Eccentric tempo (bodybuilding)

Slow lowering (eccentric) tempos are often used in bodybuilding to increase the amount of hypertrophy stimulated per set. But how do they work?

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#1. Does a slower eccentric tempo cause more hypertrophy?

Some studies have reported that using a slower eccentric tempo during normal strength training can increase gains in muscle size.

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Yet, not all studies have reported significant effects, which suggests that either [1] the effect varies between lifters, or that [2] the effect is relatively small and may not make that big of a difference.

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Even so, the effect has been observed to a greater degree in some muscles than in others, which may be because some muscles are more susceptible to stretch-mediated hypertrophy than others.

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Indeed, in the above study, the vastus medialis displayed a greater response to the slow lowering (eccentric) phase than the vastus lateralis, and the vastus medialis is known to have a much longer descending limb than the vastus lateralis, so is more susceptible to stretch-mediated hypertrophy.

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#2. How does a slower eccentric tempo cause more hypertrophy?

The greater dosage of passive mechanical tension during the slow lowering (eccentric) phase is likely made possible by three factors: [1] the fairly flat eccentric force-velocity relationship of single muscle fibers, [2] the slightly greater external forces required to lower a weight more slowly, and [3] the reduced fatigue that occurs in the lowering phase of a normal strength training exercise compared to the lifting phase. The flat eccentric force-velocity relationship of single muscle fibers means that activated muscle fibers produce similar forces at all lowering tempos, and the slightly greater external forces required to lower a weight more slowly means that more muscle fibers are activated.

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Additionally, fatigue is minimal during lowering (eccentric) phases in conventional strength training. Hence, the extra time spent lowering the weight does not really interfere with the ability of the lifting (concentric) phase to stimulate muscle growth, so long as the duration is sensible.

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The relatively low level of fatigue during the lowering (eccentric) phase of normal strength training exercises occurs for two reasons. Firstly, it occurs because eccentric contractions are very efficient and hence use little ATP. Since they use less ATP, they produce far less metabolite accumulation. Thus, the extra amount of metabolite-related fatigue is small during slow lowering (eccentric) phases.

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Secondly, it occurs because the lowering (eccentric) phase of normal strength training reps involves a low level of muscle activation. Since it involves a low level of muscle activation, the fast twitch fibers are not activated and therefore any calcium ion-related fatigue cannot easily occur. The accumulation of calcium ions inside the slow twitch fibers is prevented by the larger number of mitochondria inside the more oxidative, slow twitch fibers. Therefore, the amount of calcium ion-related fatigue is also relatively small in slow lowering (eccentric) durations.

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#3. Can the slow eccentric phase duration be excessively long?

When the lowering (eccentric) phase duration is lengthened to 6 seconds, the number of reps that can be performed to failure in the lifting (concentric) phase is reduced. This indicates that there is fatigue occurring in the lowering (eccentric) phase that negatively affects the concentric phase stimulus. This is not something that we want to do because it is only in the concentric phase that we are able to train the muscle fibers of the high-threshold motor units.

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Conclusions

Slow lowering (eccentric) tempos during normal strength training are often used by bodybuilders. Indeed, some studies have shown that they enhance the amount of muscle growth that can be achieved. It seems likely that this enhanced muscle growth occurs by way of a greater dosage of active and passive mechanical tension. This extra dose of mechanical tension is made possible because [1] the eccentric force-velocity relationship is fairly flat (such that total mechanical tension is similar across a range of tempos, and the only factor that differs is the duration of time for which the mechanical tension is experienced), because [2] the slower lowering duration means that slightly higher external forces are produced and so greater motor unit recruitment is required (which leads to a stimulus being applied to more muscle fibers in the lowering phase), and because [3] the fatigue in the lowering phase of a normal strength training exercise is small (owing to the relatively low level of motor unit recruitment and the efficient nature of eccentric contractions) such that longer time spent in the lowering phase does not produce fatigue that interferes with mechanical tension produced in the lifting (concentric) phase. Yet, more fatigue does occur if the slow lowering tempos are longer than 4 seconds, and this fatigue will impair the stimulus achieved in the lifting phases. Therefore, less than 4 seconds is likely an optimal duration for slow lowering tempos for hypertrophy.​