Why slowing down the eccentric phase of your strength training exercise does not cause more hypertrophy
Many bodybuilders continue to tell us that the downward or lowering (eccentric) phase of the exercise is at least as important (if not more important) than the upward or lifting (concentric) phase. Yet, this is physiologically impossible. Let us explore why this must be the case.
What is the main difference between the lifting (concentric) and the lowering (eccentric) phases of an exercise?
In the lifting (concentric) phase of an exercise, the working muscle fibers contribute force to the lift by the formation of actin-myosin crossbridges. In contrast, during the lowering (eccentric) phase, the working muscle fibers contribute force by [1] the formation of actin-myosin crossbridges, and [2] the passive tension generated when we stretch structures inside the muscle fiber that resist lengthening, such as titin.
Consequently, the force produced by each muscle fiber during the lowering (eccentric) phase is greater than the force produced by each muscle fiber in the lifting (concentric) phase. In animal studies, the increase in force per fiber owing to the addition of the passive tension varies but can quite easily reach 100% or even more. And in human muscle fibers assessed in vitro, the increase in force per fiber is 80–90%. Since these numbers are assessed relative to isometric contractions (in which crossbridge force is higher than in concentric contractions), we might expect that the extra force produced by a muscle fiber in the lowering (eccentric) phase is approximately the same as the force produced by each muscle fiber in the lifting (concentric) phase. In other words, muscle fibers produce twice as much force in the lowering (eccentric) phase compared to in the lifting (concentric) phase.
Since the force per muscle fiber is twice as high in the lowering (eccentric) phase as in the lifting (concentric) phase, this means that the level of motor unit recruitment and therefore the number of activated muscle fibers must be approximately 50% in the lowering (eccentric) phase as in the lifting (concentric) phase. This is not speculation but arithmetic. And since we need to activate a muscle fiber in order to cause either radial growth by the addition of myofibrils or longitudinal growth by adding sarcomeres, this tells us that the lowering (eccentric) phase can only provide a hypertrophy stimulus to the bottom 50% of the motor unit pool (and we know that the first 30% of the motor unit pool cannot grow since it represents the low-threshold motor units that are used during daily life activities). Evidently, some more simple arithmetic tells us that there is only 20% of the muscle that can be trained in the lowering (eccentric) phase.
N.B. In vivo studies show that maximal eccentric torque measured in a dynamometer is approximately 1.4 times maximal concentric torque on average. Clearly, this is a lot lower than the levels observed in vitro. The reason for this discrepancy is the reduced level of motor unit recruitment that we can achieve during maximal eccentric efforts (most likely because of the increased cognitive load that we experience while the brain struggles to coordinate the unfamiliar eccentric contraction). This does not affect the calculations for the single muscle fiber force because the lower motor unit recruitment is the factor responsible for the lower eccentric force, which obviously reduces the number of activated muscle fibers.
What happens when we slow down the lowering (eccentric) phase of an exercise?
The force-velocity relationship of individual muscle fibers in the lowering (eccentric) phase is fairly flat. Therefore, it does not matter how quickly or slowly we move, the force per muscle fiber is always relatively similar. Yet, the level of motor unit recruitment necessarily increases slightly when we slow down the lowering (eccentric) phase of the repetition, simply in order to apply more braking force to the weight and reduce its downward speed. (Obviously, there would be a big reduction in motor unit recruitment if were to literally just drop the weight, but we are talking about different speeds of controlled lowering phases here).
Nevertheless, we cannot ever exceed approximately 50% of the motor unit recruitment that we achieve in the lifting (concentric) phase, because we necessarily reach failure in the lifting (concentric) phase when we hit 100% of motor unit recruitment. Once we hit failure in the lifting (concentric) phase, the subsequent lowering (eccentric) phase motor unit recruitment level will still only be 50% of that value. Thus, it is not possible to claim that slowing down the lowering (eccentric) phase of the repetition can help us bypass the limitation imposed by the load on the bar.
That being said, so long as we do not produce excessive fatigue during the lowering (eccentric) phase of the repetition, we can exposure the working muscle fibers to a higher dosage of (active and passive) mechanical tension and thereby cause them to grow to a greater extent. This is why some studies have reported that slowing the lowering (eccentric) phase of the repetition can cause extra hypertrophy in comparison with faster lowering (eccentric) phases. Whether this is really going to be meaningful in the long-term, however, is unlikely. The activated muscle fibers are necessarily located in the bottom half of the motor unit pool, which is why we see preferential slow twitch muscle fiber growth after using slow lowering (eccentric) phases, and this means that the long-term capacity for the hypertrophy of these muscle fibers is very limited.
Importantly, slowing down the lowering (eccentric) phase of the repetition to the point where (calcium ion-related) fatigue mechanisms develop will quite likely have a very negative impact on hypertrophy. In contrast to the metabolically inefficient concentric contractions, eccentric contractions do not produce much metabolite-related fatigue (which does not suppress the mechanical tension generated by each muscle fiber). Instead, they mainly produce calcium ion-related fatigue (which does suppress mechanical tension). For this reason, if we start to lose repetitions to failure because of our slow lowering (eccentric) phases, this necessarily indicates that the lowering (eccentric) phase fatigue is reducing our ability to generate mechanical tension and this will mean that we perform fewer stimulating reps over the course of the entire set.
What does this mean?
Since the force per muscle fiber is twice as high in the lowering (eccentric) phase as in the lifting (concentric) phase, this means that the level of motor unit recruitment and therefore the number of activated muscle fibers must be approximately 50% in the lowering (eccentric) phase as in the lifting (concentric) phase. Since muscle fibers must be activated in order to cause hypertrophy, this greatly limits the ability of the lowering (eccentric) phase to stimulate muscle growth. Since we always reach failure on the lifting (concentric) phase of the repetition, we cannot increase this level of motor unit recruitment using a slower lowering (eccentric) phase. Therefore, the best we can do by slowing down this phase is to apply a little more stimulus to a small number of slow twitch muscle fibers at the very bottom of the high-threshold motor unit pool, whose capacity for growth is very limited.
