Why you should not underestimate the muscular benefits of bean protein
The biological value of protein from beans is not that good when you compare it with high-quality animal proteins. If you are vegan and want to build muscle mass, can you still use protein from beans? If you've read the research researchers at Arizona State University have published in Nutrients, you're inclined to answer this question with 'yes'. Even though the subjects in that study did not train.Study
The researchers experimented with 25 vegetarians and vegans aged 18-55. The subjects did not participate in sports and did little exercise. Those who moved more than 150 minutes per week were not even eligible to participate in the trial.

For 8 weeks, the researchers gave half of the subjects 18 grams of bean protein isolate every day. To be precise, the subjects received Just Egg from Just Inc. [ju.st] Contrary to what the name suggests, Just Egg does not contain protein from eggs, but protein from mung beans.The other group was given a biscuit every day that provided only 4 grams of protein. Biscuit and protein supplements had identical caloric content.
Before and after the supplementation period, the researchers determined the body composition and muscle strength of the subjects.
At the end of the supplementation period, the subjects who had received bean protein had gained a few ounces of extra muscle mass. In addition, they had become a bit stronger. This had not happened with the subjects in the other group. However, these differences between the groups were not statistically significant.

Click on the figure below for a larger version.

When the researchers looked at the subjects individually, they saw that 45 percent of the bean protein group had gained more than half a kilogram of muscle mass. This was the case for only 7 percent in the control group.

When the researchers swept the two groups together, looking at the changes in their lean body mass and muscle strength, they saw (as you might expect) a clear association between the two. An increase in lean body mass was accompanied by an increase in muscle strength.
"Since greater muscle mass allows for greater force production capabilities and muscular strength, future research should focus on quantifying the appropriate protein intake for individuals consuming mainly plant proteins", write the researchers.

Nutrients. 2019 Oct 11;11(10):2423.