The takeaway: Although not the most fun way to train, low load training can be as effective as traditional muscle building approaches. For people who have joints that ache on some exercises using a heavy weight, low load training may be a viable option that nets similar results on a load easier on the joints.
The details: Most people wanting to build muscle have used the approach of higher loads at 8-12 reps that bodybuilders have used for decades. There have been recent studies showing muscle growth can occur across various loads.
Low load training is typically using around 30% of your 1 rep max (1RM). With that load, it typically takes about 25-30 reps and training to/near failure to see similar growth rates.
Putting muscle fibers under strain is what fuels growth. The typical approach of a heavier load (about 80% 1RM) recruits most the muscle fibers from near the start. Low load training does eventually recruit the majority of muscles…it just that happens towards the end of the rep range.
The pro on low load training: lower weight is often easier on the joints. The con: It is not fun; training to/close to failure over 30 reps each set is grueling. Also, it becomes tougher to progressively overload over time.
A new study recently published (“Acute Effects of Different Training Loads on Affective Responses in Resistance-trained Men.” International journal of sports medicine. 2019 Sep 9. [https://www.ncbi.nlm.nih.gov/pubmed/31499564]) confirmed a lot of this. The study compared subjects’ perceived exertion and discomfort during low-load training (25-30 reps) vs traditional load training (8-12 reps). The subjects noted more “displeasure” with the low-load training session.
Low load is not going to be something most people would adhere to as a full-time program when they can get the same results with a more enjoyable program. It doesn’t mean it doesn’t have value – it may be worth mixing in a traditional routine for exercises that higher loads cause joint discomfort.