Training intensityIntensity · Percentage of 1RM

Training intensity is how demanding a load is relative to your maximum. In strength training it is normally written as a percentage of 1RM: a set at 80% means the bar is loaded to 80% of the most you could lift once. Confusingly, cardio uses the same word for a percentage of maximum heart rate or VO2 max, which is why it is worth saying which you mean.

It matters because load and reps trade off against each other. Heavy, low-rep work at high percentages drives maximal strength through largely neural adaptations; moderate percentages taken close to failure drive size. Programmes vary intensity deliberately rather than staying at one point on that scale.

In practice most people estimate their 1RM from a heavier set rather than testing a true single, then work backwards to the percentages a programme calls for. Effort-based measures like RPE and RIR are often more useful day to day, because your true maximum moves with sleep, stress and fatigue.