I am not sure if this is the right place for this question. What are the effects of too much exercise?

One of the pups from my last litter is very gangly and weedy compared to his brothers and sisters. He is much smaller than I would have expected an 8 month old puppy to be and just looks a bit weedy.

I always advise my puppy owners not to do a lot of exercise. I was speaking to this lads owner today about it and she said she only takes him on a very short walk about 1/2 a mile. Then while we were chatting it came out that she plays fetch with him with a tennis ball for at least 30 minutes plus a day. She says he never gets tired. Would you say that this is too much for a young pup? could this contribute to his poor development?  Is it too late to reverse the effects? if she stops with the ball playing is it possible for him to catch up?

I don't know about whether it s affected his development and obviously it depends how strenuous the fetch game is, but I think it's too long to play fetch with a dog that age.

I'm not sure what breed you're talking about and can only comment on my own, where the general rule of thumb is 5 minutes of enforced exercise per month of age up to a year old. Throwing a ball for a pup can seem like a good way of exercising it but it can play havoc with young joints and isn't anything I'd encourage. You say he looks weedy, has she been following a correct worming regime?

Way back in 1984 I had my first puppy. My previous dog had been an adult rescue. When he was something like a year old, I met the breeder at a show with his littermate. (My dog's littermate, not the breeder's......) Same sex.  The breeder's dog was a LOT bigger than mine, a LOT heavier and had MUCH thicker legs -his were like tree trunks, my dog's were like Whippet legs in comparison (okay exaggeration -these were Golden Retrievers). The breeder told me I had clearly exercised my pup too much. This was the first time I ever heard about not overdoing it with pups -and unfortunately the breeder didn't tell me BEFORE I went ahead with plenty of walks etc.

your description is exactly like my pup. So is it too late at 8 months to undoo the damage? If I can persuade her to drop  the ball throwing  could he fill out?

I agree that over-exercising can do this to dogs but I always give my pups plenty of free-running on grass when young.  I just make sure they are getting plenty of very good food and have never had this problem with large breeds.  I did have a cocker who wasn't a very good eater and at 5 months, she went through a horrible, gangly stage due to exercising without eating the proper amounts/food.
I do think nutrition has far more to do with this than over-exercising.

Which breed?

I think you may well be right cracar. Our current pup, a GR, is fed a much higher quality food then our previous GR pups (foods have improved hugely since we last had a pup). She is not over exercised on lead walking which, I believe, is where the five minute rule applies, but has a good run per day in fields close by occasionally with another dog. She's a much chunkier looking dog then previous and is remaining that way despite growing fast - has enormous paws!

I believe, is where the five minute rule applies, but has a good run per day in fields close by occasionally with another dog.

I think this is where people get confused isn't it? Continual walking without rest should be kept to the golden rule of 5 mins per month, but free running in a back garden etc should be left to the pup, a pup will have a mad spurt and then rest. 'Forced' games are making a pup do more than it's body and legs are ready for.

It is perfectly ok for a pup to have it's 10 min lead walk, then if not turning back, to carry it the rest of the way if you have other dogs to walk too, but it is also ok to then put the pup down again for a play with other dogs if sitting in a field, woodland etc for free play and then carry the pup back home.  It is the continual walking and the 'forced' play which can cause damage, along with allowing pups to run up and down stairs. :-)