Research to date suggests that, when using six to eight repetitions to failure at 75-80% 1RM, there is little significant difference between training with single and multiple sets in terms of increasing either strength or muscle size. Any small differences that have been recorded indicate that a single set completed to failure encourages strength gains but subsequent sets have a slightly greater impact on muscle size. What is clear is that the law of diminishing returns applies, so you have to question if the marginal improvements in size justify the extra time and effort expended.
Like everything else in life, bodybuilding does not remain static and several cutting edge experts have redefined the boundaries of achievement. Increasingly, serious athletes are using methods that extend the set beyond the point of failure. This involves forcing the muscle to perform more work despite having experienced failure in the previous rep. In practice, you perform one last forced repetition with the help of a training partner.
This obviously calls for great commitment and high motivation but the rewards include better mass gains thanks to the greater muscular overload. Such intensive training places additional importance on the need to lift with sound technique and to incorporate sufficient recovery time into your muscle building program.