growth.
However, it is not correct to state that only heavy loads produce high levels of mechanical tension. In fact, both heavy loads and light loads can produce high levels of mechanical tension on muscle fibers to make them grow, because the force-velocity relationship is the only factor that determines force production (and therefore mechanical tension) on individual muscle fibers.
Let me explain.
What is mechanical tension?
Mechanical tension is the type of force that tries to stretch a material.
During strength training, muscles experience stretching forces when they try to shorten, but are resisted when they do so. They also experience stretching forces when they lengthen while we are holding a load, but these forces are comparatively smaller.
For example, we stand up from a sitting position by activating the hip and knee extensors and causing them to shorten. As they shorten, they experience a stretching force acting against them, which results from the resistance imposed on the body by gravity and inertia.
Importantly, the stretching force experienced by the muscle is equal and opposite to the force that the muscle itself exerts on the body.
In order for us to get out of a sitting position, the muscle force we produce must be greater than our bodyweight due to gravity. How much greater it is determines our acceleration from the chair. If we apply a much larger muscle force, we will spring out of our seat very quickly, but if the muscle force is only slightly greater than bodyweight, we will get up slowly.
How do muscles produce force?
There are two ways in which muscles can increase the amount of force that they produce during a lifting (concentric) muscle action: (1) changes in the number of active muscle fibers, and (2) changes in the amount of force that each muscle fiber produces.
An increase in the number of active muscle fibers is accomplished by an increase in the level of motor unit recruitment.
Motor units are recruited in size order from small to large. Small motor units control only a dozen or so muscle fibers, while large motor units control thousands. When a motor unit is recruited, all of the previously recruited motor units must first be recruited, and remain active.
An increase in the amount of force that each muscle fiber produces is achieved by the force-velocity relationship.
Muscle fibers exert high forces when they shorten slowly, but low forces when they shorten quickly. This is because slow shortening speeds allow lots of actin-myosin crossbridges to form at the same time, and actin-myosin crossbridges are what allows each muscle fiber to produce force. In contrast, fast shortening speeds cause the actin-myosin crossbridges inside muscle fibers to detach at a faster rate, and this leads to fewer simultaneous crossbridges being formed at any one time.
Importantly, the force-velocity relationship is the only factor that determines the amount of force exerted by each muscle fiber. In contrast, both the force-velocity relationship and the level of motor unit recruitment affect the amount of force exerted by the whole muscle.
This has implications for the forces (and therefore the mechanical tension) that (1) the muscle, and (2) the muscle fiber experience.
How do we control the amount of force that muscles produce when lifting light loads?
We control the amount of force that muscles produce by increasing the amount of effort we exert. When we exert a high effort, this triggers a high level of motor unit recruitment. When we exert a submaximal effort, only a small fraction of the motor units are recruited.
If we lift a light load with a maximal effort (in unfatigued conditions), the weight moves very quickly. During this lift, our motor unit recruitment levels are very high, and therefore most of the muscle fibers are active.
However, despite the high levels of motor unit recruitment, overall muscle force is not particularly high, because each muscle fiber only exerts a small amount of force when it shortens quickly. This is why we produce much lower forces during maximal effort vertical jumps compared to in one-repetition maximum back squats. The force-velocity relationship causes the amount of muscle force exerted to be lower when we are moving more quickly.
Conversely, if we lift a light load with a submaximal effort (again, during unfatigued conditions), the weight moves comparatively slowly. During this lift, our motor unit recruitment levels are low, and only a proportion of the muscle fibers are active.
In this case, overall muscle force is even lower, because the external force is lower than if we moved the same weight very quickly. The external force is equal to the weight due to gravity plus the force required to accelerate the mass, and the slower peak bar speed means that the acceleration is less while the weight of the barbell is the same. However, each active muscle fiber can (and therefore does) produce a high force, because it is shortening slowly.
What happens to mechanical tension?
When we lift a light load very quickly, muscle force is fairly low despite the fact that we are exerting maximum effort. Therefore, the stretching force or mechanical tension experienced by the whole muscle-tendon unit is low. This happens because the force that can be exerted by each individual muscle fiber is very low, because of the force-velocity relationship.
When we lift a light load deliberately slowly, muscle force is even lower, because we are causing the mass to accelerate less. Therefore, the stretching force or mechanical tension experienced by the whole muscle-tendon unit is even lower. However, since the level of motor unit recruitment is greatly reduced during a submaximal effort, the number of active muscle fibers is greatly reduced, and each individual muscle fiber exerts a high force due to its favorable location on the force-velocity relationship.
As you can see, the mechanical tension experienced by whole muscles is very different from the mechanical tension experienced by each muscle fiber. Muscle force (and therefore mechanical tension) decreases from the fast lift with a light load to the deliberately slow lift with a light load, but individual muscle fiber force (and therefore mechanical tension) increases substantially from the fast lift to the deliberately slow lift.
What happens to mechanical tension as we fatigue?
When we lift a light load and experience fatigue, the weight gradually moves slower and slower until it stops completely. If we start the set using a maximal speed, then the decrease in speed is quite large, but if we start the set with a slow tempo it is comparatively small.
While lifting this light load and experiencing fatigue, the amount of motor unit recruitment may change. If we start the set using a maximal speed, then it will remain high throughout, but if we start the set with a slow tempo it will increase dramatically.
By the end of the set, regardless of our starting point, we finish with high levels of motor unit recruitment and a slow bar speed. These changes happen in order for us to be able to continue producing the same level of force to lift the weight, despite the accumulation of fatigue.
Importantly, the level of motor unit recruitment reached is very likely just as high (or nearly as high) as when lifting heavy loads, and the slow bar speed that results is essentially just as slow as when lifting a very heavy weight.
Overall muscle force is low throughout the set, because the external force is low. However, the force on each muscle fiber changes throughout the set. When using a slow tempo, the muscle fibers controlled by high-threshold motor unit will not even be active at the start of the set, and are only recruited towards the end. When they are finally recruited, they do experience high forces, since they will be shortening slowly. When using a fast tempo, all muscle fibers will be active throughout the set, but they will only experience high levels of mechanical tension toward the end when bar speed slows down and muscle fiber shortening velocity becomes similar to that achieved when lifting heavy weights.
What does this mean?
Hypertrophy happens when individual muscle fibers experience mechanical loading, not when whole muscle-tendon units experience mechanical loading.
Therefore, the levels of muscle force is irrelevant for our understanding of how muscle growth works.
Ultimately, therefore, only the force-velocity relationship matters.
However, this suggests that we should be able to produce hypertrophy of the muscle fibers controlled by low-threshold motor units by moving at deliberately slow tempos. Indeed, this would happen, if those muscle fibers were responsive to mechanical loading.
However, research suggests that very oxidative muscle fibers cannot easily grow after strength training, are not very responsive to a strength training stimulus, and do not contribute to muscle growth. This explains why training using deliberately slow tempos does not stimulate greater hypertrophy than using self-selected or fast tempos.
What is the takeaway?
Heavy loads do not produce high levels of mechanical tension on muscle fibers to make them grow. When muscles shorten slowly, they produce high forces in accordance with the force-velocity relationship, and this leads to the high levels of mechanical loading on individual muscle fibers that trigger hypertrophy (although only those fibers controlled by high-threshold motor units respond to this stimulus by growing).
We can cause muscles to shorten slowly either by deliberately moving slowly, by requiring them to exert a high muscle force (with a heavy load), or through fatigue. However, only exerting a high muscle force (with a heavy load) and fatigue involve high levels of motor unit recruitment at the same time, and therefore cause the responsive muscle fibers controlled by high-threshold motor units to grow.