mabey after a while it makes you lose muscle mass but I just started mountain biking about 4 1/2 months ago and I have gotten 100% stronger, proved to myself by getting up stuff that I couldnt get up when I started (by the way feels great!) so wouldnt this mean I am gaining muscle mass? When I started I just got over a severe asthma period of about 2 years where I couldnt do anything but watch TV and play video games so I was basically a walking peice of fat.