The motto is, 'failure is not an option', but failure is the goal of strength training.
You want to bring your muscles to temporary failure or to the point when you could not complete another successful repetition (good form & full range of motion) on a given exercise. Exhausting the muscle in this fashion is originally thought to be the stimulus necessary to cause adaptations in muscle strength and size.
But, should you reach failure with every set you do?
In the past, I have recommended that clients to approach every set like it was the last and push to failure. Recently, I came across a 2006 study by Izquierdo et al. on, 'hormonal response to resistance training to exercising to failure' that suggests the opposite.
Here are highlights from the 11-week study that compared results of exercising to failure or not to failure:
Both groups resulted in similar gains in:
1 repetition maximum
muscle power output of arm and leg extensors
maximal number of repetitions in the squat
However, not-to-failure group showed greater increases in:
resting testosterone levels
greater reductions in cortisol (stress hormone) levels
1.) Taking each set to failure when trying to increase muscular strength, power, hormonal response may not be as important as once felt.
2.) Taking each set to failure, may actually make you more susceptible to overtraining and to decreased hormonal and muscle adaptations .