However, a 2012 research that was published in the Journal of Applied Physiology questioned that notion. Kinesiology researcher Stuart M. Phillips, PhD, from McMaster University, who led the study, and his team stated that there is very little evidence that heavier loads can induce higher muscle hypertrophy than lighter loads.
Given that everyone consumed the same amount of post-workout meals immediately after the training session, the researcher found that there were not much differences in muscle hypertrophy in the quadriceps in all three groups:
“After 10 wk of training, the quadriceps muscle volume increased significantly in all groups (P < 0.001) to 1,676 ± 198, 1,651 ± 213, and 1,633 ± 198 cm3 in the 30%-3, 80%-1, and 80%-3 groups, respectively.”
While all groups showed muscle growth, no one method is superior than the other in this study.
Now that does not mean your clients should just stick with light weights that you often see in fitness advertisements and stock photos. The amount of weight they should use would vary per individual, but it should still be challenging enough to make them grunt or sweat a bit. If your clients want to go heavy at about 70% to 80% 1RM, go for it.
Remember that your clients' goals are more important than your own preferred training methodology. Adapt to your clients.