New studies have shown that low intensity high repetition training is equally effective in increasing muscle mass as high intensity low repetition training which goes against the traditional opinion of most trainers.
A study carried out by Mc Master university performed a variety of experiments changing different elements of a resistance workout, intensity, volume and muscle time under tension.
They have found that you get similar results from low intensity high volume training that you do from high intensity low volume training of course there are advantages and disadvantages to both approaches if you find it hard to lift weights and don't like to push yourself to the very limit then the high intensity training won't be for you however if you don't have hours to train the low intensity training also won't be an effective way to train as this is very time consuming.
This study claims that the important thing is working to the point of fatigue not how many reps it takes to reach this point and that groups doing three sets of 80% of their 1RM and groups doing three sets of 30% of their 1RM had significant gains in muscle mass with little difference between the groups although the heavier group did gain more strength.
They also had a group doing 1 set of 80% of their 1RM this groups saw about half the muscle gain that the other groups did which is hardly surprising.
This study isn't necessarily perfectly accurate and I'm not sure of their sample size or composition but it is an indication that if muscle gains are your objective you don't have to go the typical gym rat route and lift big heavy things.