If you ask a personal trainer, bodybuilder, or fitness expert, chances are they will tell you that intensity (lifting heavy weights for low reps) is key to stimulating muscle growth, while volume (lifting light weights for high reps) is required for definition. It’s commonly accepted that low-intensity, long-duration aerobic exercise generally cannot result in significant muscle gains, if any at all. Wikipedia echoes this widespread belief:

“It is generally believed that if more than 15 repetitions per set are possible, the weight is too light to stimulate maximal growth.”

This is why the commonly prescribed regime for muscle mass is 8-12 reps at >70% of one repetition maximum, while for shape and definition the prevailing approach is that of 15-25 reps to failure using lighter weights.

But is there any research to support the notion that heavy weights are indeed required for muscle increase? Has anybody ever proven that lighter weights are similarly, if not more effective, in stimulating muscle growth? The truth is that current beliefs on the effectiveness of heavy weight training are based mainly on empirical evidence. From a physiological point of view, we know very little as to how heavier training loads would result in greater muscle hypertrophy than lighter weights lifted to the point of fatigue.

Research has come to challenge and stir debate on what we have so far considered a requirement for muscle hypertrophy. According to recent studies, it appears that heavy resistance (high intensity) is not a pre-requisite for muscle hypertrophy. In fact, volume, not intensity, could be where the secret to fast muscle growth lies.

High Load vs Low Load

In a 2010 study, researchers from the McMaster University (Canada), and the University of Nottingham (UK) attempted to determine how intensity and volume affect muscle growth. Specifically, they compared which of the two –intensity or volume – triggers the highest protein synthesis, anabolic signalling and gene expression in the muscle tissue.

In this study, the researchers asked 15 men to perform 4 sets of leg extensions either with high intensity or with high volume. All men were familiar with this type of exercise as they engaged in lower body resistance training for more than 6 months before the experiment.
The participants performed the 4 sets of leg extensions, resting for 3 minutes between each set, in two different ways as far as the intensity and volume are concerned. In the first way, the intensity was set at 90% of 1 repetition maximum (1RM) and the exercise was performed until failure (90FAIL). In the second way, the intensity was set at 30% of 1RM until failure (30FAIL). Failure was reached when an additional full repetition could not be completed due to fatigue.

As it was expected, in the 90FAIL condition the weight the participants could lift was heavy (80kg) but they could lift it for only a few times (5 reps on average) in each set. In the 30FAIL condition, though, where resistance was set 3 times lower (28kg), the times the weight was lifted increased 5 fold (24 reps). The exercise volume of each condition, which was determined by multiplying the repetitions accomplished by the load (kg) lifted, was higher in the 30FAIL condition (1073) than in the 90FAIL one (710).

Protein Synthesis Rate Depends More on Volume than Load

We know that weight lifting stimulates the production of skeletal muscle proteins. As a result, our muscles grow (hypertrophy). Therefore, by measuring the rate of protein synthesis we can indirectly assess how exercise may affect muscle size.
There are two types of muscular hypertrophy: Myofibrillar, which refers to the contractile component of the muscle cell and results in strength increase rather than size increase, and Sarcoplasmic which refers to the non-contractile component of the muscle cell and results in size increase rather than strength increase.

Researchers examined whether intensity (90FAIL) or volume (30FAIL) resulted in greater activation of protein synthesis in the muscle cells of the participants’ quadriceps. Specifically, they looked at both myofibrillar and sarcoplamic protein synthesis. The protein synthesis was measured at rest (before the exercise), at 4 hours-, and 12 hours-post exercise.

As the graph shows, at 4 hours, myofibrillar protein synthesis was elevated in both conditions to almost the same degree. However, protein synthesis was sustained at 24 hours only in the 30FAIL protocol.

Similar results were obtained when sarcoplasmic protein synthesis, and several genes and proteins involved in anabolic signaling were studied.

What does this mean? Simply, low-load high volume weight lifting (30FAIL) is more effective at activating muscle protein synthesis than high-load low volume weight lifting (90FAIL).

Does this mean that low intensity high volume resistance training makes a muscle bigger and stronger, too? Perhaps, it does. In order to find out if this unexpected effect of low-load high volume on protein synthesis leads to increased muscle size and performance, researchers did another experiment.

Resistance Training Load does not Determine Muscle Gains

Published in July 2012 in the Journal of Applied Physiology, this second research employed 18 men. They performed 3 sets of leg extensions 3 times a week for a period of 10 weeks. Subjects trained their legs in each set to the point of fatigue with 30% of 1RM (30RM) or 80% of 1RM (80RM) emulating a condition of high volume and high intensity training, respectively. Pre- and post-training measures of the participants’ leg’s strength and muscle volume were carried out.

As the graph shows, after 10 weeks, both legs that trained with high intensity and legs that trained with high volume exhibited significant increase in quadriceps volume. What’s interesting though is that this increase was the same between the two conditions, amounting to 7%.

But volume was not the only thing that changed in the subjects’ legs. Performance (muscle function) increased, too. Specifically:

• The maximal load that could be lifted (1RM) after the training increased in both conditions. The increase in 1RM strength was greater in the 80RM condition than the 30RM.
• Participants in both conditions exhibited the same increase in the number of repetitions at 80% of 1RM.
• Interestingly, only legs in the 30RM condition exhibited an increase in the number of repetitions that could be performed with 30% of 1RM.
• The size of both type I and type II fibers increased equally with the heavy and light weights.

What Does This All Mean for You?

Of all these findings, the most interesting and unexpected is that hypertrophy in the 80RM (high intensity) and 30RM (high volume) conditions was equivalent. This finding flies on the face of many experts who recommend that heavy loads are necessary in order to optimally provide the signal for muscle protein synthesis. Of course, these studies do not eliminate the importance of load in the process of muscle increase. They just prove that protein synthesis is not entirely dependent on the load, as we have so far thought. The exercise volume is an important determinant, too. In fact, it appears that one can achieve better muscle stimulation by sacrificing the load (intensity) for the sake of volume.

Intensity and volume are mutually exclusive. If you increase the one, the other automatically decreases. This is human physiology. Which of the two do you focus on for muscle increase? This new piece of research shows that working at the lower end of your maximum intensity (low-load contractions performed with numerous repetitions until failure) stimulates muscle growth equally to (if not more than) working at your higher end of intensity (high-load contractions performed for fewer repetitions).

How Can The Findings Be Interpreted?

It has been proven that an isometric contraction performed at 30% of 1RM will recruit fewer muscle fibers than a contraction preformed at 80% of 1RM. Then, why do light weights produce the same hypertrophy as heavy weights? Shouldn’t a high load be more effective? Well, one should not neglect the effect of “working until failure“.

According to the researchers, when you perform a high repetition set of light weight lifting until exhaustion, you reach a point where the first motor units that engaged in the motion get fatigued and produce less force or cease firing altogether. This necessitates the involvement of new motor units in order to sustain force generation. As more and more fibers get involved and your muscle reaches the point of ultimate fatigue, nearly all motor units have participated in the motion. Based on this notion, lighter weights lifted to failure can produce the same amount of muscle fiber activation as heavier loads lifted to failure.

This theory is supported by the fact that, in the second experiment, both type I and II fibers increased equally regardless of the intensity. Traditionally, we know that high resistance (anaerobic) training activates type II muscle fibers while low resistance (aerobic) training activates type I fibers. The closer one gets to their strength limits, the more type II (fast switch) fibers one activates. Yet, according to the researchers’ speculation, performing a set as light as 30% of your 1RM until fatigue can progressively result in maximal motor unit activation as more and more fibers are called on duty to replace the ones that fatigue. The end result is that the muscle gets stimulated to a degree comparable to high load (intensity) activation.

These new findings shake the long held view of how resistance training induces muscle anabolism. Apparently, the traditional dogma of “lift heavy for muscle gains” is, at the very least, not the only way to stimulate muscle growth effectively.

What are your thoughts on the role of intensity and volume? When it comes to muscle hypertrophy, what has best worked for you: 20 reps to failure or 8 reps to failure?