Muscle tension is critical for building high levels of strength. Reps to failure with little rest between sets may burn, but it will not make you as strong as lifting heavy weights and resting long enough between sets to come back and do it again. A group of Brazilian scientists conducted a study in the Journal of Science Medicine Sport and reaffirmed this basic principle of strength athletics. They showed that rest intervals of at least 3-5 minutes between sets produce the greatest increases in upper and lower body strength. If strength and power are your goals, then rest long enough to recover between sets.