The need for using heavy weight in order to build muscle is a fairly controversial subject. Often, information regarding this subject gets muzzled by people with opinions on either side of the fence.
Some say using heavy weight is necessary while others attribute muscle growth to other factors such as total volume and frequency.
As it turns out, both are probably right, it just depends on context.
Building muscle is a way of adapting to the stress of exercise
Building muscle is a fairly complex process that has many different variables.
The leading theory behind how you actually build muscle regards increasing something called protein synthesis, and increasing it often.
When you exercise, you place your body under stress. As a result, the body increases a process called protein synthesis; a rate of constructing new components of muscle that leads to an increase in muscle size and strength.
When you first begin exercising, this process happens pretty easily. Almost any stress you place your body under will result in this adaptation process.
However, once you begin to be more and more adapted to the stress of working out, it becomes increasingly difficult to trigger this increase in protein synthesis.
When you lift weight, your body has a threshold for increasing the rate of protein synthesis, and thus muscle. Basically, if you don't push past this threshold, you wont trigger a response from the body to build muscle. This is because it's not a large enough stressor to force an adaptation.
The Form of Stress You are Under, Determines How You Build Muscle
As it turns out, triggering the process of protein synthesis, isn't always dependent on the weight you use, but the forms of stress you place on the muscle.
A study by Schoenfeld, et al in 2015 compared two groups executing the same exercises. One group used 30-50% of their 1 Rep Max (1 RM), while the other used 70-80% of their 1 RM.
Surprisingly, their was no difference between the groups in terms of increasing muscle size.
When you lift weight, your body has a threshold for increasing the rate of protein synthesis, and thus muscle. Basically, if you don't push past this threshold, you wont trigger a response from the body to build muscle. This is because, it's not a large enough stressor to force an adaptation.
Interestingly, based on the amount of weight you are using, the threshold varies.
If you are using weight that is light (30-50% of 1 RM), the response is dependent on approaching muscular failure (i.e. doing a lot of repetitions). If you are using heavier weight, you'll need to approach failure with a heavier weight, at least above 60% of your 1 RM.
The Weight You Use Affects More Than Just Building Muscle
While using light and heavy weight correctly appears to result in the same amount of muscle, your intended goal matters significantly.
For instance, if your main goal is to increase strength, you'll need to use heavier weight (higher than 60% of 1 RM).
While the study by Schoenfeld didn't show significant differences between increases in muscle size, the study did reveal that the group using heavier weight showed the greatest increases in strength. Which isn't surprising. If you expect to be able to lift heavy weight, you'll need to do so often.
Additionally, the lower weight group showed much more increase in muscular endurance when compared to the heavy weight group.
The conclusion? The weight you use, drives the adaptation when you've surpassed the relative threshold.
Additional Notes on Building Muscle
Earlier I mentioned that these responses are an adaptation to stress. While this is true, eventually you'll adapt and the weight and/or reps you used will not produce the same response it once did. This is when you need to employ the concept of progressive overload or constantly either increasing, weight, repetitions or both.
When using a lighter weight, you should consider using a technique called Blood Flow Restriction. By occluding muscles near the proximal end of the limb and using a lighter weight, lifted to failure, you can promote muscle growth. The catch is you can receive similar adaptations while using lighter weights and fewer reps.
Do You Need To Lift Heavy To Build Muscle?
Depending on your goal, you may need to use heavy weight. When performed correctly, both light and heavy weight can result in building muscle, but the ability of that muscle will vary based on your training. The most important thing is to make sure you are triggering an adaptation response.
If you are using lighter weight as opposed to heavy, you'll need to make sure you approach muscular failure in order to induce an increase in protein synthesis.