Why does strength training with high reps improve muscular endurance?

Researchers recently discovered that we could increase muscle size by training using light weights, with a high number of repetitions (reps), so long as we took sets to muscular failure (the point where we can no longer perform one more rep).

What’s more, they found that this type of training with light weights and a high number of reps could produce similar muscle growth to traditional strength training with heavy loads.

But even though training with heavy weights and light weights to failure produce similar muscle growth, they do *not* produce identical effects in every respect.

In fact, training using light weights with a high number of reps causes proportionally greater increases in “repetition strength” (which is our ability to perform a number of reps) but smaller increases in “maximum strength” (which is our ability to lift the heaviest possible weight), compared to training with heavy weights.

Why does this happen?

Well, somewhat surprisingly, we still do not really know for sure. But there are a few possibilities. Here are the most likely ones.


#1. Changes in muscle fiber type

Some researchers have suggested that muscular endurance is improved more after using light weights for high reps because of a proportionally greater increase in type I muscle fiber area.

In theory, this makes sense, because type I fibers are less fatiguable than type II muscle fibers. So if type I fiber area increased more after training with light weights for high reps, then our muscular endurance should improve.

Also, this idea is supported by studies showing that endurance athletes are typically able to do a higher number of reps to failure with a given percentage of one repetition-maximum (1RM) in comparison with strength athletes, and they also display a greater proportion of type I fibers. So having a larger proportion of type I fibers is related to better repetition strength.

Unfortunately, most studies have shown that whether we use heavy weights for a low reps, or light weights for high reps, the changes in type I and type II fiber area are similar.

So the effect of type I fiber proportion on muscular endurance may be largely genetic, because it does not seem to be modified very much by strength training.


#2. Changes in muscle capillarity

Capillaries are blood vessels that deliver blood from the arteries into the muscles. Muscle capillarization measures are therefore assessments of the number of capillaries that either surround or come into contact with each individual muscle fiber.

Muscle capillarization is an important determinant of oxygen delivery to the muscle fibers, and consequently of oxidative capacity, which influences muscle endurance. Indeed, it has long been established that aerobic exercise causes the formation of new capillaries in a process called angiogenesis.

For a long time, it was thought that capillarization did not increase after strength training, or only occurred to a minor degree. Now, we think that meaningful angiogenesis does occur, but only to achieve a constant ratio between the number of capillaries and the muscle cross-sectional area or fiber perimeter, which are the best measures of the ability of the blood vessels to provide oxygen to the muscle.

Yet, whether angiogenesis happens to a greater extent after training with light weights and high reps is still unknown. And since recent studies are showing that capillarization measures are quite closely related to fiber size before and after strength training, it seems unlikely that a larger degree of angiogenesis causes the greater gains in repetition strength after training with light loads and high reps to failure.


#3. Increases in buffering capacity

The buffering capacity of a muscle is its ability to deal with the hydrogen ions that are produced during ATP hydrolysis and anaerobic glycolysis, which is necessary for fuelling muscle fibers during sets of strength training.

Buffering capacity has been linked to superior repeated sprint performance in a range of populations, but it does not seem to be improved after after training with light weights and high reps, even when muscular endurance itself is improved.


#4. Increases in the rate of ion transport

In resting muscle cells, sodium ions are maintained at low levels, while potassium ions are maintained at high levels, which leads to an electrical potential across the membrane of the muscle fiber.

When the central nervous system sends a signal to the neuromuscular junction, this causes a electrical potential to propagate along the muscle fiber, which moves these ions through the membrane. After this happens, a “sodium-potassium pump” is activated to reverse the process.

Researchers have found moderately-strong relationships between the changes in muscular endurance and changes in the concentrations of this sodium-potassium pump after strength training, which indicates that this could be an important mechanism by which muscular endurance is enhanced.

Yet, we still do not know whether training with light weights and high reps would produce greater improvements in this factor than traditional strength training.


What is the takeaway?

We know surprisingly little about the ways in which “repetition strength” or muscular endurance is improved after training, and we know even less about how it is affected differently by training with either light loads and high reps or heavy loads and low reps.

Muscle fiber type, capillarization, buffering capacity, and the rate of ion transport have all been investigated and remain possibilities, but a greater deal of work is needed before we can identify which of these are actually contributing to the effects we observe.