What is the minimal load that you need to use to build muscle?
Recent research looking into the minimum load required to stimulate muscle growth shed some light on this question. The authors were able to come up with a suggested threshold.
4 groups performed leg and bicep exercises with different 1RM % loads. The results show that even with loads as low as 40% you can still get significant results comparable to training with 80%.
The threshold at which gains diminished was 20% of 1RM.