I'm 5'7 and now 170.
I stepped on the scale for the first time yesterday in 4 months. Since I started mtn biking heavily, I've slimmed in my stomach area, however my legs have noticeably bulked, and my upper body seems more sculpted as well. So I guess I’m pretty happy with my results so far. The only strange thing is, I was expecting to see a loss in the weight column.. I definitely don't look or feel fatter than I was before, and I’m definitely in better shape than I was 4 months ago.. Is it possible that I gained 10 pounds of muscle in that time? Has anyone experienced anything similar to this?

I just started riding about 1 month ago. I'm 6' tall and I've gone from a 34 waist to a 32. I was quite surprised when I stepped on the scale and found out I've gone from 205 lbs to 210!

Pretty much like your experience. My legs are bigger, upper body "looks better", but I weigh more. I know that after a 20 or 30 mile ride I am crazy hungry, so I'm eating a lot.

Anyway, you're not alone.

yes it is possible. remember muscle weighs more than fat.. The old baseline is how do your clothes fit, if they are a little loose then you are going in the right direction..

If you can find a gym that can do a body fat exam, this would be helpful in the future..

It is certainly possible to lose fat without losing weight. It is much better to be up 20 pounds of muscle and down 10 pounds of fat (Net: +10lbs) than to just do nothing and weigh a bit less.

If you feel better and look better, you're doing the right thing. I'm in a similiar boat as you, I was 5'9" 170lbs, started cycling, after one month I was down to 160, after two months I was back at 170 but with a lot more tone and a lot less gut.

No, it's not possible to gain 10 lbs of muscle in 4 months. Unless you use the Barry Bonds method.

A pound or two of that extra weight may be fluid. Your blood volume increases when you become aerobically fit.

The only juice I consume is the orange kind

You're kidding, right?

It's possible to gain a 1-1.5 pounds of muscle a week.

Only at the beginning of a weight training program can you put on muscle that quickly. I've read that the most muscle weight gain you can expect in a year is 13 lbs. And that's on a weight training program, not a bicycle training program.

It's true that "muscle weighs more than fat", but there's no way you've gained that much pure muscle in that short of a time period.

Most likely you're feeling very hungry from all the riding, and overcompensating (eating too much). If your goal is to lose weight, you're going to have to carefully monitor and measure what goes in (food/calories) versus what goes out (energy into cycling).

Same thing happened to me I thought maybe I was eating too much but everything is fitting better and my legs are bigger especially my calves so I would not worry , I started drinking protein shakes too so I know the more protein in your diet the more muscle you will put on ..

If you're thinner and fitter- everythings going well. Some people, especially folks who had plenty of muscle in an earlier part of thier lives, will put it back really quickly in a short time. Or people with the genes for it. I 've been back on the bike for six weeks after 4 years away and my legs are ripped. Of course I was a speed skater for 15 years and have already done the worst of the work. The body remembers.
If your heart rate is recovering well and you're having fun, not bonking, just go with it. Any trainer who puts you in a weight room will tell you- the numbers go up before they go down. It takes a longer time to change your metabolism to a fat burning machine than to get nice muscles going, because you need that muscle to eat the fat.
Just wear your helmet religiously . have fun!

You don't gain muscle from cycling,if any thing you lose it. my legs look bigger but in fact they are smaller then before i started cycling. they look big because they are cut, not fat on them. if you lost all your belly fat doing no ab work you would have a six pack. no if your lifting then yea maybe you did gain muscle.

I just completed a one week biking vacation, doing 100 mile/day at over 17 MPH average.
My weight went up 5 lb.
My waist is unchanged. I do not know if I gained muscle or not.
What I do know is what I ate. I made sure to compensate for the calories burned. I felt great.
My wife said I over compensated.
A previous similar trip resulted in weight loss and I felt like sh-----.

Serious mountain biking is a great workout - more upper body strength is used, and all that up and down off the seat, using your legs as shock absorbers, etc, builds more overall muscle and tones a lot of your muscles. Also great for balance. I think it is a better overall body workout than dedicated road biking, but still would find it hard to believe you have gained 10 lbs of muscle in 4 months. Maybe half that, and as Terry mentions, more fluid volume, maybe even some increase in bone density. Who knows, but keep it up - the important thing is you're getting results.

I certainly agree with the above.

Two years ago I was about 230 lbs. Got into riding more (about 10-12 miles a day average) and seemed to lose some weight. Got back into running, continued biking, and for quite a while I seemed stuck between 205-210. But all that time people kept telling me I was losing weight, and I know my speed and endurance (in both sports) was improving. So I decided not to worry about what number the scale said. I changed my primary goal to improving my ability. When I did that I started thinking of myself as an athlete, rather than a guy trying to lose weight. I stopped exercising, started training, etc. This way, I find I am much less likely to get discouraged.

I still weigh myself daily and record the numbers, but I do so to see overall trends. I want to get down around 165-175 (currently I hover 189-193) but I don't fret if I gain a few pounds for a while.

People have to realize that not everyone is the same, Genetics play a big part in the way you put on muscle and lose fat , I have been working out at a jym for five years and have seen some people who get much faster results whether it be putting on muscle or losing weight than others. Same with mountain biking if you are doing alot of hill climbs with your bike especially if you are using a lower gear to climb you will probably start putting on muscle on your legs instead of making leaner with higher rpm.... age has a big part in it too you have alot more testosterone in your system in your late teens early twentys ...How old are you ? SFV Rider.