if you want to gain muscle mass you will have to perform strength training, period. cardio fitness and strength are different and develop in different ways. you also have to eat a lot of protein and carbs to fuel those workouts and build mass. if you only want to gain that mass in your legs, then a structured training plan on the bike can accomplish this, but you will lose or limit your upper body strength. if you want to change the composition of your body overall, you will have to do both. remember, you can't just watch the scale when you are gaining muscle, weight will lie to you in terrible ways.

are you male or female? you also don't list your height. if you're a male, 54kg is crazy tiny if you want to appear muscular. for example, a lean and muscular male of 6' in your age range would be 180+ lbs. even a lean and muscular female at 5'6" would be at least 120 lbs.

tell us more, m or f? height? what is your current diet? what are your overall goals? what are these "exercises" that you're doing? plyo? weights?