The study shows claims there is an increase in "strength endurance". They gave each person a 42.5 KG barbell in a smith machine to squat. The placebo group increased their squatting by 9 reps at the end of the study. The astaxanthin group increased their squats by 27 reps at the end of the study. So at the same strength output, they could do it more times = strength endurance.
The astaxanthin group also gained half the weight of the placebo group. (1 vs. 2.1)...not sure this really applies too much to muscle building, but it may be helpful to runners or even sprinters.