I've added 300 push ups per day for the last few months to my routine. Is this building muscle mass?
Most likely if you're able to do 300 a day then the answer is YES, probably, but NOT significantly, it isn't build any significant mass. If your able to do 300 push ups then, for your body weight / strength ratio, it's more than likely below 15% of your 1RM in say bench press. Therefore, this activity is best described as an aerobic or endurance training without adding much in strength or mass.
When you go beyond >15% of your 1RM then there is little measurable increase. With the traditional Push up, you're lifting around 65% of your body weight. I assume that if you're doing 300 that you're probably doing 50-75+ in each set. There has bee considerable amount of research on this subject. This article explains it very well.
In order to build mass you need to be lifting at a higher weight / lower rep ratio. With extreme of the bulk building exercise philosophy, typically this means lifting weights that are around 85%-95% of your 1RM with a lot of sets resting around 2-3 min between and targeting around a maximum of 6 in each set. You could adjust it to as low as 50% of your 1RM, though there may be some, there wouldn't be much of an increase in mass.
Thant said push ups can increase muscle mass under certain circumstances. It's all relative to your 1RM and your body weight. For example, if the person doing push ups can only do =<25 at a time. This follows the high weights/less reps model. So, if you're only able to do less than 25 per set than this could definitely have a measurable impact on mass/strength.
Nutrition also plays an important factor. For example it your not getting enough protein your body will not have enough resources to build mass with. See protein calculators like this one for more info. Personally, I think it over estimates how much you need but it gives you a general idea.
Some endurance activities will have some measurable effect on strength but when it is this low of a percentage relative to your 1RM, it will have very little effect on mass/strength.
To sum it up, it's all relative you your 1RM how much any given exercise will potentially increase your mass. The lower the percentage of weight relative to 1RM the lower the mass and strength gains. The gains in strength will decrease exponentially as the weight goes down.
For More Information See: