Bodybuilding experts sometimes recommend that we “keep a constant tension on the muscle” during each set of a workout, in order to maximize hypertrophy.
When making this recommendation, they are usually either (1) suggesting that each repetition should be done with a deliberately slow lifting tempo, or (2) that there should be no pause between each repetition. Sometimes, they are suggesting both of these things.
However, in the context of conventional bodybuilding training with moderate loads (65–85% of 1RM or 5RM — 15RM), neither of these factors affect the amount of muscle growth that occurs after strength training, because of the ways in which motor units are recruited, and because of the factors that determine the mechanical loading on individual muscle fibers.
Let me explain.
How does altering lifting tempo affect hypertrophy?
When bodybuilding experts recommend “keeping a constant tension on the muscle” during a repetition performed with a moderate load, they almost always specify a submaximal lifting tempo.
When a moderate load is lifted as fast as possible (with maximal effort), there is an acceleration phase followed by a deceleration phase. This means that muscle force is high in the first half of the rep, and low in the second half.
It is not possible to “keep a constant tension on the muscle” and also use a maximal effort, except when we are very fatigued.
How does lifting tempo affect motor unit recruitment, and the mechanical loading on individual muscle fibers?
#1. Performing reps as fast as possible
If we perform a single rep with a moderate load as fast as possible, this requires a maximal effort. If we deliberately perform the same rep at a slower speed, this requires only a submaximal effort.
Performing single reps of a moderate load with a maximal effort has three key features.
Firstly, motor unit recruitment is very high. The central nervous system determines the force that is produced against any given load by altering the degree of motor unit recruitment. Therefore, maximal efforts involve maximal levels of motor unit recruitment. This means that more high-threshold motor units are recruited at maximal levels of effort, compared to at submaximal levels of effort.
Secondly, since maximal efforts lead to faster bar speeds (and therefore faster muscle fiber shortening velocities), individual muscle fibers must necessarily exert lower forces at maximal efforts compared to at submaximal efforts, because of the force-velocity relationship. This means that the mechanical loading experienced by each individual muscle fiber is smaller at maximal efforts compared to at submaximal efforts.
Thirdly, when reps are performed with maximal effort, there is a long acceleration phase at the start of the movement, followed by a long deceleration phase. In the acceleration phase, the force produced by the muscle on the barbell must be equal to the sum of the force due to gravity (F = mg) plus the force required to accelerate the mass of the barbell (F = ma). In the deceleration phase, the force produced by the muscle on the barbell must be smaller than the force due to gravity (F < mg), so that the barbell naturally slows to a halt at the top of its range of motion. This means that the muscle only exerts a high force in the first part of the exercise (at the point when the muscle is lengthened), after which it reduces force production, by reducing motor unit recruitment.
In summary, when we perform a single rep with a moderate load as fast as possible (and therefore with maximal effort), motor unit recruitment is very high due to the high level of effort, but the mechanical loading experienced by each individual muscle fiber is lower than at submaximal efforts (and slower contraction velocities), because of the force-velocity relationship. Whole muscle force also reduces partway through the rep, because of the need for a deceleration phase, and this is accomplished by reducing motor unit recruitment.
#2. Performing reps slowly and under control
If we perform a single rep with a moderate load deliberately slowly, this involves a submaximal effort. Performing single reps of a moderate load with a submaximal effort has three key features.
Firstly, motor unit recruitment is relatively low. The central nervous system determines the force that is produced at any given speed (i.e. against any given load) by altering the degree of motor unit recruitment. Therefore, submaximal efforts involve lower levels of motor unit recruitment than maximal efforts, which means that they do not recruit the high-threshold motor units that control the large numbers of highly responsive muscle fibers that grow after strength training.
Secondly, since submaximal efforts lead to slower bar speeds (and therefore slower muscle fiber shortening velocities), individual muscle fibers will exert higher forces at submaximal efforts compared to at maximal efforts, because of the force-velocity relationship. This means that the mechanical loading experienced by each individual muscle fiber is higher at submaximal efforts compared to at maximal efforts. However, since these muscle fibers are attached to low-threshold motor units, which tend not to grow very much after strength training, this is not relevant for hypertrophy.
Thirdly, when reps are performed with a submaximal effort, there is only a very short acceleration phase at the start of the movement, and only a very short deceleration phase at the end. This means that the force exerted is approximately equal to the force due to gravity (F = mg) throughout the whole exercise range of motion, which causes a proportional decrease in force when the muscle is lengthened, and a proportional increase in force when the muscle is shortened. Importantly, however, this tension is only applied to those muscle fibers that are controlled by the recruited motor units, which must necessarily exclude the high-threshold motor units, because the effort is submaximal.
In summary, when we perform a rep deliberately slowly (with a submaximal effort), motor unit recruitment is relatively low because of the lower level of effort, but the mechanical loading experienced by each individual muscle fiber is higher than at maximal efforts (and faster contraction velocities), because of the force-velocity relationship. Whole muscle force remains constant (at both long and short muscle lengths) throughout the rep, because there are only short acceleration and deceleration phases, and this keeps a more consistent tension on the working muscle fibers. However, since these muscle fibers are not the ones that are controlled by high-threshold motor units, this does not enhance hypertrophy.
What happens as we fatigue over a set?
Over the course of a set, fatigue accumulates, which has two effects.
Firstly, fatigue causes a reduction in the force produced by each working muscle fiber. This causes additional motor units to be recruited, which activates additional, fresh muscle fibers. These additional muscle fibers compensate for the reduced force being produced by the fatigued ones.
This is a key stage in the process by which hypertrophy occurs, since only the muscle fibers of high-threshold motor units contribute in a meaningful way to muscle growth. This is because high-threshold motor units each control exponentially more muscle fibers than low-threshold motor units, and the muscle fibers they control are far more responsive to the strength training stimulus than those of low-threshold motor units.
Secondly, fatigue causes a reduction in the shortening velocity of the working muscle fibers. This causes the force to be produced by each muscle fiber to be higher than at faster shortening velocities, because of the force-velocity relationship. (The force produced by fatigued muscle fibers is still reduced, but the force produced by any new muscle fibers activated due to additional motor units being recruited is high).
This is also a key stage in the process by which hypertrophy occurs, since mechanical loading of single muscle fibers is what causes them to increase in volume. Reducing muscle fiber shortening velocity therefore exposes the new muscle fibers that are activated due to additional motor units being recruited to high levels of mechanical loading.
#1. Performing reps as fast as possible
When we perform single reps with a moderate load as fast as possible in a non-fatigued state, this involves:
- A maximal effort, and therefore high motor unit recruitment
- A fast bar speed (and therefore a fast muscle fiber shortening velocity), which means low levels of force produced by each individual muscle fiber (and therefore low levels of mechanical loading on each muscle fiber), because of the force-velocity relationship
- A long acceleration phase, and a similarly long deceleration phase, which reduces the motor unit recruitment in the second half of the rep
However, when we perform multiple reps as fast as possible with a moderate load, fatigue accumulates, and this situation changes.
So long as we continue to exert a maximal effort, motor unit recruitment remains high. However, fatigue decreases muscle fiber shortening velocity. This increases the mechanical loading on each individual muscle fiber. Also, the amount of force that can be produced in excess of the force due to gravity reduces, so the durations of the acceleration and deceleration phases reduce, which leads to long period of constant (and high) mechanical tension on the activated muscle fibers of high-threshold motor units, which is what triggers hypertrophy.
#2. Performing reps slowly and under control
When we perform single reps of a moderate load slowly and under control in a non-fatigued state, this involves:
- A submaximal effort, and therefore lower levels of motor unit recruitment
- A slow bar speed (and therefore a slow muscle fiber shortening velocity), which means high levels of force produced by each individual muscle fiber (and therefore high levels of mechanical loading on each muscle fiber), because of the force-velocity relationship
- A short acceleration phase, and a similarly short deceleration phase, which maintains motor unit recruitment at similar levels throughout the rep.
However, when we perform multiple reps slowly and under control with a moderate load, fatigue accumulates, and this situation changes.
As the load becomes harder and harder to lift, the effort required to perform the exercise at the specified tempo necessarily increases. The submaximal effort becomes a maximal effort, because of the accumulated fatigue. As a result, motor unit recruitment becomes high. Muscle fiber shortening velocity was already slow, and the acceleration and deceleration phases were already short, so there is a long period of constant (and high) mechanical tension on the activated muscle fibers of high-threshold motor units, which is what triggers hypertrophy.
N.B. Heavy loads
The above analysis refers to moderate loads. However, the role of fatigue can also be performed by heavy weights. When relative load is high (>85–90% of 1RM), motor unit recruitment is complete regardless of tempo, the difference in bar speed between maximal and submaximal efforts is fairly minimal, and the durations of the acceleration and deceleration phases are small.
How does taking no pause between reps affect hypertrophy?
Most of the time when we are lifting weights, we subconsciously or consciously take a short pause between each rep. Short pauses have the effect of allowing the muscles a brief period of time in which to recover from the fatigue that eventually causes us to terminate the set.
Cluster sets can involve deliberately taking short pauses between each rep, often of between 10–15 seconds. This training method slows down the rate at which fatigue accumulates, and allows sets to be prolonged for longer than would otherwise be possible. Compared to cluster sets, fatigue accumulates more quickly in conventional sets (and sets are completed faster).
There are two main differences between sets performed with no pauses between reps, and sets performed with pauses (or even short rests) between reps. Firstly, taking no pauses between reps leads to reduced total reps per set compared to taking pauses. Secondly, taking no pauses between reps leads to greater reductions in blood oxygenation.
But do either of these factors affect hypertrophy?
#1. Differences in volume
Some people have observed that since inter-rep rest periods permit greater numbers of reps, this allows greater volumes to be performed, which may lead to greater muscle growth, since greater volumes are often linked to greater hypertrophy in a dose-responsive way.
However, this suggestion fails to understand the nature of the relationship between volume and hypertrophy.
The literature shows that greater volumes (number of sets to failure) produce more hypertrophy, and this result is dose-responsive up to quite high volumes. Yet, this result is not affected by the number of reps that are done in each set, when using moderate or moderately light loads. When the weight is between 5RM and 30RM, the amount of hypertrophy that is achieved is the same, although training with lighter loads involves several times more volume (sets x reps) and volume load (sets x reps x weight) than training with moderate loads, and greater increases in both volume (sets x reps) and volume load (sets x reps x weight) over a training program.
In other words, volume (sets x reps) and volume load (sets x reps x weight) are completely unrelated to hypertrophy.
This is because the only reps that produce hypertrophy during conventional strength training are those that involve a high level of motor unit recruitment at the same time as a slow muscle fiber shortening velocity, and these are probably the final five (stimulating) reps of any set performed to failure, when lifting a moderate load. Consequently, it is likely that the differences in volume (sets x reps) and volume load (sets x reps x weight) between sets performed with and without pauses with moderate loads will have no effect on the resulting muscle growth that occurs.
#2. Differences in blood oxygenation
When performing reps with a deliberately slow tempo and with no pause between them, there is a greater reduction in blood oxygenation, compared to when performing reps at a normal speed with 1-second pauses. This greater reduction in blood oxygenation is likely related to the restriction of blood flow caused by the constantly contracting muscle.
Some researchers have suggested that hypoxia might lead to the recruitment of high-threshold motor units, in order to maintain force production despite a lack of energy availability (although this might be just as easily explained by the presence of any form of fatigue). Others have pointed out that hypoxia leads to increased reactive oxygen species (ROS) production. ROS are often increased in response to strength training, and have often been linked to metabolic stress, but whether they play a key role in hypertrophy or muscle damage repair is unclear.
Even so, it is worth noting that other measures of metabolic stress are not heightened when using slow tempos. Blood lactate levels are largely the same regardless of whether reps are performed with a deliberately slow tempo and no pause, or a normal tempo with pauses, indicating that metabolite accumulation is not different. (Moreover, some studies have shown that slower tempos actually produce smaller increases in post-workout blood lactate than faster tempos, when no pauses are taken between reps).
On balance, it seems likely that many of the elevated post-workout responses that occur as a result of hypoxia are related to muscle damage. Fatiguing conditions performed in hypoxic conditions involve a sustained excitation-induced influx of calcium ions under conditions of low energy status, and this activates proteases known as calpains and phospholipases. These proteases break down the ultrastructure of the muscle cell and the sarcolemma, which leads to muscle fiber damage.
It is true that some research has been conducted showing that performing strength training under hypoxic conditions (including with blood flow restriction) may be slightly superior for muscle growth, in comparison with conventional strength training. However, training under hypoxic conditions and under conditions of external blood flow restriction typically does often enhance metabolite accumulation compared to normal strength training, so is probably not the same as training with constant tension.
What is the takeaway?
Bodybuilding experts often recommend that we “keep a constant tension on the muscle” during each set of a workout, in order to maximize hypertrophy. Superficially, this sounds plausible, but since deliberately slow lifting tempos involve submaximal levels of motor unit recruitment, this tactic cannot enhance muscle growth, since it fails to activate the large numbers of highly responsive muscle fibers that are controlled by high-threshold motor units. Only once fatigue accumulates towards the end of a set (and makes the submaximal effort maximal) are the muscle fibers of these high-threshold motor units stimulated to grow.