Why does lowering tempo affect muscle growth, but lifting tempo does not?
Contrary to the claims of hundreds of bodybuilding articles written over the last decade, the tempo of the lifting phase of an exercise does not affect the amount of muscle growth that is triggered by a workout.
However, research indicates that we can increase the hypertrophy that results from a workout when we slow down the lowering phase of an exercise.
Why does this happen?
What is tempo?
The tempo of an exercise is the speed that we choose to move. It describes the duration of time that each part of a repetition takes to perform. There are four parts to each repetition:
- The lowering (eccentric) phase
- The pause between the lowering and lifting phases
- The lifting (concentric) phase
- The pause between the lifting and lowering phases
Tempos are usually written as four-figure numbers, describing the number of seconds taken in each phase.
For example, when a lift is done with a 4-second lowering phase, a 1-second pause between lowering and lifting phases, a 2-second lifting phase, and no pause between the lifting and lowering phases, this would be written as 4/1/2/X. We usually write “X” to describe a phase in which there is either no pause, or the phase is done as fast as possible.
If we want to figure out how lifting (concentric) and lowering (eccentric) tempos affect hypertrophy, we first need to understand what stimulates hypertrophy, and how different lifting and lowering speeds might alter that stimulus.
What stimulates hypertrophy?
Hypertrophy is the result of single muscle fibers increasing in volume. This increase in volume can happen either though increases in fiber length or increases in fiber diameter.
Either way, increases in fiber volume are triggered by a mechanical loading stimulus. This stimulus is produced by the muscle fiber itself. The forces that the muscle fiber produces need to be above a certain threshold, because low forces do not stimulate hypertrophy.
Each muscle contains several thousand fibers. These fibers are arranged into groups of motor units, which are recruited in order of size, from small, low-threshold motor units to large, high-threshold motor units.
Importantly, only the muscle fibers of high-threshold motor units grow after experiencing a mechanical loading stimulus. We know this not only because of experiments involving strength training, but also because most types of aerobic exercise involve applying a high volume of mechanical loading to the fibers of low-threshold motor units, but their muscle fibers do not often grow very much in response. In fact, they sometimes even reduce in size.
Essentially, hypertrophy only occurs when the muscle fibers controlled by high-threshold motor units experience a mechanical loading stimulus that exceeds the required level of force.
How do muscle fibers produce a mechanical loading stimulus?
The mechanical loading stimulus that leads to hypertrophy can be created either when a muscle fiber is shortening (or remaining a constant length) or when a muscle fiber is lengthening.
Muscle fibers shorten when we lift a weight, and lengthen when we lower a weight. Lifting while fibers are shortening, and lowering while fibers are lengthening, are called concentric and eccentric contractions, respectively.
When fibers shorten, they only produce force actively, through the formation of actin-myosin crossbridges. When muscle fibers lengthen, they produce force actively, through the formation of actin-myosin crossbridges, and also passively, since the structural elements of the fiber resist being stretched.
Regardless of how forces are produced by muscle fibers (and therefore the mechanical tension that is experienced), muscle growth is stimulated. However, the type of hypertrophy differs according to the proportional contribution of active and passive elements to the total force.
When the mechanical tension experienced by the fiber is produced more by passive elements (the structural parts of the fiber, including titin), fibers increase in volume mainly by increasing in length, by adding sarcomeres in series. When the mechanical tension experienced by the fiber is produced more by active elements (the actin-myosin crossbridges), the fiber increases in volume mainly by increasing in diameter, by adding myofibrils in parallel.
This means that lengthening (eccentric) contractions produce greater increases in muscle fiber length, while shortening (concentric) contractions produce greater increases in muscle cross-sectional area (fiber diameter).
How is the mechanical loading stimulus determined in the lifting (concentric) phase?
In the lifting (concentric) phase, muscle fibers are stimulated while they are shortening. To achieve the required level of force for hypertrophy to occur, the muscle fibers need to work at a slow speed, because the shortening speed of a fiber is the primary determinant of the force it produces.
Slow fiber shortening speeds allow fibers to produce high forces because they involve more simultaneously attached actin-myosin crossbridges.
When a fiber shortens slowly, each crossbridge inside the fiber can remain attached for a long time, because it does not need to detach as quickly. This slower detachment rate allows more crossbridges to remain attached at the same time during the contraction, and this greater number of simultaneously attached actin-myosin crossbridges allows a higher force to be produced.
How is the mechanical loading stimulus determined in the lowering (eccentric) phase?
In the lowering (eccentric) phase, muscle fibers are stimulated while they are lengthening. To achieve the required level of force for hypertrophy while the muscle fibers are lengthening, the working speed of a muscle fiber is less relevant, although it does affect how the force is produced.
Faster lengthening velocities cause attached actin-myosin crossbridges to detach more quickly, and this reduces the active force exerted at faster speeds. This reduction in force at faster speeds is probably smaller than in the lifting (concentric) phase, because of the elongation of myofilaments that occurs when fibers are stretched.
However, passive forces are also produced during fiber lengthening, by titin and other structural elements inside the muscle fiber. These passive elements have viscoelastic properties, which means that the forces they produce are higher at faster lengthening velocities.
These increasing passive forces compensate for the reduced active forces, as fiber lengthening speed increases. This is why the force-velocity relationship of eccentric contractions is flatter than the force-velocity relationship for concentric contractions.
Even so, differences in the contribution of active and passive forces to total mechanical tension influence the degree to which fibers adapt by increasing in length or diameter. At faster lengthening velocities, more force is produced by passive elements, which causes increases in volume to occur more through increases in fiber length. At slower fiber lengthening velocities, more force is produced by active elements, which means that increases in volume occur more through increases in fiber diameter.
N.B. Practical effects of architectural gearing
In practice, it may be quite difficult to bring about fast *fiber* lengthening velocities during strength training, because the architectural gear ratio is twice as high in eccentric contractions than in concentric contractions.
Architectural gearing refers to the extent to which the fibers rotate while the muscle shortens or lengthens. The rotation of fibers allows them to shorten at slower speeds than the whole muscle. When the architectural gear ratio is very high (which it is during eccentric contractions), each fiber changes length much more slowly than the whole muscle. This high gear ratio means that even when a muscle is lengthening quickly, each muscle fiber may not be lengthening very quickly at all.
What determines how motor units are recruited in the lifting (concentric) phase?
Motor unit recruitment levels are determined by the level of effort, while factors inside the muscle and inside each muscle fiber determine the amount of force that corresponds to that level of effort.
When muscle fibers are shortening, motor unit recruitment (and effort) must be increased to lift a heavier object, to move a light object at a faster speed, or to lift a light object more times while fatigued.
However, only lifting a heavy object and lifting a light object repeatedly while fatigued involve a high level of force exerted by the muscle fibers of high-threshold motor units, because these are the only situations in which fiber shortening velocity is slow enough to allow the fibers to produce high forces.
Consequently, only lifting a heavy object and lifting a light object repeatedly while fatigued produce hypertrophy, even though moving a light object very quickly also involves high levels of motor unit recruitment.
What determines how motor units are recruited in the lowering (eccentric) phase?
Movement speed has a smaller impact on the level of motor unit recruitment when fibers are lengthening, compared to when they are shortening. This is because the force that a fiber can exert is less dependent upon its velocity when it is lengthening, compared to when it is shortening.
Force is less dependent upon lengthening velocity than upon shortening velocity because passive forces produced by titin and other structural elements increase with increasing velocity, which compensates for the decreasing active force generated by actin-myosin crossbridges.
Since the force per fiber does not alter substantially with lengthening velocity, there is no need for motor unit recruitment to change, when lengthening velocity is increased or decreased. In fact, motor unit recruitment probably only alters markedly in response to the level of force required, or to the amount of fatigue that is present.
N.B. Comparing recruitment in eccentrics and concentrics
The level of motor unit recruitment required to produce a force while fibers are lengthening is smaller than when producing the same force while fibers are shortening. This is because force is produced by both active and passive elements when the fiber is lengthening, but only by active elements when the fiber is shortening.
Therefore, if we perform movements that involve the same level of force in coupled lowering (eccentric) and lifting (concentric) phases, the level of motor unit recruitment involved in the lowering (eccentric) phase will typically be lower than in the corresponding lifting (concentric) phase.
How does tempo affect the hypertrophic stimulus?
#1. In the lifting (concentric) phase, when fibers are shortening
If we produce a given lifting (concentric) force deliberately slowly, this reduces the number of active motor units. Slower fiber shortening speeds mean greater force produced by each fiber, so fewer fibers need to be activated for a given level of force.
If we produce the same lifting (concentric) force as fast as possible, this increases the number of active motor units. Faster fiber shortening speeds lead to less force produced by each fiber, so more fibers need to be activated for a given level of force.
Hypertrophy requires the fibers controlled by high-threshold motor units to experience a mechanical loading stimulus that exceeds a given level of force. Producing a given lifting (concentric) force deliberately slowly can fail to recruit high-threshold motor units, while moving as quickly as possible can fail to load fibers with a sufficiently high force.
In both cases, a mechanical loading stimulus is only achieved if the weight is heavy (which involves full recruitment and a slow fiber shortening speed) or once fatigue arises, as this increases recruitment and makes fiber shortening speeds similar regardless of whether we are moving deliberately slow or trying to move as fast as possible.
Consequently, the choice of what tempo to use when producing a given lifting (concentric) force does not influence the amount of muscle growth that results.
#2. In the lowering (eccentric) phase, when fibers are lengthening
If we produce a given lowering (eccentric) force at different speeds, this does not have as large an effect on the number of motor units that are recruited, because the force exerted by each fiber remains far more constant, partly due to the high level of architectural gearing, and partly due to an increase in passive force compensating for the lower active force.
Whatever the tempo, high-threshold motor units will only be recruited if the force required is sufficiently high, or if enough fatigue is present such that recruitment is increased because of a reduction in force exerted by low-threshold motor units.
Even so, lowering (eccentric) tempo does affect how muscle growth occurs, and may also affect now much muscle growth occurs.
A slow lengthening tempo increases the amount of active force production, which stimulates greater increases in fiber diameter, while a fast lengthening tempo increases the amount of passive force production, which stimulates greater increases in fiber length.
This could mean that greater gains in muscle size are recorded after eccentric training with a slow tempo, because most hypertrophy researchers use changes in muscle cross-sectional area to assess muscle growth, which tends to reflect changes in fiber diameter rather than fiber length.
However, a slow lengthening tempo may genuinely increase the amount of muscle growth that occurs overall, by increasing the total time exposed to a given level of tension. This is possible partly because the force exerted by each fiber is not altered by the movement speed, and therefore motor unit recruitment is not changed by tempo, and partly because the fatigue resistance of lengthening contractions is higher than of shortening contractions, which means that slow lengthening tempos do not lead to a substantially reduced number of reps, as often occurs when using slow lifting tempos.
#3. In coupled lifting and lowering movements
This happens because the maximum force we can exert in eccentric-only contractions is approximately 25–30% greater than in concentric-only contractions. We can exert these higher forces when muscles are lengthening because of the higher forces produced by each muscle fiber, which is enabled by the contribution of passive elements to total force production.
This means that when we lift 85% of our one repetition-maximum (1RM), that weight is actually only 65% of our eccentric-only 1RM, when we come to lower it.
This has important implications for hypertrophy.
Since motor unit recruitment is less in the lowering (eccentric) phase than in the lifting (concentric) phase, the lowering (eccentric )phase of normal strength training exercises logically cannot stimulate as much muscle growth as the paired lifting (concentric) phase.
To recruit the same number of high-threshold motor units in the lowering (eccentric) phase of coupled eccentric and concentric contractions as in the corresponding lifting (concentric) phase requires eccentric overload, in which a larger force is applied to the barbell during the lowering phase, perhaps by using weight releasers.
What is the takeaway?
Motor unit recruitment is less in lowering (eccentric) phases than in lifting (concentric) phases during normal strength training. This makes the stimulating effect of the lowering phase smaller than of the lifting phase. However, we may still be able to alter the type and amount of muscle growth that we achieve, by slowing down the tempo of the lowering phase.
Slow lengthening tempos increase the amount of active force production, which stimulates greater increases in fiber diameter, and this could be why some research has recorded greater increases in muscle cross-sectional area after slow lengthening tempos, compared to fast lengthening tempos. We would probably find that the beneficial effects of slow lengthening tempos were smaller, if we measured changes in muscle volume instead of muscle cross-sectional area.
Even so, slow lengthening tempos might still genuinely increase the amount of muscle growth that occurs overall, by increasing the total time exposed to a given level of tension. Unlike in the lifting (concentric) phase, time under tension can be increased in the lowering (eccentric) phase without reducing motor unit recruitment levels, and the high levels of fatigue resistance in the lowering phase make this an ideal way to increase time under tension without also reducing the number of stimulating reps.