Chris Beardsley is a biomechanics researcher. He is also the editor of Strength and Conditioning Research. He is based in Loughborough, Leicestershire.
Most trainees in the gym looking to get bigger stick to a time-honoured ‘ideal’ rep range for hypertrophy, which most experts claim is around eight to 12 reps. Since most of us looking for maximum size gains train to muscular failure, or at least within a rep or two of muscular failure, choosing a rep range means we are also making an assumption about the optimal load that we should train with, as well as just the number of reps.
In the case of training in the range of eight to 12 reps we are assuming that we need to use what researchers define as ‘heavy’ loads. These heavy loads are around 60%-65% of one-rep max, or around a 15-rep max. By powerlifting standards that’s not heavy, but that’s not the point. The point is that we have a way of talking about loads that are commonly used and those not commonly used.
We made a mistake
When researchers first started looking at resistance training nobody believed it was even possible to gain muscle with weights lighter than 15-rep max loads. So researchers originally called everything above that weight ‘heavy’ and anything below that weight ‘light’ to distinguish between them. It was assumed that light weights were useless for hypertrophy. That was a mistake.
Research has since shown that meaningful gains in muscle size can be achieved with so-called light loads. In fact, studies have found that in many cases, the amount of muscle gained with light loads is similar to the amount of muscle gained with heavier loads. So if you’re not making use of this knowledge, you could be missing out.
Clearing up terminology
Researchers, strength and conditioning coaches and bodybuilding experts all tend have different ways of talking about how heavy a load is for each trainee. Some talk about percentages of one-rep max or rep-maxes, while others use phrases like intensity or relative load. The term intensity has a long and distinguished heritage but has become confusing because of its association with effort. At the end of the day it really doesn’t matter that much what we call the weight on the bar. We just need a sensible way of relating that weight to our maximum strength on that exercise.
Mechanisms of hypertrophy
Although lots of research has been performed looking at the mechanisms of hypertrophy and how such mechanisms might be affected differently by heavy or light loads, the truth is that we really don’t know what signals muscles to grow. We have a broad idea but there is certainly no clearly-defined molecular map showing the exact pathway from the external loading stimulus all the way to the growing muscle fiber. Lots of research is being done in this area and there will be some big discoveries in the coming years that will really help to move our understanding forwards.
At the moment it is generally accepted that muscles are stimulated mainly by the application of external mechanical load and by the generation of internal metabolic stress. By looking at these mechanisms we can assess how heavy and light loads might affect hypertrophy differently.
Mechanical load is the tension developed in the muscle fibers when they either shorten to lift a weight up or lengthen to lower it back down again. Tension is developed in either phase of the lift, although it relies upon slightly different structures. In the lifting phase, what are called the contractile elements are responsible for shortening the muscle fibers. In the lengthening phase, passive elements bear some of the strain as well. So there are two reasons why heavier loads might cause a greater hypertrophic stimulus, by exposure to a larger amount of mechanical loading.
Firstly, heavier loads lead to a greater absolute tension than lighter loads. Secondly, because of the force-velocity relationship, when a greater force is generated at a joint, velocity at that joint must be lower, so long as maximal effort is applied. As a result, heavier loads require slower bar speeds and longer repetition durations, which means more time exposed to mechanical loading.
However, there is also a way in which lighter loads might lead to greater exposure to mechanical loading. Since lighter loads allow us to perform more repetitions, they generally permit a greater volume of work – load multiplied by repetitions – to be performed, which means more time exposed to mechanical loading.
The second major stimulus for hypertrophy is thought to be exercise-induced metabolic stress. This results from resistance training relying heavily upon anaerobic glycolysis for the production of ATP, or from exercise performed under ischemic conditions and which actively prevents venous return, such as blood flow restriction training. Anaerobic glycolysis leads to the build-up of metabolites including lactate, hydrogen ions, inorganic phosphate ions, and creatine, hence the term metabolic stress.
Importantly, any load above around 40% of maximum isometric force can occlude venous return and produce metabolic stress through the prevention of venous return and the consequent build-up of metabolites. So there is no need for heavier loads to trigger this effect. Lighter loads will do the job just as well. Therefore, there is a clear way in which lighter loads might lead to a greater hypertrophic stimulus through exposure to a larger amount of metabolic stress. Lighter loads, especially significantly lighter loads, allow sets that last much longer, and consequently present a far greater exposure to metabolic stress.
Weighing lighter loads
So although our gut reaction is to assume that heavy loads are essential for hypertrophy, there are some good reasons why light loads might also allow us to gain muscle too. And while we can speculate about how each of the above mechanisms might affect hypertrophy indefinitely – and researchers frequently do – there is a better way to find out what is going on.
By looking at studies that compare what actually happens to muscle size when two groups of subjects train for several weeks with either heavy or light loads, we can see whether light loads are really what they are cracked up to be. So far, there have been 12 studies comparing heavy and light loads. Of these, eight found that light loads were just as good as heavy loads, while four found that heavy loads were better. In most cases, however, the heavy load showed slightly better gains.
A formal combination of most of these studies, called a meta-analysis, was last year published in the European Journal of Sport Science and came to the exact same conclusion: there is not as much difference between heavy and light loads as we once thought. While the meta-analysis could not detect the difference in hypertrophy between the light and heavy loads statistically, there was a small numerical difference in favour of the heavy load groups.
Two extra points
If you are still unconvinced by the idea of using lighter loads, here are two bonus points. Firstly, so far as we can tell, each of us are largely stuck with the prevailing fiber type proportion in our muscles. So we have type I fibers and type II fibers and we can make each type bigger but converting one type to the other doesn’t really happen except under extreme circumstances.
While resistance training causes increases in both type I and type II muscle fibers, some studies show that training with lighter loads might cause greater increases in type I muscle fibers, while training with heavier loads might cause increases in type II muscle fibers. So mixing up both heavy and light loads could be the perfect growth solution.
Secondly, analysis of lower-body compound lifts such as squats and deadlifts shows that light or heavy loads causes a difference in how hard each of the muscles is working relative to the others. So lighter loads in compound lower-body lifts tend to be better for the quads, while heavy loads tend to be better for the glutes and hams. Again, mixing it up means that everything gets targeted at different time. Science is always interesting but what matters is what happens when it is applied. So here is the bottom line.
What it all means
Volume matters for hypertrophy. It really matters. Increase your training volume and you will get bigger. Is that guaranteed? Yes, so long as you can recover from it and don’t get injured. That’s where lighter loads can be useful. Research shows us that while they are slightly less effective compared to heavy loads, they can still work.
So use lighter loads as part of your routine. Back-off sets, light days, or just drop the weight on your second or third exercise for a body part and do more reps. You can really pump up the volume while reducing the load on your joints. What’s more the variety could be the spice that your training programme really needs.