High-rep training to failure increases muscle protein synthesis more than high-weight, low-rep training. Danish researchers found that failure training was not necessary to maximize muscle activation. They compared muscle activation levels with electromyography (measures electrical activity of the muscles) during heavy weight, low-reps sets with sets to failure. Initial activation was greatest when lifting heavy weights. Muscle activation increased progressively during the failure sets, but reached a maximum at 3 to 5 repetitions before achieving failure. This study showed that pushing muscles to the max is an effective way to activate and train muscle fibers, but it is not necessary to push muscles to absolute failure. The take-home message is to train hard, but not too hard.
Source: (Journal Strength Conditioning Research, 26: 1897-1903, 2012)